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Geological Survey of Finland

Geological Survey of Finland

Bulletin 405 • Monograph

Sm–Nd and U–Pb isotope geochemistry of the Palaeoproterozoic mafic magmatism in eastern and northern Finland

Hannu Huhma, Eero Hanski, Asko Kontinen, Jouni Vuollo, Irmeli Mänttäri and Yann Lahaye

2018Geological Survey of Finland

Geological Survey of Finland, Bulletin

The Bulletin of the Geological Survey of Finland publishes the results of scientific research that is thematically or geographically connected to Finnish or Fennoscandian geology, or otherwise related to research and innovation at GTK. Articles by researchers outside GTK are also welcome. All manuscripts are peer reviewed.

Editorial BoardProf. Pekka Nurmi, GTK, ChairDr Stefan Bergman, SGUDr Asko Käpyaho, GTKDr Antti Ojala, GTKDr Timo Tarvainen, GTK, Scientific Editor

Instructions for authors available from the Scientific Editor.

GEOLOGICAL SURVEY OF FINLAND

Bulletin 405

Sm–Nd and U–Pb isotope geochemistry of the Palaeoproterozoic mafic magmatism in eastern and northern Finland

by

Hannu Huhma, Eero Hanski, Asko Kontinen, Jouni Vuollo, Irmeli Mänttäri and Yann Lahaye

Unless otherwise indicated, the figures have been prepared by the authors of the publication.

Layout: Elvi Turtiainen Oy

Espoo 2018

Huhma, H.1) , Hanski, E.2), Kontinen, A.3), Vuollo, J.4), Mänttäri, I.1) & Lahaye, Y.1) 2018. Sm–Nd and U–Pb isotope geochemistry of the Palaeoproterozoic mafic magmatism in eastern and northern Finland. Geological Survey of Finland, Bulletin 405, 150 pages, 128 figures, 1 table and 11 appendices.

The extensive isotopic studies performed at the Geological Survey of Finland (GTK) since the early 1970s have shown that mafic magmas in the Karelia province of the Fennoscandian Shield were emplaced in several stages, including ca. 2.5 Ga, 2.44 Ga, 2.3 Ga, 2.22 Ga, 2.15 Ga, 2.12 Ga, 2.05 Ga, 2.0 Ga, 1.88 Ga and 1.78 Ga. Most of the rock associations formed during these events are related to episodes of shield-wide extension and rifting of the Archaean lithosphere and may be re-garded as examples of ancient large igneous provinces. The Sm–Nd whole-rock and mineral data produced by GTK on Palaeoproterozoic mafic rocks in the Karelia province consist of ca. 800 analyses from ca. 100 rock units. The Sm–Nd mineral ages from well-preserved samples are mostly consistent with the available U–Pb zircon ages and provide reliable estimates for the initial isotope composition of the rocks in question. These data, together with geochemical and other geological information, are used to constrain the age and origin of mafic magmas and the evolution of the lithosphere.

The initial εNd values in the studied mafic rocks range from very positive to strongly negative and suggest that some of them were derived from a depleted mantle source, whereas others record a large contribution from old enriched lithosphere. Long-term mantle heterogeneity is evident from the isotopic data on high-REE mantle-derived rocks. Nearly chondritic initial εNd values were obtained from the 2.6 Ga Siilinjärvi carbonatite, the 2.0 Ga Jormua OIB and 1.8 Ga lamprophyres, whereas the 2.0 Ga Laivajoki and Kortejärvi carbonatites have yielded clearly posi-tive initial εNd values of +2.5. Further evidence for a depleted mantle source (by εNd = +4) is provided, for example, by the 2.0–2.1 Ga komatiites and some basalts in Lapland and 2.3 Ga dykes in eastern Finland. The positive εNd(T), particularly at 2.0–2.1 Ga, may indicate major attenuation of the lithosphere, which eventually allowed material from convective mantle to ascend uncontaminated to the surface.

Deep-crustal contamination of ultramafic magma may explain many features of the studied mafic-ultramafic rocks, such as the 2.44 Ga layered intrusions with an εNd value of –2. Occasionally, contamination of country rock material at the final emplacement site of an intrusion may have been important, for example in the case of the 2.06 Ga Kevitsa mafic intrusion, showing an initial εNd value from –3.4 to –6.4.

The age and initial Nd isotope composition, together with other relevant infor-mation, provide tools for correlating dykes, intrusions and mafic extrusive units within the Fennoscandian Shield. The data are useful in constraining the ages of the Karelian lithostratigraphic units and their correlation. The results can also be used in correlating events between different cratons, particularly across the Atlantic to the Canadian Shield.

Electronic appendices are available at http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_405_appendix_1_11.xlsxElectronic table is available at http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_405_table_1.xlsx

Keywords: Karelia Province, mafic magmas, gabbros, dykes, isotopes, absolute age, U/Pb, Sm/Nd, Proterozoic, Finland, Fennoscandian Shield

1) Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland2) Oulu Mining School, P.O. Box 3000, FI-90014 University of Oulu, Finland3) Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland4) Geological Survey of Finland, P.O. Box 77, FI-96101 Rovaniemi, Finland

E-mail: [email protected]

ISBN 978-952-217-394-2 (PDF)ISSN 0367-522X (print) ISSN 2489-639X (online)

2

mailto:[email protected]

http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_405_appendix_1_11.xlsx

http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_405_appendix_1_11.xlsx

http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_405_table_1.xlsx

http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_405_table_1.xlsx

CONTENTS

DEDICATION ........................................................................................................................................................ 6

PREFACE ...............................................................................................................................................................7

1 INTRODUCTION ............................................................................................................................................ 8

2 ANALYTICAL METHODS ............................................................................................................................... 82.1 Nd isotope analysis ...............................................................................................................................82.2 U–Pb isotope analysis ......................................................................................................................... 10

3 LAPLAND ...................................................................................................................................................... 123.1 Geological background ........................................................................................................................ 123.2 The 2.4–2.5 Ga intrusions in Lapland ............................................................................................... 16

3.2.1 Tshokkoaivi intrusion .............................................................................................................163.2.2 Koitelainen intrusion and associated felsic volcanic rocks .................................................. 173.2.3 Peuratunturi and Koulumaoiva intrusions ............................................................................193.2.4 Lehtomaa intrusion ..................................................................................................................213.2.5 Onkamonlehto dyke ............................................................................................................... 22

3.3 The 2.22 Ga Palovaara intrusion ........................................................................................................243.4 The 2.15 Ga intrusions ........................................................................................................................25

3.4.1 Rantavaara intrusion ............................................................................................................... 253.4.2 Tanhua intrusions ................................................................................................................... 26

3.5 The 2.05 Ga intrusions ........................................................................................................................293.5.1 The Kevitsa intrusion .............................................................................................................. 293.5.2 Kevitsa dykes.............................................................................................................................323.5.3 The Moskuvaara intrusion ...................................................................................................... 343.5.4 The Puijärvi and Satovaara intrusions ...................................................................................35

3.6 The 2.0 Ga intrusions in Kittilä..........................................................................................................363.7 The 1.8 Ga Tainio and Lotto intrusions ............................................................................................ 40

3.7.1 Tainio intrusion ....................................................................................................................... 403.7.2 Lotto dyke ..................................................................................................................................41

3.8 Intrusions with unknown age ............................................................................................................ 413.8.1 Väkkärävaara intrusion ............................................................................................................413.8.2 Värriö intrusion ....................................................................................................................... 42

3.9 Volcanic rocks ......................................................................................................................................43

4 TAIVALKOSKI BLOCK IN THE LENTUA COMPLEX AND KUUSAMO SCHIST BELT .............................. 454.1 Geological background ........................................................................................................................454.2 The 2.44 Ga Koillismaa layered intrusion suite ...............................................................................474.3 Dykes in the Lake Pääjärvi and Suoperä areas, Russia .................................................................... 51

4.3.1. Gabbro-norite dyke A1412 Pääjärvi ........................................................................................514.3.2 “Older Fe-tholeiitic dyke” A1414 Pääjärvi ............................................................................51

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4.3.3 “Younger Fe-tholeiitic dyke” A1492 Pääjärvi ......................................................................534.3.4 Orthopyroxene-phyric dyke A1465 Pääjärvi .........................................................................534.3.5 Fe-tholeiitic Oulanka dyke ..................................................................................................... 544.3.6 Gabbro-norite dyke, A1415 Suoperä ...................................................................................... 54

4.4 The 2.3 Ga Karkuvaara intrusion ......................................................................................................554.5 Dykes in the Taivalkoski town area ...................................................................................................57

4.5.1 A1466 Taivalkoski ................................................................................................................... 574.5.2 A1471 Taivalkoski .................................................................................................................... 574.5.3 A1797 Törninkuru .................................................................................................................... 584.5.4 A1796 Kallioniemi .................................................................................................................... 584.5.5 A1800 Murhiniemi ...................................................................................................................604.5.6 A1798 Kontioluoma .................................................................................................................604.5.7 A1794, A1795 Tilsanvaara ........................................................................................................614.5.8 A1802 Koivuvaara .....................................................................................................................614.5.9 A1801 Hirsikangas .....................................................................................................................61

4.6 Volcanic rocks in the Kuusamo schist belt ......................................................................................62

5 PUDASJÄRVI COMPLEX AND THE PERÄPOHJA SCHIST BELT ............................................................... 635.1 Geological background ........................................................................................................................635.2 The 2.44 Ga Kemi, Penikat, Kilvenvaara and Siikakämä intrusions .............................................655.3 Loljunmaa gabbro-noritic dyke ....................................................................................................... 685.4 Tholeiitic dykes, A1410 Uolevinlehto, Pudasjärvi ............................................................................ 695.5 The 2.44 Ga Vengasvaara intrusion ...................................................................................................705.6 Palomaa dyke, A1743 ........................................................................................................................... 715.7 Tervonkangas dyke A1808 ................................................................................................................. 715.8 The 2.13–2.14 Ga dykes in the Peräpohja schist belt, A1214 Koppakumpu and A2087 Kuusivaara ...............................................................................................................................735.9 Rytijänkkä dyke A854 .........................................................................................................................745.10 Volcanic rocks ......................................................................................................................................75

6 KUHMO BLOCK IN THE LENTUA COMPLEX ............................................................................................ 766.1 Geological background ........................................................................................................................766.2 The 2.4 Ga boninite-norite Viianki dyke, A1356 .............................................................................786.3 The 2.3 Ga dykes, Lohisärkkä A1914, Kovavaara A1361, Karhuvaara A1672 ..................................796.4 The 2.2 Ga Rasiaho dyke A261 ........................................................................................................... 806.5 The 2.15 Ga Petronjärvi dyke A1363 ...................................................................................................836.6 The 2.1 Ga Kapea-aho dyke A1212 ......................................................................................................836.7 The 2.0–1.95 Ga dykes, Kivikevätti A1409, Puuropuro A1673, Peräaho A1519, Kivimäki A1460 846.8 Dykes in the Veitsivaara area, A1489b & A1489c ............................................................................ 896.9 Dykes in the Romuvaara area............................................................................................................ 90

7 KAINUU SCHIST BELT ................................................................................................................................907.1 Geological background ...................................................................................................................... 907.2 The 2.44 Ga Junttilanniemi plutonic-volcanic complex (A1595-6)...............................................937.3 The Kapustakangas intrusive suite (A1373) ..................................................................................... 967.4 The 2.22 Ga intrusions in the Kainuu schist belt .............................................................................977.5 The 1.95 Ga Jormua ophiolite ............................................................................................................1017.6 Volcanic rocks .................................................................................................................................... 105

8 IISALMI COMPLEX .....................................................................................................................................1058.1 Geological background ...................................................................................................................... 1058.2 The 2.3 Ga dykes, Humppi A135, Siunaussalmi A1369, Petäiskangas A1362 ...............................106

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8.3 The 2.13 Ga Nieminen dyke, A1223 and A1368 ................................................................................1108.4 The 2.06 Ga Otanmäki intrusion, A1381 ........................................................................................... 1118.5 The ca. 2.0 Ga dykes Koirakoski A1875, Jäkäläkangas A1838 .........................................................1128.6 The 1.89 Ga Lapinlahti intrusion ......................................................................................................1148.7 Volcanic rocks in the Siilinjärvi area ................................................................................................115

9 THE OUTOKUMPU AREA ........................................................................................................................... 116

10 TOHMAJÄRVI VOLCANIC COMPLEX AND A BASEMENT DYKE ............................................................. 11810.1 Oravaara gabbro A398 ........................................................................................................................11810.2 Purola dyke A1231 ...............................................................................................................................119

11 CARBONATITES AND LAMPROPHYRES ...................................................................................................12011.1 Geological background ...................................................................................................................... 12011.2 The 2.61 Ga Siilinjärvi carbonatite ....................................................................................................12111.3 The Sokli carbonatite .........................................................................................................................12111.4 The 2.0 Ga Laivajoki and Kortejärvi carbonatites .......................................................................... 12211.5 Lamprophyres .................................................................................................................................... 123

12 DISCUSSION ................................................................................................................................................12412.1 Episodic rifting stages of the Archaean lithosphere ...................................................................... 12412.2 Range of εNd – evidence for heterogeneous mantle and crustal contamination ......................... 12712.3 The 2.44–2.50 Ga intrusions and dykes ......................................................................................... 13612.4 The 2.3 Ga mafic rocks ...................................................................................................................... 13812.5 The 2.22 Ga intrusions ...................................................................................................................... 13912.6 The 2.1–2.15 Ga mafic rocks ............................................................................................................. 14012.7 The 1.95–2.06 Ga mafic rocks .......................................................................................................... 140

13 CLOSING REMARKS .................................................................................................................................. 141

ACKNOWLEDGEMENTS ...................................................................................................................................142

REFERENCES ....................................................................................................................................................142

5

Geological Survey of Finland, Bulletin 405Hannu Huhma, Eero Hanski, Asko Kontinen, Jouni Vuollo, Irmeli Mänttäri and Yann Lahaye

DEDICATION

We want to dedicate this work to Dr Olavi Kouvo, the founder of isotope geology in Finland, who recently passed away at the age of 96. He started his isotope geological career at Princeton University in the United States in the mid-1950s. There, he had an opportunity to work with eminent pioneers of geochronology, including George Tilton, George Wetherill and Paul Gast. His doctoral thesis in 1958 was based on U–Pb, Rb–Sr and K–Ar isotope analy-ses conducted on minerals from Finland. His results, particularly the U–Pb data on zircon, revolutionised our understanding of Finnish Precambrian geology.

In the early 1960s, Olavi Kouvo established the isotope laboratory at the Geological Survey of Finland, Espoo, applying the know-how he had acquired during his visit to the Carnegie Institution of Washington. Since then, active collaboration of the isotope laboratory with field geologists at the survey and in universities and mining companies has been the driving force in collecting systematic high-quality sample materials and isotope data from Finland. His interest was not only in dating rocks, but in the overall understanding of geological processes and evolution. Related to this, his enthu-siasm, contacts and the respect he had within the scientific community at home and abroad greatly helped in realizing the initiatives to set up facilities and personnel for Sm–Nd and stable isotope studies at the Geological Survey of Finland.

We are confident that the current paper would have been of special interest to Olavi Kouvo, noting that he was among the first geochronologists to use the U–Pb zircon method on zircon to date coarse-grained mafic igneous rocks.

66

PREFACE

The isotope laboratory at the Geological Survey of Finland (GTK) was established in the early 1960s by Dr Olavi Kouvo. From the beginning, the principal method has been U–Pb dating, mainly applied to zircon. The age data produced at the laboratory have been fundamental in establishing the age relation-ships of the main geological units in the Finnish part of the Fennoscandian Shield.

Subsequently, contributions from other meth-ods have also been important. The Pb–Pb isotope analyses on sulphides by Matti Vaasjoki since the 1970s focused on ore genetic studies. Sm–Nd and stable isotope methods were set up in the 1980s by Hannu Huhma and Juha Karhu, respectively. The Sm–Nd data have contributed significantly to the understanding of crustal genesis, and C-isotopes in sedimentary carbonates have been used in pal-aeoenvironmental studies.

In addition to zircon, important results have been obtained by applying the U–Pb method to other minerals, e.g., monazite, which can be used to con-strain the timing of metamorphism. As a whole, our understanding of the geological evolution of Finland greatly relies on the results produced by the GTK isotope laboratory.

The main objective of this volume is to pub-lish isotope data on Palaeoproterozoic mafic rocks from the Karelia province in eastern and northern Finland. The Sm–Nd method has mainly been used in studying the genesis of crustal rocks, but also in obtaining geochronological information on igne-ous rocks. Alteration has been a frequent problem in the Sm–Nd dating of ancient metamorphosed mafic igneous rocks from the Finnish bedrock, but

reasonably good results have been obtained by using relict primary igneous phases when clean mineral separates could be produced. The emphasis of this volume is on presenting Sm–Nd isotope data on well-preserved samples from a large number of mafic to ultramafic intrusions and dykes occur-ring in the Karelia province, and on providing tools for constraining the age and origin of the Palaeoproterozoic mafic magmatism and for mod-elling the geological evolution of the lithosphere of the shield.

In these studies, co-operation with several geol-ogists from GTK and Finnish universities has been very important. Without systematic mapping and research performed by geologists with good local geological knowledge, especially in Lapland, many rocks with primary igneous phases would not have been found and studied. In this context, it is also appropriate to acknowledge the staff of the iso-tope laboratory at GTK, including Matti Karhunen, Leena Järvinen, Tuula Hokkanen, Lasse Heikkinen and Arto Pulkkinen, who have made a major con-tribution to sample processing and isotope meas-urements. Special thanks go to Tuula Hokkanen, who has performed most of the final purification of minerals and chemical processing, and to Arto Pulkkinen for most of the recent mass spectrom-etry. Discussions with Hugh O’Brien and the late Matti Vaasjoki at the isotope laboratory, as well as many other colleagues over the years, are greatly acknowledged.

Hannu Huhma

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Geological Survey of Finland, Bulletin 405Sm–Nd and U–Pb isotope geochemistry of the Palaeoproterozoic mafic magmatism in eastern and northern Finland


1 INTRODUCTION

Abundant isotope data have been obtained from mafic rocks in Finland since the pioneering studies by Olavi Kouvo in the 1960s, when he established the U–Pb zircon dating laboratory at GTK in Espoo. The Sm–Nd method was set up in the early 1980s, and from the beginning has been widely used to date and study the origin of mantle-derived mafic rocks. Numerous studies have utilised these results, but in many cases, the primary data have remained unpublished or incompletely published (e.g., Vuollo & Huhma 2005, Huhma et al. 1995). One of the aims of this paper is to publish all these data. An impor-tant contribution is also provided by recent laser ablation ICP-MS analyses conducted on many of the previously studied samples, from which multigrain ID-TIMS analyses had yielded discordant and het-erogeneous results. In addition to the data produced at GTK, some U–Pb data on baddeleyite analysed at the Royal Ontario Museum, University of Toronto, are also included in this volume.

This paper reports ca. 400 previously unpublished Sm–Nd analyses on mafic rocks from eastern and northern Finland (Appendix 1). These are combined with ca. 200 previously published Sm–Nd analyses to constrain the origin of mafic magmatism and related geological evolution. The previously unpub-lished U–Pb data include ca. 220 analyses by TIMS, ca. 240 analyses by SIMS (NORDSIM and SHRIMP) and ca.1200 analyses by LA-MC-ICPMS. The ICPMS

work also involved ca. 200 analyses of a control sample (A382).

The Sm–Nd results, together with published data, cover approximately 90 intrusions and 20 mafic volcanic formations within the Karelia prov-ince in Finland (Fig. 1). In addition, new U–Pb zir-con or baddeleyite ages are given for ca. 30 samples (no Nd available for these). In presenting the iso-tope results for mafic rocks, we group the studied rocks largely based on their areal distribution in different geographical units, such as Lapland and the Outokumpu and Tohmajärvi areas, or geologi-cal units, such as Palaeoproterozoic schist belts, including the Peräpohja, Kuusamo and Kainuu schist belts, and Archaean basement areas, such as the Taivalkoski and Kuhmo blocks (Vuollo & Huhma 2005) in the Lentua complex and Pudasjärvi and Iisalmi complexes. Carbonatites and lamprophyres are considered in a chapter of their own. The study sites are located within the borders of Finland, with the exception of six mafic dykes that occur in the Pääjärvi and Suoperä areas in Russia, east and southeast of Kuusamo (Fig. 38).

After reporting the data, with possible comments on their significance in the interpretation of local geological relationships, the results are briefly summarised and discussed in order to constrain the age and origin of the Palaeoproterozoic mafic magmatism within the Karelia province.

2 ANALYTICAL METHODS

2.1 Nd isotope analysis

Most Sm–Nd studies in this paper were performed at the Geological Survey of Finland using the fresh-est samples that were available. Standard proce-dures were used for crushing and the separation of plagioclase and pyroxene, and the final purifica-tion often involved hand-picking. For the Sm–Nd analyses, mineral concentrates were washed ultra-sonically in warm 6 M HCl for 30 min and rinsed several times in water. The samples (150–200 mg) were dissolved in HF-HNO3 using Savillex screw-cap Teflon beakers or sealed Teflon bombs (felsic whole rocks) for 48 h. A mixed 149Sm–150Nd spike was added to the sample prior to the dissolution. After careful evaporation of fluorides, the residue was dissolved in 6 M HCl so that a clear solution

was achieved. Samarium and Nd were separated in two stages using a conventional cation exchange procedure (7 ml of AG50Wx8 ion exchange resin in a bed of 12 cm length) and a modified version of the Teflon-HDEHP (hydrogen diethylhexyl phos-phate) method developed by Richard et al. (1976). The measurements were carried out in a dynamic mode on a VG SECTOR 54 mass spectrometer using Ta-Re triple filaments. Nd ratios were normalised to 146Nd/144Nd = 0.7219. The long-term average 143Nd/144Nd ratio for the La Jolla standard is 0.511850 ± 0.000010 (standard deviation for 220 measure-ments during 1996–2010). The Sm/Nd ratio of the spike was calibrated against the Caltech mixed Sm/Nd standard (Wasserburg et al. 1981). Based on

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Fig. 1. Map showing the localities of the studied Palaeoproterozoic mafic rocks in the Karelia province. Stars and dots – intrusions; Red star – Sm–Nd (±U–Pb, 86 targets), green star – volcanic rocks (Sm–Nd, 22 targets), red dot – Sm–Nd in this paper, but age not well constrained (6), blue star – U–Pb in this paper (no Nd data avail-able, 29), black open star – U–Pb age published elsewhere (78). Symbol size correlates with age: >2380 Ma (large symbol); other break values 2280, 2180, 2080 and 1930 (<1930 Ma small symbol). Base map – geological map of the Fennoscandian Shield (Koistinen et al. 2001).

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several duplicate analyses, the error of the 147Sm/144Nd ratio was estimated to be better than 0.4%.

Measurements performed on the rock stand-ard BCR-1 provided the following values: Sm = 6.58 ppm, Nd = 28.8 ppm, 147Sm/144Nd = 0.1380, 143Nd/144Nd = 0.51264 ± 0.00002. The blank was 30–100 pg for Sm and 100–300 pg for Nd. The εNd values were calculated using λ147Sm = 6.54 · 10-12 a-1, 147Sm/144Nd = 0.1966, and 143Nd/144Nd = 0.512640 for the present CHUR. Depleted-mantle model ages

(TDM) were calculated after DePaolo (1981). Linear regression calculations and plotting of isotope data on isochron diagrams were performed using the Isoplot/Ex 3.0 programme of Ludwig (2003).

A few older Sm–Nd analyses carried out using an old technique and a non-commercial mass spec-trometer (Huhma 1986) are included in this paper. Compared with more recent analyses, they tend to yield slightly larger errors in 143Nd/144Nd, but based on duplicated newer analyses, are consistent within error.

2.2 U–Pb isotope analysis

Sampling for the isotope studies was mostly car-ried out in conjunction with extensive mapping projects, and the samples should thus be well chosen and relevant in solving major geological problems. Samples for the isotope studies were washed, crushed, cleaned from light minerals using a Wilfley table, and treated with methylene iodide and Clerici® solutions for separation of the heavy minerals. Non-magnetic heavy fractions were sep-arated with a Frantz isodynamic separator. Zircon grains were selected for analyses by hand-pick-ing. Some of the fractions were air abraded (Krogh 1982) for ID-TIMS (isotope dilution thermal ion-isation mass spectrometry) and U–Pb analyses, and for some more recent analyses, zircons were treated using the chemical abrasion (CA) method by Mattinson (2005). When applying the CA-TIMS technique, we largely followed the steps described by Schoene et al. (2006), in which zircon was placed in a furnace at 900 oC for 60 hours in beakers before being transferred to Teflon microcapsules, placed in high-pressure vessels, and leached in 29M HF for 12 hours. The decomposition of minerals and extraction of U and Pb for multigrain ID-TIMS anal-yses mainly followed the procedure described by Krogh (1973). 235U-208Pb-spiked and -unspiked isotope ratios were measured using a VG Sector 54 or non-commercial mass spectrometers at the Geological Survey of Finland in Espoo. The mea-sured lead and uranium isotope ratios were nor-malised to the accepted values of the NBS 981 and U500 standards. The performance of the ion counter was checked by repeated measurements of the NBS 983 standard.

Common-lead corrections were carried out using the age-related Pb isotope composition of the Stacey and Kramers (1975) model and errors of 0.2 for 206Pb/204Pb and 208Pb/204Pb and 0.1 for

207Pb/204Pb. The measured Pb blank was 10– 50 pg, but probably higher during the few ancient analyses included. The U–Pb age calculations were performed using the PbDat and the Isoplot/Ex pro-grammes (Ludwig 1991, 2003). Some U–Pb analy-ses on baddeleyite were obtained from the Royal Ontario Museum, University of Toronto, following the methods of Krogh et al. (1987).

For SIMS and LA-ICP-MS analyses, zircon grains were hand-picked under a binocular microscope, mounted in epoxy resin, sectioned approximately in half and polished. To target the analysis spots, back-scattered electron (BSE) and cathodolumi-nescence (CL) images of the zircon grains were taken using SEM. Half of the SIMS U–Pb analy-ses were performed using the Nordic Cameca IMS 1270 at the Swedish Museum of Natural History, Stockholm (NORDSIM facility). The spot diameter for the 4 nA primary O2- ion beam was 25 μm, and oxygen flooding in the sample chamber was used to increase the production of Pb+ ions. Three counting blocks, each including four cycles of the Zr, Pb, Th and U species of interest, were measured from each spot. The mass resolution (M/ΔM) was 5400 (10%). The raw data were calibrated against a zircon standard (91500; Wiedenbeck et al. 1995) and corrected for modern common lead (T = 0; Stacey & Kramers 1975). For the detailed analytical proce-dure, see Whitehouse et al. (1997). All the errors in age reported in the text and figures are given at the 2σ level. For some samples, the dating was carried out at VGESEI in St Petersburg using SHRIMP II and the analytical methods described by Larionov et al. (2004).

The measurements by LA-MC-ICPMS were per-formed utilising the Nu Plasma HR multicollector ICPMS at the Geological Survey of Finland in Espoo. A technique very similar to that described by Rosa

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et al. (2009) was applied, with the exception that a New Wave UP193 Nd:YAG laser microprobe was used. Samples were ablated in He gas (gas flow = 0.2– 0.3 l/min) using a low-volume teardrop-shaped (<2.5 cm3) laser ablation cell (Horstwood et al. 2003). The He aerosol was mixed with Ar (gas flow = 1.2 l/min) in a Teflon mixing cell prior to entry into the plasma. The gas mixture was optimised daily for maximum sensitivity. All analyses were conducted in static ablation mode. The ablation conditions were the following: beam diameter generally 25 μm, pulse frequency 10 Hz and beam energy density 1.4 J/cm2. A single U–Pb measurement included 30 s of on-mass background measurement, followed by 60 s of ablation with a stationary beam. Masses 204, 206 and 207 were measured in secondary electron multipliers and 238 in the extra high-mass Faraday collector. The geometry of the collector block does not allow simultaneous measurement of 208Pb and 232Th. Ion counts were converted and reported as volts by the Nu Plasma time-resolved analysis soft-ware. 235U was calculated from the signal at mass 238 using a natural 238U/235U ratio of 137.88. Mass number 204 was used for monitoring the amount of common 204Pb. In ICP-MS analyses, 204Hg mainly originates from the He supply. The observed back-ground counting rate at mass 204 was ca. 1200 (ca. 1.3 × 10−5 V), and had been stable at that level dur-ing the year prior to the measurements. The con-tribution of 204Hg from the plasma was eliminated by on-mass background measurement prior to each analysis.

Age-related common-lead (Stacey & Kramers 1975) correction was used if the analysis showed common-lead contents above the detection limit. Signal strengths at mass 206 were typically >10−3 V, depending on the uranium content and age of the zircon. Two calibration standards were run in dupli-cate at the beginning and end of each analytical ses-sion and at regular intervals during sessions. Raw data were corrected for background, laser-induced elemental fractionation, mass discrimination and drift in ion counter gains, and reduced to U–Pb iso-tope ratios by calibration to concordant reference zircons of known age, using protocols adapted from Andersen et al. (2004) and Jackson et al. (2004). Standard zircons GJ-01 (609 ± 1 Ma; Belousova et al. 2006) and an in-house standard, A1772 (2711 ± 3 Ma/TIMS; 2712 ± 1 Ma/SIMS, Huhma et al. 2012a), were used for calibration.

The calculations were performed offline using an interactive spreadsheet programme written in Microsoft Excel/VBA by T. Andersen (Rosa et al. 2009). To minimise the effects of laser-induced elemental fractionation, the depth-to-diameter ratio of the ablation pit was kept low, and isotopi-cally homogeneous segments of the time-resolved traces were calibrated against the corresponding time interval for each mass in the reference zir-con. To compensate for drift in instrument sen-sitivity and Faraday vs. electron multiplier gain during an analytical session, a correlation of sig-nal vs. time was assumed for the reference zircons. A description of the algorithms used is provided in Rosa et al. (2009). Plotting of the U–Pb isotope data and age calculations were performed using the Isoplot/Ex 3.0 programme (Ludwig 2003). All ages were calculated with 2σ errors and without decay constant errors.

A few recent U–Pb analyses were performed using a Nu Plasma AttoM single collector ICP-MS at the Geological Survey of Finland in Espoo, con-nected to a Photon Machine Excite laser ablation system. Samples were ablated in He gas (gas flows = 0.4 and 0.1 l/min) within a HelEx ablation cell (Müller et al. 2009). The He aerosol was mixed with Ar (gas flow = 0.8 l/min) prior to entry into the plasma. The gas mixture was optimised daily for maximum sensitivity. Typical ablation conditions were the following: beam diameter 25 μm, pulse frequency 5 Hz and beam energy density 2 J/cm2. A single U–Pb measurement included a short pre-ablation, 10 s of on-mass background measure-ment, followed by 30 s of ablation with a stationary beam. 235U was calculated from the signal at mass 238 using a natural 238U/235U = 137.88. Mass num-ber 204 was used as a monitor for common 204Pb. The observed background counting rate at mass 204 was 150–200 cps, and had been stable at that level over the previous two years. The contribution of 204Hg from the plasma was eliminated by on-mass background measurement prior to each analysis. Age-related common lead (Stacey & Kramers 1975) correction was used when the analysis showed com-mon lead contents significantly above the detection limit (i.e., >50 cps). Signal strengths at mass 206 were typically 100 000 cps, depending on the ura-nium content and age of the zircon.

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3 LAPLAND

3.1 Geological background

The main units of the geology of Finnish Lap-land are the Archaean basement complexes, Palaeoproterozoic sedimentary and volcanic rocks of the Central Lapland greenstone belt, granitoids, and the Lapland granulite belt. The largest basement terrain, the Kemihaara granitoid complex, occurs in eastern Lapland, south of the Lapland granulite belt (Fig. 1). It is composed of typical Archaean TTG gneisses, granitic intrusions, small greenstone belts and a 15- to 30-km-wide and 100-km-long mica gneiss area called the Tuntsa suite (Fig. 2). The lat-ter hosts some of the Palaeoproterozoic intrusions dated in this study (Chapter 3.2.3 and 3.8.2).

The geology of Finnish Lapland is dominated by the Palaeoproterozoic Central Lapland greenstone belt (CLGB). The greenstone belt extends from the Finnish–Russian border at Salla through the Sodankylä and Kittilä areas to the north until the

Finnish–Norwegian border (Fig. 1). In the south, it is bordered by the Central Lapland Granitoid Complex and Vuojärvi suite (paragneisses between the southern granite complex and northern CLGB) and in the northeast by the Lapland granulite belt. In the division and terminology recently proposed by Nironen et al. (2016), the Vuojärvi group/suite is renamed as the Central Lapland supersuite. Descriptions of the stratigraphic successions of the CLGB are found in Lehtonen et al. (1998) and Hanski & Huhma (2005), and a large amount of geochrono-logical data was published in a volume of the Special Paper of the Geological Survey of Finland edited by Vaasjoki (2001). The stratigraphy is divided into several lithostratigraphic groups (Fig. 3) bearing witness to a sedimentary and volcanic evolution of hundreds of millions of years. In this paper, we fol-low the approach of Hanski & Huhma (2005) with

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Nu�o

Puijärvi

Keivitsa

Lehtomaa

Palovaara

Pi�arova

Satovaara

Akanvaara

Rovasvaara

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Rantavaara

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KannusvaaraSilmäsvaara

HaaskalehtoKoulumaoiva

Onkamonlehto

Peuratunturi

Selkäsenvuoma

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( in this paper )

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Fig. 2. Geological map of Lapland showing sample localities. For symbols, see Figure 1. The Sm–Nd results from the 2.44 Ga Koitelainen and Akanvaara layered intrusions were published by Hanski et al. (2001a) and from the 2.2 Ga Haaskalehto, Ahvenvaara and Silmäsvaara intrusions by Hanski et al. (2010).

remarks on the updated terminology (Nironen et al. 2016, 2017).

The development of the CLGB started around the Archaean–Proterozoic boundary, with rifting of the Archaean basement and subaereal eruption of inter-mediate to felsic volcanic rocks of the Salla Group. These were followed by the crustally contaminated komatiites and mafic lavas of the Kuusamo Group (corresponding to the Onkamo Group of Lehtonen et al. (1998) and Hanski & Huhma (2005)). The 2.44 Ga layered intrusions (Koitelainen and Akanvaara) cut Salla Group rocks, but not those of the Kuusamo Group (Manninen & Huhma 2001). The early rifting-related volcanism was followed by the deposition of siliciclastic sediments, basaltic flood basalt-type lavas, carbonate rocks and pelitic sediments of the Sodankylä Group. The quartzites of the Sodankylä Group are cut by 2.22 Ga mafic-ultramafic sills, giv-ing a minimum age for these sedimentary rocks. Subsequent deepening of the sedimentary basin resulted in the accumulation of fine-grained sedi-ments, phyllites and black schists, assigned to the Savukoski Group. The upper part of this group is composed of komatiitic and picritic volcanic rocks and their derivative basalts. The emplacement of

the 2.05 Ga Kevitsa layered intrusion (Mutanen & Huhma 2001) into black schists of the Savukoski Group constrains the age of these rocks. The ca. 2.0 Ga Kittilä Group (Kittilä suite in Finnstrati, Nironen et al. 2016), which comprises various basaltic vol-canic rocks (NMORB, OIB, IAT), chemical sedi-ments and pieces of ophiolitic mantle rocks, is thought to be an allochthonous sliver of oceanic crust (Hanski 1997) and thus in tectonic con-tact with the sedimentary-volcanic associations described above. Quartzites and conglomerates of the Kumpu Group and minor volcanic rocks cap earlier rocks with angular unconformity. These molasse-type sediments contain pebbles of ca. 1.88 Ga synorogenic granitoids.

In the Central Lapland greenstone belt, differen-tiated mafic-ultramafic bodies and mafic dykes were emplaced in several stages, including ca. 2.44 Ga, 2.22 Ga, 2.15 Ga, 2.05 Ga and 2.0 Ga (Hanski et al. 2001a). Small mafic intrusions are also associated with generally felsic syn- to post-orogenic granitic magmatism, as discussed later on. The ca. 2.44 Ga age group is represented by the Koitelainen and Akanvaara intrusions (Fig. 4). Zircon U–Pb dat-ing results obtained at GTK for these intrusions

Fig. 3. Group-level lithostratigraphy of the Central Lapland greenstone belt after Lehtonen et al. (1998) with modifications: the Kumpu and Lainio Groups have been united with the Kumpu Group, the name of the Onkamo Group has been changed to the Kuusamo Group and the Vuojärvi suite has been included. Numbers indicate ages in billions of years.

1313



(Mutanen & Huhma 2001) and other similar lay-ered intrusions from the Tornio-Näränkävaara intrusion belt (Alapieti 1982, Perttunen & Vaasjoki 2001, Maier et al. in press, this work) are consist-ent with each other and also with ages obtained for mafic layered intrusions in Russia, including the Oulanga complex and the Burakovka, Imandra, Fedorovo-Pansky and Mt. Generalskaya intrusions (e.g., Amelin et al. 1995). In contrast, slightly older ages close to ca. 2.50 Ga have been reported for the Monche, Fedorovo-Pansky and Mt. Generalskaya intrusions (Balashov et al. 1993, Amelin et al. 1995, Bayanova et al. 1999). There has been no evidence for the presence of intrusions of this age group in Finland until this study (see Chapter 3.2.1).

Sm–Nd studies have demonstrated that the Koitelainen and Akanvaara intrusions (Hanski et al. 2001c) and the bulk of the other ca. 2.44– 2.50 Ga mafic intrusions (Huhma et al. 1990, Tolstikhin et al. 1992, Balashov et al. 1993, Amelin & Semenov 1996, Karinen 2010) are characterised by negative initial εNd values from –1 to –2, thus having a clear lithospheric signature in their Nd isotope composition. The initial εNd values from the Tsipringa intrusion (Oulanga complex) and the

lower zone of the Burakovka intrusion seem to be slightly higher, from –1 to 0. The Sm–Nd results for most mafic dykes of this age group also give initial εNd values of –1 to –2, although some dykes have yielded higher values of up to +1.7 (this paper).

The Sodankylä Group quartzites are cut by 2.22 Ga, strongly differentiated mafic-ultramafic sills that are called karjalites or assigned to the gabbro-wehrlite association (GWA), based on their common occurrence in Karelian schist belts or their typi-cal lithology, respectively (Vuollo & Huhma 2005, Hanski et al. 2010). The presence of these differ-entiated sills, together with positive δ13C values in dolomitic carbonate rocks (Karhu 2005), allows the correlation of the Sodankylä Group quartzites with Jatulian-system quartzites in other Karelian schist belts, such as the Peräpohja, Kuusamo, Kainuu and North Karelia belts. Using the TIMS and SIMS methods and an electron microprobe, Hanski et al. (2010) performed a detailed geochronological and mineralogical study on zircon in the GWA sills from several schist belts in order to find reasons for the commonly observed strong discordancy of the iso-tope compositions and to clarify the real magmatic ages of the rocks. One of the most well-preserved

Fig. 4. Geological map showing major intrusions related to the several generations of mafic-ultramafic magma-tism in Central Lapland.

1414

and precisely dated examples of the 2.22 Ga sills is the Haaskalehto intrusion (Fig. 4), which is located ~20 km west of Sodankylä (Hanski et al. 2010). It seems that these sills represent a short-lived but widely distributed magmatic phase in Karelian formations in eastern and northern Finland, with similar sills also being recently recognised in the Kautokeino belt, northern Norway (Bingen et al. 2015).

The following intrusive event took place at ca. 2.15 Ga, producing a group of differentiated intru-sions in the Tanhua area and the Rantavaara sill in an area between the Koitelainen intrusion and Sodankylä town (Fig. 4). The magmatism that occurred at 2.05 Ga in Lapland has special signifi-cance, as it included voluminous komatiitic volcanic eruptions (Hanski et al. 2001b) and differentiated mafic–ultramafic intrusions with a high ore poten-tial, as testified by the Ni–Cu–PGE-bearing Kevitsa intrusion (Mutanen 1997, Mutanen & Huhma 2001, Santaguida et al. 2015) located just south of the Koitelainen intrusion (Fig. 4). Mafic intrusions with an age of ca. 2.0 Ga occur within the Kittilä allochthon (Kittilä Group) and, together with felsic porphyries, serve as good targets for obtaining geo-chronological information for this aerially extensive geological unit (Rastas et al. 2001).

Palaeoproterozoic felsic plutonic rocks in north-ern Finland are divided into two main broad groups: the ca. 1.89–1.88 Ga synorogenic plutons, includ-ing the Haaparanta Suite, and postorogenic, ca. 1.80–1.77 Ga granites (Nironen 2005). Both of these magmatic stages include a small amount of mafic rocks. An example of synorogenic mafic intrusions is the Fe–Ti oxide-bearing Karhujupukka intrusion (Karvinen et al. 1988). Postorogenic mafic rocks are represented by appinitic intrusions, which are melanocratic hornblende-rich syenite, monzonite or diorite rocks rich in volatites. Currently, more than twenty appinitic pipes and dykes have been recognised in Finnish Lapland (Mutanen 2003, Mutanen & Väänänen 2004, Mutanen 2011). The intrusions occur in swarms, with each swarm being located above a positive Bouguer gravity anomaly. This type of relationship between the occurrence of dense deep-crustal bodies and the location of appinitic intrusions seems to be the case practi-cally everywhere, in southern Finland, eastern Norrbotten in Sweden (Bergman et al. 2001), and in the wider world, suggesting that the sources of appinitic rocks were huge masses of mafic magma emplaced at the mantle–crust boundary or into the

lower crust (“magmatic underplating”) (Neilson et al. 2009). The results of seismic reflection sound-ings in Finnish Lapland support the idea of under-plated magmas.

In Chapters 3.2–3.9, we report geochronologi-cal data and Nd isotope compositions mainly for mafic intrusive and volcanic rocks from the Central Lapland greenstone belt (Fig. 2). In this context, we also publish new data suggesting that some felsic volcanism took place at ca. 2.50 Ga. The other stud-ied intrusive rocks include a granophyre from the 2.44 Ga Koitelainen layered intrusion (Fig. 4), two ca. 2.4–2.43 Ga dyke-like intrusions (Lehtomaa, Onkamonlehto) (Fig. 2), four intrusive bodies of the ca. 2.15 Ga age group (Tanhua, Rantavaara) (Fig. 4), four intrusions and one dyke having an age of ca. 2.05 Ga (Kevitsa, Satovaara, Moskuvaara, Puijärvi) (Fig. 4), and two gabbroic intrusions (Selkäsenvuoma, Pittarova) and felsic porphyries from the Kittilä ca. 2.0 Ga allochthon (Fig. 2).

Among the studied volcanic rocks from the CLGB are Savukoski Group komatiites from the Sattasvaara area and Savukoski Group mafic pil-low lavas from the Linkupalo area (Fig. 5). Some felsic rocks were also included due to problems related to the previous geochronological data. One problematic unit, the Honkavaara Formation of Lehtonen et al. (1998), contains rocks that are interpreted as intermediate and felsic volcanic rocks (Fig. 5). Even though these rocks are regarded as Palaeoproterozoic (Lehtonen et al. 1998, Lahtinen et al. 2015a), U–Pb zircon ages acquired for them are Archaean (ca. 2.72, 2.75 Ga; Rastas et al. 2001). These types of rocks are found close to the south-ern and southeastern margin of the CLGB, not far from the Central Lapland Granitoid Complex, and have been assigned to the Vuojärvi suite or Central Lapland supersuite (Nironen et al. 2016). A charac-teristic feature that they share is strong albitization (Eilu 1994).

Outside the CLGB, two intrusions (Palovaara, Tshokkoaivi, Fig. 1), ca. 2.22 and 2.50 Ga in age, were studied from the NW corner of Finland and two ca. 2.45 Ga intrusions (Peuratunturi and Koulumaoiva) from the Tuntsa suite in eastern Lapland (Fig. 2). Data are also presented for two other intrusions from the Tuntsa suite, which have an uncertain age. The youngest (1.80 Ga) mafic magmatism of this work is represented by the appinitic Tainio intru-sion in the northern part of the Central Lapland Granitoid Complex and the Lotto mafic dyke within the Lapland granulite belt (Fig. 1).

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3.2 The 2.4–2.5 Ga intrusions in Lapland

3.2.1 Tshokkoaivi intrusion

The Tshokkoaivi intrusion is located in the north-western part of Finnish Lapland (Fig. 1). It has previously been studied by Vaasjoki (1971) and Kantti (2002). The intrusion is ca. 10 km long and has a total area of ca. 14 km2. In its northern part (Tshokkoaivi fell), the intrusion runs south, turn-ing southwest at Kaamajoki. The rocks are mainly plagioclase-pyroxene cumulates and plagioclase-pigeonite cumulates (augite norite in Kantti 2002), very similar to corresponding cumulates of the Koitelainen intrusion. Ultramafic rocks (olivine-rich cumulates and olivine-bearing pyroxene cumulates) occur in the lower (eastern) part of the intrusion. Plagioclase-rich cumulates (anorthosites) are known to occur near the eastern contact (Kantti 2002), but judging from the common occurrence of anorthosites as glacial erratics, anorthositic cumu-lates should be common west of Lake Tshokkajavri, in the westernmost (and, supposedly, stratigraphi-

cally the uppermost) part of the intrusion. Here, likewise, erratics of dark iron-rich gabbros with abundant disseminated magmatic Fe–Cu sulphides indicate that such rocks, although unexposed, do occur amongst the uppermost cumulates.

Two samples of the Tshokkoaivi intrusion were collected for Sm–Nd isotope studies from a field of large in situ boulders ca. 2 km SW of the top of the Tshokkoaivi hill. The rock types represented are a pyroxene-bearing gabbro (A1317) and a poiki-litic plagioclase- and olivine-bearing pyroxenite (pyroxene cumulate) (A1318). The primary mag-matic minerals, including clinopyroxene, olivine and plagioclase, are well preserved in both samples.

The Sm–Nd data acquired from whole rock, pyroxene and plagioclase are presented in Appendix 1 and in Figure 6. The analysed rocks have a rela-tively low Sm/Nd ratio, i.e., a LREE-enriched chon-drite-normalised REE pattern, which is typical for 2.4–2.5 Ga mafic intrusions in eastern and north-ern Finland. Only three Sm–Nd analyses per sample

Fig. 5. Geological map of Central Lapland showing the sample sites (symbols in Figure 1).

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(plagioclase, pyroxene, whole rock) are available. The data for gabbro sample A1317 provide an age of 2463 ± 51 Ma and an initial εNd value of –1.9 (MSWD = 1.4, Huhma et al. 1996). The Sm–Nd analyses for the pyroxenite sample A1318 are slightly scattered and give an age of 2441 ± 72 Ma (εNd = –2.5, MSWD = 1.9). Combining all the data, an age of 2458 ± 81 Ma can be calculated. The initial εNd value is –2.1, and slight scatter is indicated by the MSWD value of 1.9.

The age of the Tshokkoaivi intrusion was con-firmed by LA-MC-ICPMS analyses conducted on an old zircon sample, A360. Twenty-five data points from zircon grains extracted from this sample define a chord with intercepts at 2499 ± 11 Ma and 409 ± 27 Ma (Fig. 7, Appendix 2). Many of these results are discordant, but analyses performed utilising the best-quality zircon grains gave concordant ages of ca. 2.5 Ga. Five analyses of strongly altered zircon grains with relatively high common lead contents suggested ages of 1.8–1.9 Ga. These spot analyses explain well the previously obtained discordant and scattered multigrain TIMS data (Fig. 7). It is also likely that the lower intercept age of ca. 400 Ma has geological significance, potentially related to the development of the adjacent Caledonian Orogen. Using an age of 2499 Ma, an initial εNd value of –1.8 can be calculated, which is a typical value for the ca.

2.44–2.50 Ga intrusions (for a summary, see Hanski 2012).

3.2.2 Koitelainen intrusion and associated felsic volcanic rocks

The Koitelainen intrusion is located in central Finnish Lapland (Figs. 2, 4), and with an aerial extent of 26 km x 29 km is the largest layered intru-sion in Finland (Mutanen 1997). An age of ca. 2.44 Ga was already obtained in the 1970s using U–Pb dating of zircon (Kouvo 1977). Mutanen & Huhma (2001) published U–Pb zircon ID-TIMS data and ages of 2439 ± 3 Ma and 2439 ± 7 Ma for two gabbro pegma-toid samples from the intrusion. They also studied a granophyre sample, A580 Kaitaselkä, which gave a clearly younger date of 2405 ± 9 Ma. Recently, zircon grains from this sample were re-analysed by LA-ICP-MS. The data from the best-quality zir-con yielded a concordia age of 2434 ± 5 Ma, which is considered an igneous age (Fig. 8). A few data values with elevated common lead tend to plot on the younger side (Appendix 2). These results pro-vide an explanation for the slightly younger upper intercept age obtained from discordant ID-TIMS data by Mutanen & Huhma (2001).

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0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

143 N

d/14

4 Nd

147Sm/144Nd

A1318 Age = 2441 ± 72 Ma

eps= -2.5 MSWD = 1.9 n=3

A1317 Age = 2463 ± 51 Ma

eps = -1.9 MSWD = 1.4 n=3

Tshokkoaivi intrusion

Age = 2458 ± 81 Ma eps = -2.1

MSWD = 1.9 n=6

U-Pb zircon age 2499±11 Ma

Fig. 6. Sm–Nd isotope data for whole-rock samples and mineral separates from the Tshokkoaivi intrusion.


1818

600

1000

1400

1800

2200

2600

TIMS

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14

206 P

b/23

8 U

207Pb/235U

A360 Haukikuru/ Tshokkoaivi gabbro data-point error ellipses are 2s

Intercepts at 2499 ± 11 Ma & 409 ± 27 Ma

MSWD = 0.92 n=25

Fig. 7. Concordia plot of U–Pb zircon data from the sample A360, Tshokkoaivi intrusion. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses as red dots.

0.2

0.3

0.4

0.5

0.6

2 4 6 8 10 12 207Pb/235U

206 P

b/23

8 U

1300

1500

1700

1900

2100

2300

2500

data-point error ellipses are 2s

Concordia Age = 2434 ±5 Ma

n=7

TIMS Intercepts at 256±25 & 2408 ± 6 Ma MSWD = 10.8, n=10

A580 Kaitaselkä granophyre Koitelainen intrusion

Fig. 8. Concordia plot of U–Pb zircon data for the granophyre sample A580, Koitelainen intrusion. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses as black dots.

In this context, we report updated dating results for the oldest Palaeoproterozoic felsic rocks in Central Lapland. These include rocks that are roughly coeval with the Koitelainen intrusion, but also rocks that are clearly older, with an age of ca. 2.50 Ga (Appendix 3). The latter are represented by the Sadinoja volcanic breccia, studied earlier by Manninen et al. (2001). It is located on the NW side of the Archaean Tojottamanselkä dome in the Koitelainen area (Fig. 5). Most of the LA-MC-ICPMS analyses of zircon from sample A206-Sadinoja were concordant and yielded an age of 2505 ± 5 Ma (Fig. 9a). Three grains are Archaean and well explain the heterogeneity in the conventional TIMS data by Manninen et al. (2001). Sm–Nd analysis of another whole rock (A685) gave an initial εNd(T) value of –3.2, which suggests a significant contribution of older lithosphere in the genesis of these rocks (Appendix 6).

The sample Yläliesijoki A659 was taken one kilo-metre NW of the sample site Sadinoja A206 from a felsic tuff in a strongly tectonised zone close to the western contact of the Koitelainen layered intrusion (Manninen et al. 2001). The LA-MC-ICPMS data on zircon yielded an age of 2426 ± 6 Ma, confirming the previous age of 2438 ± 8 Ma obtained from dis-cordant TIMS data (Manninen et al. 2001, Fig. 9b).

Another felsic rock apparently coeval with the 2.44 Ga layered intrusions is represented by the Akanvaara rhyolite A1524. The old TIMS data on zircon are discordant and slightly heterogeneous (Räsänen & Huhma 2001), whereas most of the U–Pb data obtained by LA-MC-ICPMS are concordant and yield an age of 2434 ± 8 Ma (Fig. 9c, Appendix 3). A few analyses yielded clearly younger age indications, explaining the slightly heterogeneous multigrain TIMS results. A concordant analysis giv-ing an age of 2441 ± 2 Ma was also obtained using chemical abrasion and TIMS (Appendix 5). Sm–Nd analysis of whole rock gave an initial εNd(T) value of –2.3, which is most typical for rocks related to the 2.44 Ga magmatism (Appendix 6).

Support for the existence of slightly older, 2.50 Ga crust in the surroundings of the 2.44 Ga Koitelainen intrusion is provided by new LA-MC-ICPMS data on zircon from two felsic rocks NW of the intru-sion, occurring in an area traditionally called the Vuotso complex. A TIMS age of ca. 2.49 Ga obtained by Manninen et al. (2001) for the Kaunismännikkö gneiss A157 can now be refined to 2506 ± 6 Ma (Fig. 9d, Appendix 3). Another sample (A1671) was obtained from Porttipahta, for which the bulk of

the zircon U–Pb LA-MC-ICPMS data yielded a con-cordia age of 2501 ± 5 Ma (Fig. 9e, Appendix 3). A few analyses, mostly conducted on high U-domains, gave younger age indications, which explain well the 2.45 Ga age obtained with the multigrain TIMS method (Nironen & Mänttäri 2003). The initial εNd(T) value for sample A157 is –1.0 (Appendix 6).

We also used LA-MC-ICPMS to analyse zircon from the Sakiamaa felsic rock A1432 located east of the Koitelainen intrusion (Räsänen & Huhma 2001). The discordant TIMS data published by Räsänen & Huhma (2001) suggested an age of ca. 2.44 Ga, but the concordant data acquired by MC-ICP-MS gave a younger age of 2411 ± 8 Ma (Fig. 9f, Appendix 3). The initial εNd(T) values for the Sakiamaa rocks are very negative (average –7, Appendix 6), suggesting a large contribution of very old crustal material.

Based on the recent data obtained using MC-ICP-MS, we may conclude that the rocks that have traditionally been assigned to the Rookkiaapa Formation show a span in their ages. The younger rocks are roughly coeval with the 2.44 Ga layered intrusions, but the formation also contains 2.50 Ga felsic rocks that clearly predate these intrusions.

3.2.3 Peuratunturi and Koulumaoiva intrusions

The Peuratunturi and Koulumaoiva mafic intrusions are situated in the Archaean Tuntsa suite (Fig. 2). On the aeromagnetic map, the Koulumaoiva intru-sion is located within an anomaly area of ca. 2.5 km in length and 0.5 km in width. The intrusion is not exposed but is covered by about 30 m of soil. Based on drill core data, the intrusion is mainly composed of olivine cumulates, which contain some narrow layers of gabbro. The olivine cumulates locally pre-serve the primary magmatic minerals unaltered. One such olivine cumulate (A1475) was chosen for mineral separation. The main minerals in this rock are clinopyroxene, orthopyroxene and olivine, with plagioclase occurring as an intercumulus phase.

The Peuratunturi intrusion is located ca. 20 km NE of the Koulumaoiva intrusion. On the low-alti-tude aeromagnetic map, this intrusion is associated with a weak magnetic anomaly ca. 1 km in length and 0.3 km in width. It consists of olivine-bearing gabbros, which are characterised by surprisingly well-preserved primary magmatic minerals. Several similar small gabbro intrusions have been located in the area between Peuratunturi and the Russian border. From the Peuratunturi intrusion, an olivine-bearing gabbro (A1474) was sampled for isotopic

1919



2020

2300

2500

2700

2900

3100

X - A685_TIMS 0.35

0.45

0.55

0.65

0.75

7 9 11 13 15 17 19 21 207Pb/235U

A206 Sadinoja breccia

206Pb 238U

Concordia Age = 2505 ± 5 Ma, n=27

A)

2300

2400

2500

2600

0.36

0.40

0.44

0.48

0.52

0.56

0.60

8 9 10 11 12 13

206 P

b/23

8 U

207Pb/235U

A659 Yläliesijoki felsic tuff Concordia Age = 2426 ± 6 Ma (n=20)


TIMS Intercepts at 371 ± 94 & 2438 ± 8 Ma

MSWD = 1.5 (n=3)

B)

2000

2200

2400

2600

0.28

0.32

0.36

0.40

0.44

0.48

0.52

0.56

5 7 9 11 13

206 P

b/23

8 U

207Pb/235U

A1524 Akanvaara rhyolite data-point error ellipses are 2s

LA-MC-ICPMS Concordia Age = 2434 ± 8 Ma (20/24)

CA-TIMS concordant 2441 ± 2 Ma

grain 28: length 150µm TIMS - diamonds

C)

2200

2300

2400

2500

2600

2700

2800

0.34

0.38

0.42

0.46

0.50

0.54

0.58

0.62

7 9 11 13 15 207Pb/235U

206Pb 238U

A157 Kaunismännikkö felsic gneiss Concordia Age = 2506 ± 6 Ma (n=20)


TIMS upper intercept age ca. 2.49 Ga (n=4/9)

D)

2250

2350

2450

2550

0.34

0.38

0.42

0.46

0.50

0.54

7.5 8.5 9.5 10.5 11.5 12.5

206 P

b/23

8 U

207Pb/235U


S13-57 (20140306)Concordia Age =

2501 6 Man=12/16

S-13-51 (20140331)Concordia Age =

2500 8 Man=14/16

TIMS Intercepts at 445 130 & 2451 13 Ma

MSWD = 3.6 n=5

two mounts, two sessions

A416 TIMS

A1671 Porttipahta gneiss2501 5 Ma (n=26)

E)

2000

2200

2400

2600

0.25

0.35

0.45

0.55

5 7 9 11 13

206 P

b/23

8 U

207Pb/235U

A1432 Sakiamaa felsic volcanic rock data-point error ellipses are 2s


MSWD = 2.7 (4/5)

LA-MC-ICPMS Concordia Age = 2411 ± 8 Ma

n=17 (/19)

F)

Fig. 9. Concordia plot of U–Pb zircon data for 2.5–2.4 Ga felsic rocks in Central Lapland. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots. A) Sadinoja volcanic breccia A206. The three TIMS analyses of a similar rock sample, A685 (Manninen et al. 2001), are also diplayed (as x). B) Yläliesijoki felsic rock A659. C) Akanvaara rhyolite A1524. D) Kaunismännikkö felsic rock A157. E) Porttipahta gneiss A1671. An old TIMS analysis of roughly similar gneiss A416 (Manninen et al. 2001) is also presented. F) Sakiamaa felsic volcanic rock A1432.

studies. The main minerals in this rock are pla-gioclase, clinopyroxene, orthopyroxene and olivine. The obtained mineral separates were mostly clean. However, plagioclase contained dark pigment, and in standard Franz isodynamic separation, this min-eral was collected from the magnetic fraction.

The five Sm–Nd analyses conducted on whole rock and mineral separates from the Peuratunturi gabbro defined an isochron with an age of 2448 ± 30 Ma and εNd = –1.5 (MSWD = 1.8, Fig. 10, Appendix 1). A similar set of minerals and whole rock ana-lysed from the Koulumaoiva intrusion yielded an age of 2464 ± 34 Ma (εNd = –1.0, MSWD = 1.7). These data confirm that both the Koulumaoiva and Peuratunturi intrusions are members of the 2.44–2.5 Ga age group. The slightly negative initial εNd and low Sm/Nd ratio in whole-rock samples are also consistent with the general characteristics of magmatic rocks of the 2.44 Ga events.

3.2.4 Lehtomaa intrusion

The volcanic rocks of the Salla Group are cut by a mafic–ultramafic intrusion at Lehtomaa in the Salla area (Manninen 1991, Fig. 2). The dyke-like,

NW–SE-trending intrusion has a length of ca. 5 km and a width of approximately 0.5 km. It is dif-ferentiated, with dunites (serpentinites) occurring at the assumed bottom of the intrusion and grad-ing into metapyroxenites, and these further into microgabbros and metagabbros. Locally, xenoliths of quartzite and gneiss occur within the roof-part microgabbros (Palmen 1997, Mutanen 2002).

Five samples were collected from the lower part of the intrusion for dating purposes. These included two from xenoliths (quartzite and gneiss), one from the gabbroic host of the xenoliths, one from a gab-broic contact variety and one from an ultramafic cumulate. All these samples yielded by separa-tion zircon grains, which were analysed using the NORDSIM facility in Stockholm (Appendix 4a). The U–Pb data on the xenolith samples give Archaean ages, and the data on their gabbroic host rock and contact variety also mostly indicate Archaean xenocrystic grains. In addition, several of the stud-ied zircon grains have domains that yield ages of ca. 1.8 Ga, suggesting strong hydrothermal/meta-morphic effects at this time. With regard to dat-ing the magmatic crystallization of the intrusion, the few zircon grains obtained from the ultramafic

2121


2000

2200

2400

2600

0.25

0.35

0.45

0.55

5 7 9 11 13

206 P

b/23

8 U

207Pb/235U

A1432 Sakiamaa felsic volcanic rock data-point error ellipses are 2s


MSWD = 2.7 (4/5)

LA-MC-ICPMS Concordia Age = 2411 ± 8 Ma

n=17 (/19)

F)

A1474wr A1474 wr#2

A1474opx A1474cpx

A1474plag

A1475 wr

A1475cpx

A1475plag

A1475opx

0.5102

0.5106

0.5110

0.5114

0.5118

0.5122

0.5126

0.05 0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21 147Sm/144Nd

Tuntsa belt intrusions

143Nd 144Nd

A1474 Peuratunturi Age = 2448 ± 30 Ma

eps = -1.5 MSWD = 1.8 n=5

A1475 Koulumaoiva Age = 2464 ± 34 Ma

eps = -1.0 MSWD = 1.7 n=4

Fig. 10. Sm–Nd isotope data for whole-rock samples and mineral separates from the Peuratunturi (A1474) and Koulumaoiva (A1475) intrusions.


cumulate sample (n1486 = A1525) provide the most likely answer. Seven spot analyses yielded concord-ant U–Pb compositions indicating an age of 2424 ± 5 Ma (Fig. 11, Appendix 4a). This result is also supported by a few discordant data points from the contact gabbro. This age should provide a minimum age for the Salla Group volcanic rocks hosting the Lehtomaa intrusion.

Sm–Nd analysis has been performed on sample A1525 from a gabbroic rock near the basal contact of the Lehtomaa intrusion. The analysed rock shows strong LREE enrichment and yielded an εNd(2424 Ma) value of -3.6 (Appendix 1), suggesting together with the zircon data that the magma solidified in the Lehtomaa intrusion contained abundant Archaean crustal material. This is consistent with Nd isotope data obtained in general for volcanic rocks of the 2.44 Ga age group (e.g., Hanski & Huhma 2005).

3.2.5 Onkamonlehto dyke

The NE–SW-trending Onkamonlehto dyke, at least 12 km long and up to 150 m wide, intrudes metavol-canic rocks of both the Salla and Kuusamo Groups in the Salla area (Fig. 2). No primary magmatic miner-als are preserved in the dyke, which has recrystal-lised in greenschist facies metamorphic conditions. The lowermost part of the distinctly differentiated mafic body consists of a bright green, hornblend-ite-like metacumulate layer, while the middle part

comprises medium-grained hornblende metagab-bro. The discordant and slightly scattered U–Pb data obtained by TIMS on multigrain zircon fractions from gabbro sample A1405 yielded a rough age esti-mate of 2383 ± 33 Ma (Manninen & Huhma 2001). The quality of the zircon grains suggests that the uncertainty in the result is due to alteration rather than inheritance. This has recently been confirmed by LA-MC-ICPMS data, which scatter along the concordia between 2.0 and 2.4 Ga (Fig. 12, Appendix 2). There is a clear negative correlation between the 207Pb/206Pb age and U content, suggesting that the data on high-U domains are affected by radiation damage and lead loss and do not register primary igneous crystallisation ages (Fig. 12b). A TIMS U–Pb analysis of zircon subjected to a chemical abrasion treatment gave a concordant composition at 2403 ± 3 Ma, which may be considered a minimum age for the intrusion of the Onkamonlehto dyke (Appendix 5) and also a minimum age for the Kuusamo Group volcanic rocks.

Gabbroic sample A1405, which was originally taken for U–Pb studies, has also been analysed for Sm–Nd isotopes (Hanski & Huhma 2005). The anal-ysis revealed that the rock has a high concentration of REE and a LREE-enriched chondrite-normalised pattern (Appendix 2). The data yield an εNd(2403 Ma) value of –0.7, which is which within the range gen-erally observed for 2.4–2.44 Ga mafic rocks.

2222

2000

2200

2400

2600

0.32

0.36

0.40

0.44

0.48

0.52

0.56

5 7 9 11 13

207Pb/235U

206 Pb

/238 U

0.41

0.43

0.45

0.47

0.49

8.8 9.2 9.6 10.0 10.4

Intercepts at 2416 ± 12 & 403 ± 490 MaMSWD = 2.2; n=10

n1486 Lehtomaa gabbro, zircons from pyroxene cumulate

Concordia Age = 2424 ± 5 Ma; n=7(2s, decay-const. errs ignored)

dark brown, small stubby zircon

Fig. 11. Concordia plot of U–Pb SIMS data from zircon in the Lehtomaa gabbro A1525.

2323


2000

2200

2400

2600

0.32

0.36

0.40

0.44

0.48

0.52

0.56

5 7 9 11 13207Pb/235U

A1405 Onkamonlehto mafic dyke

206Pb238U


A1405H CA-TIMSConcordia Age = 2403 2 Ma

TIMS

Concordia Age = 2395 8 MaLA-MC-ICPMS data

with U<770 ppm (n=10)MSWD (of concordance) = 1.7

A)

A1405-1a

A1405-2a

A1405-3a

A1405-4a

A1405-5a

A1405-6a

A1405-7a

A1405-8a A1405-9a

A1405-10a

A1405-11a

A1405-12a

A1405-13a

A1405-14a

A1405-15a

A1405-16a

A1405-17a

A1405-18a

A1405-19a

A1405-20a

A1405-21a

A1405-22aA1405-23a

A1405-24a

A1405-25aA1405-26a

A1405-27aA1405-28a

A1405-29a

A1405-30aA1405-31a

A1405-32a

A1405-33a

A1405-34a

A1405-35a

A1405-36a

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2100

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207 P

b/20

6 Pb

age

U (ppm)

B)

2000

2200

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0.32

0.36

0.40

0.44

0.48

0.52

0.56

5 7 9 11 13207Pb/235U

A1405 Onkamonlehto mafic dyke

206Pb238U


A1405H CA-TIMSConcordia Age = 2403 2 Ma

TIMS

Concordia Age = 2395 8 MaLA-MC-ICPMS data

with U<770 ppm (n=10)MSWD (of concordance) = 1.7

A)

A1405-1a

A1405-2a

A1405-3a

A1405-4a

A1405-5a

A1405-6a

A1405-7a

A1405-8a A1405-9a

A1405-10a

A1405-11a

A1405-12a

A1405-13a

A1405-14a

A1405-15a

A1405-16a

A1405-17a

A1405-18a

A1405-19a

A1405-20a

A1405-21a

A1405-22aA1405-23a

A1405-24a

A1405-25aA1405-26a

A1405-27aA1405-28a

A1405-29a

A1405-30aA1405-31a

A1405-32a

A1405-33a

A1405-34a

A1405-35a

A1405-36a

2000

2100

2200

2300

2400

400 600 800 1000 1200 1400 1600

207 P

b/20

6 Pb

age

U (ppm)

B)

Fig. 12. A) Concordia diagram of the zircon analyses from the Onkamonlehto dyke A1405 (error ellipse – LA-MC-ICPMS, dot – TIMS (Manninen & Huhma 2001, this study). B) Pb–Pb age vs. U concentration in zircon.


3.3 The 2.22 Ga Palovaara intrusion

One of the distinct phases of the Palaeoproterozoic mafic magmatism in Finland is represented by the ca. 2.22 Ga layered sills, which are widely present in eastern and northern Finland, concentrated along the basal contacts of the Karelian schist belt (Vuollo & Huhma 2005, Hanski et al. 2010). Abundant iso-tope data on these rocks were published by Hanski et al. (2010), who reported for them an average εNd(2220 Ma) value of 0.6.

A previously unknown occurrence of 2.2 Ga sills was recently revealed by LA-MC-ICPMS analy-ses conducted on zircon grains from an old sam-ple, A136 Palovaara, from an intrusion located a few kilometres south of the Tshokkoaivi fell in Enontekiö, NW Finnish Lapland (Fig. 1), where the intrusion occurs as a sill-like body within mafic volcanic rocks. Eight concordant analyses of the best-quality zircon domains yielded an age of 2213

2424

1300

1500

1700

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2100

2300

0.20

0.24

0.28

0.32

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0.48

0.52

2 4 6 8 10

206 P

b/23

8 U

207Pb/235U

A136 Palovaara diabasedata-point error ellipses are 2s

Intercepts at 394 230 & 2208 15 Ma

MSWD = 0.16 n=10

Concordia Age = 2219 13 Man=8

Average Pb/Pb age2213 14 Ma

MSWD = 0.1 n=8

TIMSA)

200

600

1000

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1800

2200

0.0

0.1

0.2

0.3

0.4

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0.6

0 2 4 6 8 10

206 P

b/23

8 U

207Pb/235U

A136 Palovaara diabase

BaddeleyiteIntercepts at

-72 99 & 2201 7 MaMSWD = 2.9 n=11


B)

1300

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206 P

b/23

8 U

207Pb/235U

A136 Palovaara diabasedata-point error ellipses are 2s


MSWD = 0.16 n=10

Concordia Age = 2219 13 Man=8


MSWD = 0.1 n=8

TIMSA)

200

600

1000

1400

1800

2200

0.0

0.1

0.2

0.3

0.4

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0 2 4 6 8 10

206 P

b/23

8 U

207Pb/235U

A136 Palovaara diabase


-72 99 & 2201 7 MaMSWD = 2.9 n=11


B)

Fig. 13. A) Concordia plot of U–Pb zircon data from sample A136 Palovaara. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses as red dots. B) Concordia plot of U–Pb data on baddeleyite from sample A136 Palovaara.

± 14 Ma, whereas analyses of altered zircon domains yielded discordant data with much younger Pb–Pb ages (e.g., 2a in Fig. 13a). Abundant altered zircon explains the discordance of the previous TIMS data obtained by O. Kouvo. The sample also contains baddeleyite grains. Analysis of these using a zircon standard produced strongly reversely discordant data, but with Pb–Pb ages that were nevertheless close to the 2.2 Ga indicated by the apparently well-preserved zircon domains (Appendix 2).

Baddeleyite from sample A136 has subsequently been analysed using the in-house (GTK) baddeley-ite standard A974, for which a precise TIMS age of 1256.2 ± 1.4 has been determined by Söderlund et al. (2004). These data are closer to the concordia and yield an average Pb–Pb age of 2204 ± 6 Ma (Fig. 13b, Appendix 2).

3.4 The 2.15 Ga intrusions

3.4.1 Rantavaara intrusion

The Rantavaara intrusion is a ca. 20-km-long (pos-sibly 30 km), up to 1-km-thick, steeply dipping differentiated mafic body in the Sodankylä area in central Lapland (Räsänen & Huhma 2001, Fig. 4). On the stratigraphic map of central Lapland, this intru-sion is located in an area between zones occupied by schists of the Sodankylä and Savukoski Groups (Fig. 4), but it is not clear whether the magmatic body has an intrusive relationship with the black schists of the Savukoski Group. A U–Pb age of 2148 ± 11 Ma has earlier been reported for zircon grains extracted from a coarse-grained gabbroic sample of the intrusion (Räsänen & Huhma 2001).

For Sm–Nd studies, a sample (A1337) of an ultra-mafic cumulate zone was collected from the lower part of the intrusion. The primary magmatic min-erals, i.e., pyroxene, olivine and minor plagioclase, are relatively well preserved in this sample. The pyroxene occurs as large poikilitic crystals enclosing olivine grains. The amount of plagioclase is small and it generally occurs at pyroxene grain bounda-ries. The mineral fractions used for Sm–Nd analyses were fresh and clean.

Over the years, a total of ten Sm–Nd analyses have been conducted on samples from the Rantavaara intrusion (Appendix 1). Some technical problems were involved in the older mineral analyses, and several duplicate analyses of plagioclase and pyrox-ene were therefore performed. Based on the evalu-ation of these data as a whole, the old analyses of pyroxene with large errors are of questionable value and are rejected. In general, the reproducibility of the analyses is, however, acceptable, as can be seen from the duplicate data on plagioclase. The seven mineral analyses of sample A1337 provide an isoch-ron with an age of 2236 ± 27 Ma (εNd = +3.3, MSWD = 1.0). If whole-rock analyses of the two samples

that were used for U–Pb zircon dating are included, the result is 2233 ± 25 Ma (εNd = +3.2, MSWD = 0.95, Fig. 14). It should be noted that this Sm–Nd age is not consistent with the U–Pb zircon age of 2148 ± 11 Ma reported for the Rantavaara intrusion by Räsänen & Huhma (2001). To resolve this inconsist-ency, another gabbroic sample (A1586) was taken for U–Pb zircon studies. One U–Pb TIMS analysis performed on a fraction of clear zircon grains from this sample plots exactly on the chord defined by the analyses of sample A900 in Räsänen & Huhma (2001), suggesting that the published U–Pb age of 2148 ± 11 Ma gives a better idea of the primary crystallisation age of the Rantavaara intrusion.

Because there is only a slight difference in Sm/Nd between pyroxene separates and whole-rock samples, the obtained Sm–Nd age is largely based on plagioclase. Rejecting all data on plagioclase, an age of 2168 ± 120 Ma (εNd = +3.2, MSWD = 0.74, n = 5) can be calculated. The results suggest that the plagioclase analysed from the peridotite sam-ple A1337 had a lower initial 143Nd/144Nd ratio than the pyroxene separates and whole-rock samples. This difference could be explained, for example, by minor contamination introduced into intercumu-lus melt by externally derived fluids during the late stages of crystallisation of the intrusion.

The initial εNd of value +3.2 obtained from the pyroxene and whole-rock analyses is close to con-temporaneous depleted mantle values and clearly distinct from the values obtained for most other mafic intrusions during this work. The indicated high initial εNd value is supported by the initial εHf value of +9.8 reported for zircon from sample A900 by Patchett et al. (1981). The Sm/Nd ratios for whole-rock samples indicate that the magma that solidified in the Rantamaa intrusion was char-acterised by a near flat chondrite-normalised REE pattern.

2525



3.4.2 Tanhua intrusions

The Kannusvaara gabbro is one of the mafic intru-sive bodies in the Tanhua area, eastern Lapland (Figs. 2, 4). The general geology of the Kannusvaara intrusion has been described by Mattila (1974). Some geochemical features from a drill core from the eastern (upper) part of the intrusion are pre-sented and discussed by Mutanen (1997). The intru-sion runs approximately north–south and is 1.7 km (or possibly up to 3 km) long and up to 0.6 km wide. The dip in the north appears to be steep to the west, but in the south, on the best exposed Kannusvaara hill, the intrusion has been twisted into an over-turned position, so that the diamond drill hole, col-lared in magnetite gabbros, intersected granophyres deeper down (see Fig. 32 in Mutanen 1997). The gabbros, originally plagioclase-pyroxene-magnet-ite cumulates, were later partly or wholly uralitised. The granophyre member consists of a lower biotite granophyre unit and an upper hornblende grano-phyre unit. The contact rocks (immediately east of the granophyre) are pyroxene-bearing metasedi-mentary hornfelses (Mattila 1974).

Two samples were chosen for isotopic studies. These were a granophyre, A1674, on top of the internal stratigraphy and a magnetite-bearing

gabbro, A1446, below the granophyre. The grano-phyre sample A1674 yielded abundant zircon, which occurs as long, euhedral, simple prisms with sharp edges. Zircon grains are pale and transparent, and the population appears very homogeneous. Four multigrain TIMS U–Pb analyses were carried out on zircon (Appendix 5), defining a chord that has intercepts with the concordia curve at 2116 ± 10 and 300 ± 65 Ma (Fig. 15). Recent analyses using LA-MC-ICPMS suggest a slightly older age, as most data yield a concordia age of 2148 ± 7 Ma (Fig. 15, Appendix 2). Two analyses are distinctly on the younger side, and the existence of such (metamor-phic?) domains could explain the TIMS results.

Zircon from two other gabbroic samples from the Tanhua area was also analysed using laser abla-tion MC-ICP-MS, one from the Kylälampi gabbro and the other from the Koskenkangas gabbro. The previous TIMS results for these samples (A418 and A817, Fig. 2) are discordant, suggesting ages of 2.11–2.14 Ga (Räsänen & Huhma 2001). All data on the Kylälampi gabbro sample A817 (except 5d & 5e) have low U and give similar, slightly discordant Pb–U results (Fig. 16, Appendix 2). The discordance could be due to a matrix effect in “albitite” zircon, which is different from the used standard. The Pb–Pb age of 2137 ± 5 Ma can be considered as the best age

2626

A1337

A1337 plag

A1337 px#2

A1337plag#2

A1337 px#3

A1337 plag#3 A1337 plag#4

A900

A1586

0.5114

0.5118

0.5122

0.5126

0.5130

0.5134

0.10 0.12 0.14 0.16 0.18 0.20 0.22

143 N

d/14

4 Nd

147Sm/144Nd

Age = 2233 ± 25 Ma eps = +3.2

MSWD = 0.95 n=9

Rantavaara intrusion Age = 2168 ± 120 Ma

eps = +3.2 MSWD = 0.74 n=5

(plag excluded)

Cf: U-Pb zircon age = 2148±11 Ma

Fig. 14. Sm–Nd isotope data for whole-rock samples and mineral separates from the Rantavaara intrusion.

estimate of the magmatic zircon. Two points from distinctly altered domains show high U concentra-tions (5d & 5e) and provide a slightly younger age of 2.10 Ga, which would explain the TIMS result.

All LA-MC-ICPMS data on zircon from the Koskenkangas gabbro sample A418 tend to plot above the concordia curve and give a Pb–Pb age of 2110 ± 8 Ma (Appendix 2, Fig. 17). The U and Pb concentrations are extremely high, which is likely to cause problems in calibration. In fact, there is a slight negative correlation between the Pb con-centration and Pb–Pb age (diagram in Appendix 2). The concentrations in the TIMS data were much lower, but cannot be directly compared, since they were obtained from heavier zircon. The minimum age yielded by TIMS was 2134 ± 4 Ma (Räsänen & Huhma 2001).

Because of its relatively well-preserved primary magmatic mineralogy, a sample (A1446) from the Kannusvaara intrusion was selected for Sm–Nd isotopic studies. The plagioclase separate is char-acterised by a dark pigment, but is mostly clear,

although some grains are slightly yellowish. The five Sm–Nd analyses performed on whole rock, pyroxene and plagioclase defined an age of 2089 ± 33 Ma (εNd = +2.0, MSWD = 1, Fig. 18, Appendix 1). As the whole-rock sample has a Sm/Nd ratio close to that of pyroxene, the age is largely based on the analysis of plagioclase. It has become clear in this work that there are occasionally problems in using plagioclase for dating primary magmatic crystallisation. These problems are mostly related to metamorphic alteration, which may also be the case here.

The Sm/Nd ratio for the magnetite gabbro (A1446) is chondritic and the initial εNd value clearly posi-tive, suggesting derivation from depleted mantle without major crustal contamination. In contrast, the Sm–Nd analysis from the granophyre sample A1674 yielded a slightly negative εNd(2148 Ma) value of –1.0 (Appendix 1). The REE pattern for this sam-ple is, however, quite strange for a felsic rock, as the Sm/Nd ratio is higher than in chondrites.

2727


1900

2000

2100

2200

2300

0.28

0.32

0.36

0.40

0.44

0.48

5 6 7 8 9

206 P

b/23

8 U

207Pb/235U

A1674 Kannusvaara granophyredata-point error ellipses are 2s

LA-MC-ICPMSConcordia Age = 2148 7 Ma

n=18 (/20)


n=4

Fig. 15. Concordia plot of U–Pb zircon data from the Kannusvaara granophyres, A1674. LA-MC-ICPMS analyses shown as error ellipsoids and ID-TIMS analyses as black dots.


2828

1500

1600

1700

1800

1900

2000

2100

2200

0.26

0.28

0.30

0.32

0.34

0.36

0.38

0.40

3 4 5 6 7 8

206 P

b/23

8 U

207Pb/235U

A817 Kylälampi gabbro (albitite) data-point error ellipses are 2s


MSWD = 0.82 n=14 (/16)


MSWD = 2.6 n=6

Fig. 16. Concordia plot of U–Pb zircon data from the Kylälampi gabbro A817. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses as black dots.

Fig. 17. Concordia plot of U–Pb zircon data from gabbro A418. LA-MC-ICPMS analyses presented as error ellip-soids and ID-TIMS analyses as black dots.

20402080

21202160

22002240

2280

TIMS_A418A

0.35

0.37

0.39

0.41

0.43

0.45

0.47

0.49

6.2 6.6 7.0 7.4 7.8 8.2 8.6207Pb/235U

A418 Koskenkangas gabbro

206Pb238U

LA-MC-ICPMSAverage Pb/Pb age

2110 8 Mavery high U& Pb - calibration??


TIMS A418APb/Pb age

2134 4 Ma


3.5.1 The Kevitsa intrusion

The Kevitsa (also known as Keivitsa) mafic-ultra-mafic intrusion is located within pelitic metasedi-ments and komatiitic metavolcanic rocks of the Savukoski Group in Central Lapland, only ca. 1 km south of the large 2.44 Ga Koitelainen layered intrusion (Fig. 4). The first detailed account of the geology of the Kevitsa intrusion was reported by Mutanen (1997), and a summary of the geology and exploration history was recently published by Santaguida et al. (2015). On the surface, the intru-sion has a roundish shape and an area of ca. 18 km2. The estimated maximum thickness is more than 1.5 km. The lower part of the layered succession consists of ultramafic olivine-clinopyroxene-orthopyroxene cumulates (olivine websterites and olivine clinopyroxenites), with a rather constant original modal olivine content (15–25%). The over-lying gabbroic cumulates include ferrogabbros, graphite-bearing gabbros and magnetite gabbros, which all typically contain inverted pigeonite. The upper gabbroic cumulates are topped by a layer of magnetite-bearing and sulphide-rich sodic grano-phyre; this layer is ca. 0.5 km wide along the south-

ern margin and still 30 m thick near the eastern border. Using zircon extracted from an ultramafic cumulate and the U–Pb TIMS method, Mutanen & Huhma (2001) were able to date the intrusion at 2058 ± 4 Ma.

A large disseminated Cu-Ni-PGE sulphide deposit (Keivitsansarvi), which is currently under exploita-tion, is located in the middle part of the ultramafic zone. This part of the intrusion is also characterised by the presence of ultramafic (including dunitic) and pelitic hornfelsed xenoliths. Two geochemi-cally distinct, spatially closely associated ore types can be discerned (Mutanen 1997, Yang et al. 2013): the prevalent regular ore (Cu–Ni–PGE–Au type) with Ni and Cu tenors of 4–7 wt% and 5–12 wt%, respectively, and the less abundant sulphur- and Cu-poor Ni–PGE ore with extremely high Ni tenors of up to ca. 40 wt%. There also exists some ore that is transitional between these two types. In addi-tion, Mutanen (1997) recognised an uneconomic variety with equal contents of sulphur compared to the mentioned ore types, but with very poor grades of Ni, Cu and PGE, referring to it as “false ore”. Apart from being different in their chalcophile ele-ment contents, the ore types have their own litho-

2929


A1446 wr

A1446px

A1446plag

A1446px#2

A1446plag#2

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.09 0.11 0.13 0.15 0.17 0.19 0.21 0.23

143 N

d/14

4 Nd

147Sm/144Nd

A1446 Kannusvaara Age = 2089 ± 33 Ma

eps = +2.0 MSWD = 1.06 n=5

Cf: Granophyre U-Pb zircon age 2148 ± 7 Ma

Fig. 18. Sm–Nd isotope data for whole rock and mineral separates from the Kannusvaara intrusion.


phile trace element characteristics, with the most remarkable of them being the high LREE contents of the Ni–PGE ore type (Mutanen 1997, Hanski et al. 1997).

An important aspect of the Kevitsa intrusion is that the rocks are mostly rather fresh, contain-ing primary igneous minerals (Ca-rich pyroxene, orthopyroxene, olivine, plagioclase), which allow the use of the Sm–Nd method for dating and genetic studies. A Sm–Nd mineral age of 2052 ± 25 Ma has earlier been published in some conference abstracts (Huhma et al. 1995, 1996, Hanski et al. 1997), being within error equal to the U–Pb zircon age reported by Mutanen & Huhma (2001). Previously, barren and ore-bearing samples from the Kevitsa intrusion have also been studied using other radiogenic and stable isotope systems: Hanski et al. (1997) pub-lished preliminary Os isotope data and Grinenko et al. (2003) documented extensive S and C isotope data. In this chapter, we focus on Sm–Nd isotope results and discuss in detail their relationship with other isotope and geochemical data in a separate paper (Hanski et al. in prep.).

Sm–Nd isotope data from Kevitsa are listed in Appendix 1, consisting of 33 analyses conducted on 11 mafic and ultramafic samples. In addition to the

material from the intrusion, Sm–Nd analyses have been performed on country rock samples and two dykes cutting the intrusion. Data from the dykes are presented separately in the following chapter, 3.5.2.

The results of five Sm–Nd analyses from a well-preserved gabbro sample, A1226, provide a mineral-whole rock isochron with an age of 2019 ± 26 Ma (εNd = –3.8, MSWD = 1.6, Fig. 19). The Sm/Nd ratio and concentrations in the clinopyroxene separates are close to those of the whole-rock sample. In con-trast, pigeonitic pyroxene from another gabbroic sample, A1316, has a very low REE level and con-siderably higher Sm/Nd. Six analyses of this sample yielded an age of 2067 ± 22 Ma (εNd = –3.3, MSWD = 0.19, Fig. 19). The old analysis of poorly purified pyroxene yielded an unusually high concentration of Nd and also had a large error, and has been omitted from the age and εNd calculations.

The third sample, A1390, on which several Sm–Nd analyses have been performed, represents barren pyroxenite from the ultramafic main suite. The ana-lysed fractions include whole rock, clinopyroxene, orthopyroxene and apatite, and define an isochron with a date of 2018 ± 56 Ma (εNd = –3.4, MSWD = 2.6, Fig. 19). The Sm/Nd ratio in orthopyroxene is unusually low. This peculiar sample also contains

3030

Fig. 19. Sm–Nd isotope data for three whole-rock samples and related mineral separates from the Kevitsa intrusion.

A1226

A1226 plag A1226 plag #2

A1226 cpx#1 &#2 A1316

A1316 plag

A1316 px #3

A1316 px#4 A1316 px#5

A1316 px#6

A1390 A1390 cpx #1 & #2

A1390 opx

A1390 opx#2

A1390 ap

0.5102

0.5112

0.5122

0.5132

0.04 0.08 0.12 0.16 0.20 0.24 0.28

143 N

d/14

4 Nd

147Sm/144Nd

A1226 Gabbro Age = 2019 ± 26 Ma

eps = -3.8 MSWD = 1.6 n=5

A1316 Gabbro 2067 ± 22 Ma

eps = -3.3 MSWD = 0.19 n=6

A1390 Pyroxenite Age = 2018 ± 56 Ma

eps = -3.4 MSWD = 2.6 n=6

Kevitsa intrusion

some high-U zircon, which was studied earlier by Mutanen & Huhma (2001). It turned out to be nearly concordant and yielded the above-mentioned U–Pb age of 2058 ± 4 Ma. Zircon from the same sam-ple was recently re-analysed by LA-MC-ICPMS (Appendix 2). The data tend to be slightly discord-ant and provide an average Pb–Pb age of 2046 ± 8 Ma (Fig. 20).

Some Sm–Nd analyses were also conducted on the peridotite samples A1319, HH/18.5B-92 and R337/18.5–18.9 (Appendix 1), which are all related to the regular ore type. All the obtained isotope data from these samples, combined with the data from the three other samples discussed above, plot roughly on a Sm–Nd isochron with an age of 2049 ± 26 Ma (MSWD = 5.7; εNd = −3.4, n = 23, Fig. 21). The Sm–Nd analyses on a granophyre sample (A1380) and Fe-sulphide-rich “false ore” sample (R688/34.25) also plot on the same line (Fig. 21). In contrast, the Sm–Nd data on the samples rep-resenting the Ni–PGE ore type clearly lie below the line and thus give lower initial εNd values; isotope data on the whole-rock sample R695/67.65-67.7 and related pyroxene and plagioclase separates pro-vide a Sm–Nd age estimate of 2069 ± 31 Ma (MSWD

= 0.4, εNd = –6.4), which is supported by another whole-rock sample (R713/36.6-) of the Ni–PGE ore type (Appendix 1). The REE concentrations in these Ni–PGE ore samples are very high and form a LREE-enriched chondrite-normalised REE pat-tern. In particular, the REE concentrations in the analysed pyroxene concentrate are extremely high and yield a rather unusual chondrite-normalised pattern (see Fig. 3 in Hanski et al. 1997).

In summary, the average initial εNd value of the main cumulate suite, related regular ore and “false ore” is –3.4 ± 0.3, whereas the Ni–PGE ore type provides an initial εNd value of –6.6. The duplicate analyses of a granophyre sample yielded εNd values of –2.7 and –3.4. In order to be able to evaluate the potential influence of the immediate country rocks on the isotope composition of the Kevitsa magma and ores, 6 Sm–Nd analyses were performed on samples from the surrounding pelitic schists and hornfelses. They provided εNd(2050 Ma) values from –3.7 to –6.8, thus falling between the initial εNd values measured for the Kevitsa main suite and the Ni–PGE ore type.

The obtained strongly negative initial εNd val-ues are consistent with the radiogenic Os isotope

1920

1960

2000

2040

2080

0.31

0.33

0.35

0.37

0.39

5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8207Pb/235U

A1390 Kevitsa pyroxenite

206Pb238U

LA-MCI-CPMS average Pb/Pb age 2046 8 Ma

n=8



MSWD = 2.6 n=6

TIMS average Pb/Pb age 2052 4 Ma

(data: M & H 2001)

Fig. 20. Concordia plot of U–Pb zircon data from the Kevitsa pyroxenite A1390. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses by Mutanen & Huhma (2001) as black dots.

3131



A1380 granophyre micro gabbro

R695 R695cpx

R695plag

false ore

Keivitsa country rocks A1445 felsic dyke

0.510

0.511

0.512

0.513

0.514

0.0 0.1 0.2 0.3

143 N

d/14

4 Nd

147Sm/144Nd

Kevitsa mafic intrusion Age = 2049 ± 26 Ma

Epsilon = - 3.4 MSWD = 5.7 n=23 wr, px, plag, ap from

four samples/ main rock types pyroxene A1316

plagioclase, apatite

U-Pb zircon age for Kevitsa: 2058±4 Ma

Ni-PGE ore (R695) Age = 2069 ± 31 Ma

eps = - 6.4 MSWD = 0.40 n=3

Fig. 21. Sm–Nd isotope data for whole-rock samples and mineral separates from the Kevitsa intrusion and its country rocks.

compositions reported by Hanski et al. (1997) and deviate noticeably from the composition of the con-temporaneous convective upper mantle, implying an involvement of old material with high LREE/HREE in the genesis or evolution of the Kevitsa magma. A major problem is whether these geo-chemical features are related to crustal contamina-tion or inherited from heterogeneous subcontinental lithosphere. According to Mutanen (1989, 1997), an important contaminant was reduced carbonaceous material from the surrounding black schists. This contamination involved reduction of the redox state of the magma, with part of the carbonaceous mate-rial ultimately crystallising as cumulus grains of graphite (Mutanen 1989). On the other hand, in their model of ore genesis at Kevitsa, Yang et al. (2013) suggested that contamination with sulphur-bearing country rock material already happened before the final emplacement of the Kevitsa magma in a more deep-seated magma chamber. In any case, the Nd and other isotope data are consistent with a major contribution of material from a source similar to the surrounding S- and C-rich sedimentary rocks. However, involvement of this material in the gen-esis of the Ni–PGE ore type does not fully explain

the extremely non-radiogenic Nd isotope composi-tion of this ore type, with its εNd(2050 Ma) values being mostly lower than those of the metasediments and approaching those of Archaean felsic gneisses (Fig. 22).

3.5.2 Kevitsa dykes

Several types of dykes have been found to cut the Kevitsa intrusion, varying in composition from ultramafic and mafic to felsic (Mutanen 1997). Sample A1445 (from drill core) represents a medium-grained dioritic dyke having sharp, wind-ing contacts to the surrounding olivine pyroxenite. This sample has earlier yielded a U–Pb zircon age of 2054 ± 5 Ma, indicating that the dyke is contempo-raneous with its host intrusion (Mutanen & Huhma 2001). The Sm–Nd isotopic results furnished by sample A1445 are given in Appendix 1 and plotted in Figure 21 together with other Sm–Nd data from the Kevitsa intrusion. As shown by the isochron dia-gram, the isotope composition of the diorite dyke lies very close to the line defined by the samples from peridotite related to the regular ore type and yields an initial εNd value of –3.7.

3232

3333


A1366 A1366#2

A1366px A1366px#2

A1366plag#2

0.5135

0.5137

0.5139

0.5141

0.5143

0.5145

0.25 0.27 0.29 0.31 0.33

143 N

d/14

4 Nd

147Sm/144Nd

A1366 Kevitsa dyke Age = 1916 ± 67 Ma

eps = +5.1 MSWD = 0.3 n=5

Fig. 23. Sm–Nd isotope data for whole rock and mineral separates from the Kevitsa LREE-depleted dyke.

diorite dyke

-15

-10

-5

0

0 10 20 30 40

Nd-

epsi

lon

(206

0)

Nd (ppm)

Kevitsa whole rocks

granophyre

Ni-PGE ore schists

false ore

main suite gabbros & peridotites

* DM

Archean gneisses

Fig. 22. εNd vs. Nd (ppm) diagram showing whole-rock data from the Kevitsa intrusion: main suite – red dot, false ore – green square, Ni-PGE ore – blue triangle, granophyre – red diamond, diorite dyke – diamond, and country rock schists – x. DM shows the composition of magma derived from the depleted mantle. Data on some Archaean gneisses in Finland are presented for reference (+, data from Huhma 1986, O’Brien et al. 1993, Hölttä et al. 2000, Hanski et al. 2001c, Mutanen & Huhma 2003).


Some narrow (ca. 4–5 m) dykes with olivine-rich ultramafic central parts display prominent flow differentiation. Mutanen (1997) called them olivine gabbro diabases. These dykes have a chemi-cal composition very distinct from their host ultra-mafic rocks, displaying strongly LREE-depleted chondrite-normalised REE patterns. One such ENE-trending dyke was sampled (A1366) for min-eral separation and Sm–Nd isotope studies. Some problems were encountered in the mineral separa-tion, as plagioclase turned out to be slightly mag-netic. The analysis of plagioclase also revealed a relatively high REE level and unusually high Sm/Nd, in fact higher than in the whole-rock sample (Appendix 1). This composition is probably more representative of the inclusions in plagioclase than the mineral itself. Nevertheless, five analyses con-ducted on this olivine-bearing gabbro dyke yielded an isochron age of 1916 ± 67 Ma (εNd = +5.1, Fig. 23). The highly positive initial εNd value suggests that the magma was derived from depleted mantle without any contamination from the old enriched lithosphere. The obtained age seems to indicate that the dyke is clearly younger than the Kevitsa intrusion. However, the age of ca. 1920 Ma is rather

strange for a mafic magmatic phase in northern Finland. In addition, the chemistry of the dyke shows similar features to those of the ca. 2050 Ma Ti-enriched komatiites of the Savukoski Group in Central Lapland (Hanski et al. 2001b). Assuming an age of ca. 2050 Ma for the dyke, individual samples would have initial εNd values of between +2.8 and +3.9, which are consistent with the values measured for these komatiites.

3.5.3 The Moskuvaara intrusion

The Moskuvaara intrusion is located ca. 10 km south of Kevitsa, where it has intruded into black schists of the Savukoski Group. Sample A1436 represents the gabbroic portions of the intrusion. It yielded a small quantity of turbid small simple zircon prisms. The earlier multigrain TIMS data are discordant and heterogeneous, suggesting an upper intercept age of ca. 2.0 Ga (Appendix 5, Fig. 24). The LA-MC-ICPMS data are also scattered and partly discord-ant, with Pb–Pb ages of ca. 1.9–2.05 Ga (Appendix 2). Rejecting compositions with elevated common lead (206Pb/204Pb <3000), an age of 2039 ± 14 Ma can be calculated (n = 15). The effects of alteration

3434

1400

1600

1800

2000

2200

0.16

0.20

0.24

0.28

0.32

0.36

0.40

0.44

2.5 3.5 4.5 5.5 6.5 7.5207Pb/235U

A1436 Moskuvaara gabbro

206Pb238U


A1436 Intercepts at 250 230 & 2039 14 Ma

MSWD = 2.9 n=15 (/27)(data with 206Pb/204Pb>3000)


MSWD = 22 n=4

Fig. 24. Concordia plot of U–Pb zircon data from the Moskuvaara gabbro A1436. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses as black dots.

are obvious in the zircon Pb-U system, and the true magmatic age is probably ~2.05 Ga, i.e., close to the age of the Kevitsa intrusion.

The Sm–Nd analysis of A1436 yielded an εNd(2040 Ma) value of –5.2. A clearly negative ini-tial εNd value of –4.2 has also been obtained for the roughly coeval 2055 ± 5 Ma (Räsänen & Huhma 2001) Rovasvaara gabbro further south (A820, Appendix 1).

3.5.4 The Puijärvi and Satovaara intrusions

Recently, gabbros from the Puijärvi and Satovaara areas were selected for isotope studies by FQM FinnEx Ltd. The Puijärvi intrusion is one of the mafic-ultramafic bodies west of the large 2.44 Ga Koitelainen layered intrusion. Sedimentary and volcanic rocks enclose the gabbro, from which two samples, A2288 and A2289, were processed for mineral separation. Both samples yielded zircon that is turbid and shows domains of alteration in BSE images (Fig. 25).

The analyses by LA-MC-ICPMS revealed that zircon from A2288 has generally high U, and the results tend to be slightly reversely discordant. This may well be due to difference between the clear,

good-quality standard zircon compared with the turbid, high-U material of the sample (Fig. 25, Appendix 2). The Pb/Pb isotope ratio is less sensi-tive to this difference and, excluding one analy-sis, an average Pb–Pb age of 2035 ± 8 Ma can be calculated for zircon A2288. An analysis (13b) of a clearly altered domain is discordant and suggests the effects of a younger event.

The results from the other Puijärvi sample, A2289, are consistent with those of sample A2288, suggesting an age of 2028 ± 8 Ma. The data from these samples (and the Satovaara sample considered next) show a clear negative correlation between the U concentration and apparent (Pb–Pb) age, suggesting some secondary Pb loss (diagram in Appendix 2). Thus, the ages reported above may be slightly too young for magmatic crystallisation.

The Satovaara intrusion occurs close to the Kevitsa intrusion (Fig. 4). An extensive, NE–SW- trending shear zone separates the two intrusions from each other, although the displacement along this structure is probably minor judging from the close similarity of the sedimentary and vol-canic rocks on both sides of the tectonic zone. The Satovaara intrusion is differentiated, with a perido-tite basal unit grading into an upper gabbroic unit.

3535


1700

1800

1900

2000

2100

2200

0.26

0.30

0.34

0.38

0.42

0.46

4 5 6 7 8

206 P

b/23

8 U

207Pb/235U

A2288 Puijärvi gabbro data-point error ellipses are 2s

Average Pb/Pb age 2035 8 Ma

n=17

Fig. 25. Concordia plot of U–Pb zircon data from the Puijärvi gabbro A2288.


For isotopic studies, a gabbroic sample (A2290) was chosen from drill core SAT-007, where hydrother-mal alteration and serpentine overprinting of the peridotite unit is not intensive.

Only a few zircon grains were found from the sample. The U–Pb results are similar to those obtained from the Puijärvi samples. The average Pb–Pb age is 2025 ± 8 Ma, but in this case, the negative correlation between the date and U con-

centration also leaves room for speculation that the real magmatic age may be slightly older (Fig. 26, Appendix 2).

The initial εNd values for the Puijärvi and Satovaara samples are close to zero, and thus distinct from the Ni-ore-bearing Kevitsa intrusion (Appendix 1). The isotope results from the Puijärvi and Satovaara intrusions were earlier reported in a conference abstract (Peltonen et al. 2014).

3.6 The 2.0 Ga intrusions in Kittilä

Much of the Kittilä greenstone area represents juvenile ca. 2.0 Ga Palaeoproterozoic oceanic crust with initial Nd isotope compositions close to that of coeval depleted mantle (Hanski & Huhma 2005). The age is based on U–Pb zircon data from felsic porphyries and gabbroic rocks (Rastas et al. 2001) and supported by Sm–Nd results from mafic vol-canic rocks, including analyses of primary pyrox-ene (Vesmajärvi Formation, Hanski & Huhma 2005). Further age constraints have been obtained from the Selkäsenvuoma gabbroic dyke A1563 (Fig. 2). This sample was collected from a large outcrop area, where the gabbroic rock occurs as narrow screens between fine-grained, parallel mafic dykes

(sheeted) (see Fig. 16 in Lehtonen et al. 1998). Mafic pillow lavas in the vicinity are chemically related to the mafic dykes.

The mineral separation of sample A1563 yielded a small amount of anhedral, brown and fairly tur-bid zircon, quite normal for gabbroic rocks. The CA-TIMS U–Pb analysis of zircon gave a concordant composition and, together with a slightly discordant analysis of air-abraded zircon, provides an age of 2008 ± 3 Ma (Fig. 27, Appendix 5). Subsequently, zircon was analysed using LA-MC-ICPMS, and rejecting a few points with elevated common lead, a concordia age of 2002 ± 8 Ma (n = 21) can be calculated (Appendix 2). Thus, the zircon popula-

3636

19401980

20202060

21002140

21802220

0.33

0.35

0.37

0.39

0.41

0.43

0.45

5.6 6.0 6.4 6.8 7.2 7.6 8.0

206 P

b/23

8 U

207Pb/235U

A2290 Satovaara gabbro

Average Pb/Pb age 2025 8 Ma

n=10


Fig. 26. Concordia plot of U–Pb zircon data from the Satovaara gabbro A2290.

tion appears to be homogeneous and the CA-TIMS result 2008 ± 3 Ma can be considered as the best age estimate for the gabbroic rocks and the whole mafic rock suite in the area.

The new Sm–Nd analysis conducted on the whole-rock sample of the Selkäsenvuoma gab-broic dyke (A1563) yielded an initial εNd value of +2.8 (Appendix 1), which is close to the isotope compo-sition measured for most volcanic rocks from the Kittilä Group (Hanski & Huhma 2005).

Employing laser ablation MC-ICP-MS, we also confirmed earlier TIMS results for zircon from two felsic porphyries (A581 Veikasenmaa and A893 Kiimarova) and a gabbroic sample (A1273 Kulkujärvi) from the Kittilä Group (Figs. 28–30, Appendix 2). All results are consistent with the TIMS ages published by Rastas et al. (2001). Previously unpublished discordant TIMS data on a gabbroic rock sample, A721 Jolhikko, are also consistent with an age of ca. 2.0 Ga (Appendix 5).

In contrast to clearly positive initial epsilon Nd values in rocks of the Kittilä Group, a 2.0 Ga gabbro cutting the Savukoski Group at Pittarova (A1272, Rastas et al. 2001) gives an εNd(2000 Ma) value of –0.4 (Appendix 1).

The Tuulijoki gabbro (A1565) in the western Kittilä area (Fig. 2) represents a rock that contains a small amount of heavily altered zircon. Four of the five U–Pb analyses by laser ablation MC-ICP-MS yielded ages of ca. 1.79 Ga, which is considered to register a major metamorphic event (Fig. 31a, Appendix 2). One analysis plots above the concordia curve, suggesting an age of ca. 2.0 Ga and provid-ing an explanation for the TIMS analysis, which resulted in a Pb–Pb age of 1.82 Ga (Appendix 5). The data are too few to determine the magmatic age of the rock.

Subsequently, baddeleyite was analysed using a Nu Attom SC-ICP-MS. The U–Pb data are scattered and discordant and the laser beam very probably hit domains that also contain zircon. The data do not provide any reliable age, but the rock may well belong to the 2.0 Ga group, occurring within the Savukoski Group basalts (Fig. 31b, Appendix 2). West of this locality in the Saarenpudas area, Kolari, an age of ca. 2.03 Ga has previously been reported (sample A964, Hiltunen 1982).

3737


1850

1950

2050

2150

0.28

0.32

0.36

0.40

0.44

4.8 5.2 5.6 6.0 6.4 6.8 7.2 7.6 8.0

206 P

b/23

8 U

207Pb/235U

A1563 Selkäsenvuoma gabbrodata-point error ellipses are 2s

LA-MC-ICPMS Concordia Age2002 8 Ma, n=21

TIMS A1563Cconcordant

2008 3 Ma

Fig. 27. Concordia plot of U–Pb zircon data from the Selkäsenvuoma gabbro A1563, Kittilä. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses as red dots.


3838

18401880

19201960

20002040

20802120

21602200

0.28

0.32

0.36

0.40

0.44

5.0 5.4 5.8 6.2 6.6 7.0 7.4 7.8

206 P

b/23

8 U

207Pb/235U

A581 Veikasenmaa felsic porphyrydata-point error ellipses are 2s

LA-MC-ICPMSConcordia Age2007 5 Ma

n=20


MSWD = 0.17 (n=4)

Fig. 28. Concordia plot of U–Pb zircon data from the Veikasenmaa felsic porphyry A581, Kittilä. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses by Rastas et al. (2001) as red dots.

Fig. 29. Concordia plot of U–Pb zircon data from the Kiimarova felsic porphyry A893, Kittilä. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses by Rastas et al. (2001) as red dots.

19201960

20002040

20802120

0.32

0.34

0.36

0.38

0.40

0.42

5.4 5.8 6.2 6.6 7.0

206 P

b/23

8 U

207Pb/235U

A893 Kiimarova felsic porphyry

LA-MC-ICPMSConcordia Age

2005 6 Man=8



n=4

3939


18401880

19201960

20002040

20802120

0.30

0.32

0.34

0.36

0.38

0.40

0.42

5.0 5.4 5.8 6.2 6.6 7.0

206 P

b/23

8 U

207Pb/235U

A1273 Kulkujärvi gabbro/ zircondata-point error ellipses are 2s

LA-MC-ICPMSConcordia Age = 1986 14 Ma

n=11

Rastas et al 2001TIMS coarse grain zircon Intercepts at

1363 99 & 1986 10 Maanalysis H concordant 1986 3 Ma

Fig. 30. Concordia plot of U–Pb zircon data from the Kulkujärvi gabbro A1273, Kittilä. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses by Rastas et al. (2001) as dots.

900

1100

1300

1500

1700

1900

2100

A1565-1aA1565-2a

A1565-3a

A1565-4a

A1565-5a

A1565A TIMS (+4.3 turbid)

0.1

0.2

0.3

0.4

0.5

1 3 5 7

206 P

b/23

8 U

207Pb/235U

A1565 Tuulijoki gabbrodata-point error ellipses are 2s


MSWD = 0.67 n=4

A)

0.05

0.15

0.25

0.35

0.45

0 2 4 6 8207Pb/235U

206 P

b/23

8 U

1000

1800

Intercepts at 432±97 & 2058±36 Ma

MSWD = 3.6 n=21


A1565 Tuulijoki gabbro KittiläBaddeleyite ± zircon

B)

900

1100

1300

1500

1700

1900

2100

A1565-1aA1565-2a

A1565-3a

A1565-4a

A1565-5a

A1565A TIMS (+4.3 turbid)

0.1

0.2

0.3

0.4

0.5

1 3 5 7

206 P

b/23

8 U

207Pb/235U

A1565 Tuulijoki gabbrodata-point error ellipses are 2s


MSWD = 0.67 n=4

A)

0.05

0.15

0.25

0.35

0.45

0 2 4 6 8207Pb/235U

206 P

b/23

8 U

1000

1800

Intercepts at 432±97 & 2058±36 Ma

MSWD = 3.6 n=21


A1565 Tuulijoki gabbro KittiläBaddeleyite ± zircon

B)

Fig. 31. A) Concordia plot of U–Pb zircon data from the Tuulijoki gabbro A1565, Kittilä. LA-MC-ICPMS analyses presented as error ellipsoids and ID-TIMS analyses as dots. B) Concordia plot of U–Pb baddeleyite data (using Attom) from the Tuulijoki gabbro A1565, Kittilä.

A) B)


3.7 The 1.8 Ga Tainio and Lotto intrusions

3.7.1 Tainio intrusion

The appinitic intrusive rocks occurring in the Central Lapland granitoid area commonly give rise to distinct magnetic anomalies. One of such intrusions is the Tainio gabbro in the Pasmajärvi area (map sheet 2642 12; Väänänen 2004) (Fig. 1). The stock- or pipe-like intrusion is distinguish-able as a round, zoned magnetic anomaly ca. 3 km in diameter. The outer magnetic ring is caused by small-grained contact gabbro rich in Fe-Ti oxides and apatite. Towards the centre, the rocks are coarser in grain size, mainly consisting of plagio-clase, pyroxenes, biotite and hornblende. The minor and accessory minerals include quartz, magnetite, ilmenite, apatite, sulphides, carbonate and zircon. Gabbro autoliths and cavities partly filled by zeolite and carbonate minerals have also been observed, with the latter feature suggesting that the magma was rich in volatiles. The U–Pb zircon age of the Tainio intrusion is 1796 ± 4 Ma (Väänänen 2004), and the body hence represents one of the youngest igneous rock suites in western Finnish Lapland (the Lohiniva Suite). Crystal fractionation has produced

variation in the chemical composition of the suite, but all rocks are enriched in incompatible trace ele-ments. Typical rocks are gabbroic in composition with MgO 6–9 wt.%, total FeO 7–9 wt.%, Na2O 3.5 wt.%, K2O 1.5 wt.%, Cr 300 ppm and Ni 100 ppm (Mutanen & Väänänen 2004, Väänänen 2004).

Primary igneous plagioclase and pyroxene, together with amphibole, are the main minerals and were separated and used for Sm–Nd studies. The Sm–Nd reconnaissance work was carried out on the same gabbroic sample (A1665) from which the zircon age of 1796 ± 4 Ma was obtained. The three analyses available provide an age of 1774 ± 54 Ma (εNd = –5.2, MSWD = 1.6, Fig. 32), which is consistent within error with the zircon age. A peculiar feature of the analyses is the Sm/Nd ratio measured for the “pyroxene” concentrate, which is lower than the ratio in the whole-rock sample (Appendix 1). The analysis by XRD revealed that in addition to pyrox-ene, this material also contains some amphibole. Amphibole was probably formed in reaction between pyroxene and the melt (Mutanen & Väänänen 2004), and should thus be practically coeval with the other main minerals.

4040

A1665

A1665plag

A1665 px+amph

0.5105

0.5107

0.5109

0.5111

0.5113

0.5115

0.03 0.05 0.07 0.09 0.11 0.13

143 N

d/14

4 Nd

147Sm/144Nd

A1665 Tainio gabbro Age = 1774 ± 54 Ma

eps = -5.2 MSWD = 1.6 n=3

Zircon age 1796±4 Ma

Fig. 32. Sm–Nd isotope data for whole rock and mineral separates from the Tainio gabbro A1665.

The low initial εNd value suggests major involve-ment of old LREE-enriched lithosphere in the gen-esis of the Tainio gabbro and related intrusions. The geochemical features are best explained by interaction of mantle-derived magma with crustal material at great depths, which is compatible with the magmatic underplating model discussed above.

3.7.2 Lotto dyke

A mafic NNW–SSE-trending swarm of olivine-bearing diabase dykes cuts the Lapland granulite belt in the Lotto area, Inari. Individual dykes of the swarm can be traced for several kilometres on the low-altitude aeromagnetic map as elongated positive anomalies (Mutanen 2011). The Lotto dyke sampled for this study preserves its primary min-

erals, albeit variably altered. Sample A1916 for iso-tope studies was taken from a drill core (R302, 41.1 m) intersection in which the rock mainly consists of plagioclase, clinopyroxene, orthopyroxene and olivine. Minor minerals include biotite, chromite, ilmenite, magnetite, sulphides and, as alteration products, amphibole, serpentine, talc, carbonate and chlorite.

The Sm–Nd data on whole rock, plagioclase and a light-coloured, clear-looking heavy pyroxene frac-tion define an isochron with an age of 1805 ± 44 Ma and a low initial epsilon Nd value of -5.2 (Fig. 33, Appendix 1). An analysis conducted on another pyroxene concentrate, which has a relatively low density and higher REE level than the high-density fraction, plots below the isochron, probably due to associated metamorphic amphibole.

3.8 Intrusions with unknown age

3.8.1 Väkkärävaara intrusion

Several outcrops indicate a gabbro intrusion at Väkkärävaara, south of the Sattasvaara komatiitic rocks (Fig. 5). Based on an interpretation of aero-geophysical maps, it seems likely that schists of the Matarakoski Formation (Savukoski Group) surround

the gabbroic rocks. The gabbro in the outcrops is cut by an ultramafic dyke. Sample A1715 collected for isotope dating contains few grains of zircon, which are turbid and of poor quality. The U–Pb data obtained by LA-MC-ICPMS mostly have high com-mon lead and yield scattered and mostly discordant results (Fig. 34, Appendix 2). Two distinct grains

4141


A1916 wr

A1916 plag

A1916 px#2

A1916 px(+amph?)

0.5108

0.5112

0.5116

0.5120

0.5124

0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21

143 N

d/14

4 Nd

147Sm/144Nd

A1916 Lotto dyke Age = 1805 ± 44 Ma

eps = -5.2 MSWD = 1.00 n=3

Fig. 33. Sm–Nd isotope data for whole rock and mineral separates from the Lotto mafic dyke A1916.


give Archaean ages and are clearly xenocrystic. Most data plot on a chord having intercepts with the con-cordia curve at 2.1 and 0.4 Ga, which is supported by one concordant analysis at 2.1 Ga (A1714-4a). However, one spot is concordant at 2.0 Ga, but this zircon grain is very high in U and the result is prob-ably less reliable for dating the primary magmatic crystallisation event. Although an age of ca. 2.1 Ga is possible, the available data do not allow reliable dating of this gabbro.

Sm–Nd analysis of A1715 indicated nearly chon-dritic LREE/HREE for the sample and yielded an εNd(2100 Ma) value of –3.0 (Appendix 1).

3.8.2 Värriö intrusion

The Värriöjoki intrusion complex is a large ultra-mafic rock formation in eastern Lapland (Törmänen et al. 2007, Fig. 2). The surrounding rocks belong to the Archaean Tuntsa suite, mainly consisting of quartz-feldspar and mica gneiss with amphibo-lite and amphibole-chlorite schist intercalations (Juopperi & Vaasjoki 2001). According to Törmänen et al. (2007), the rocks in the centre of the intrusion complex are mostly dunites, whereas peridotitic and pyroxenitic rocks dominate in the outer part. The primary magmatic minerals are replaced by ser-

pentine, chlorite, talc and amphiboles. A narrow band of chlorite schist usually occurs between the complex and its country rocks.

In order to constrain the age of the Värriö com-plex, twelve samples were collected from four drill cores representing different blocks of the intrusion complex. All samples have a low level of REE and a variably LREE-enriched chondrite-normalised REE pattern. The Sm–Nd analyses provide a range of Sm/Nd ratios, but the isotope data are scattered (Fig. 35, Appendix 2). Regression of all data gives a date of ca. 1.8 Ga, and it is possible that meta-morphic effects have disturbed the Sm–Nd system.

However, the results give some constraints on the initial Nd isotope composition of the magma. The average εNd value calculated using an age of 2440 Ma is -2.2 ± 1.6, and becomes lower if younger ages are used (e.g., -4.6 ± 1.0 if the age is 2050 Ma). It seems evident that a major Archaean LREE-enriched com-ponent is present in the system. These apparent epsilon values are similar to the 2.05 Ga Kevitsa intrusion, as well as the Koulumaoiva intrusion located ca. 10 km east of Värriö, for which the Sm–Nd mineral age is 2464 ± 34 Ma. In fact, based on the geological setting, a correlation with these 2.4 Ga rocks would be conceivable (Fig. 2).

1000

1400

1800

2200

2600

3000

A1715-10aA1715-2a

A1715-1a

A1715-5a

A1715-6aA1715-3a

A1715-8bA1715-12a

A1715-8a

A1715-9a

A1715-4aA1715-7a

A1715-11a

0.0

0.2

0.4

0.6

0 4 8 12 16 20

206 P

b/23

8 U

207Pb/235U

A1715 Väkkärävaara metagabbrodata-point error ellipses are 2s

Analysis A1715-4a Concordia Age = 2110 20 Ma

Fig. 34. Concordia plot of U–Pb zircon data obtained by LA-MC-ICPMS from the Väkkärävaara gabbro A1715.

4242

3.9 Volcanic rocks

Felsic volcanic rocks from Honkavaara in the Sodankylä area contain Archaean zircon with an average age of ca. 2.7 Ga (Fig. 5, Rastas et al. 2001). Recent LA-MC-ICPMS analyses have revealed that the zircon populations are heterogeneous, showing a range of Archaean ages. These rocks are cut by 2.2 Ga mafic sills and have recently been assigned to the Central Lapland (Vuojärvi) supersuite, which is considered Palaeoproterozoic in age (Lahtinen et al. 2015a, Nironen et al. 2016). The Honkavaara rocks were one of the special targets of a large project focusing on volcanic rocks in Lapland (Lehtonen et al. 1998) and were also analysed using the Sm–Nd method (in 1991). Both felsic and mafic rocks are enriched in LREE, providing Archaean model ages and negative initial εNd values at any Palaeoproterozoic age (Appendix 6).

Preservation of primary clinopyroxene in the komatiitic rocks at Jeesiörova (Sattasvaara Formation, Savukoski Group) in the Kittilä area allowed Hanski et al. (2001b) to determine a direct Sm–Nd isotope age for these rocks. Seven pyrox-ene–whole rock pairs yielded an average age of 2056 ± 25 Ma and a range of initial εNd values from +2 to +4. In contrast to samples from Jeesiörova, most mafic and ultramafic rocks of the Sattasvaara

Formation elsewhere are more altered and prob-lematic for isotope studies. Some clinopyroxene was found in the komatiites from Mikkuurova (Sattasvaara, Fig. 5), but there were difficulties in separating pure mineral fractions. Analyses of pyroxene concentrates together with whole-rock samples yielded age estimates of ca. 1.8–1.9 Ga, which reflects the influence of metamorphism. The komatiitic whole-rock samples from Mikkuurova have low REE concentrations, are depleted in LREE and yield εNd(2060 Ma) values of ca. +4 (Appendix 6), all features that are also typical for the Jeesiörova area komatiites.

The three samples taken from a 5-m-thick mafic dyke cutting tuffitic komatiites in the northern slope of Sattasvaara hill provide an example of secondary REE fractionation. These samples show a large range in Sm/Nd (and also other chemical parameters) and yielded an age of ca. 1.84 Ga (Fig. 36, Appendix 6).

Excluding dyke rocks, the 41 published and unpublished Sm–Nd analyses conducted on komatiitic rocks and pyroxene concentrates from the Sattasvaara Formation and the three analyses of Peuramaa area picrites are consistent with an age of ca. 2.06 Ga, with a clearly positive initial εNd (Fig. 37).

4343


0.5112

0.5114

0.5116

0.5118

0.5120

0.5122

0.10 0.12 0.14 0.16 0.18 147Sm/144Nd

Värriö ultramafic intrusion

143Nd 144Nd

Age = 1768 ± 250 Ma eps = -6.1

MSWD = 13 (n=12)

Fig. 35. Sm–Nd isotope data for whole-rock samples from the Värriö intrusion.


13718 (210-1/LVP-88)(1

13719 (210-2/LVP-88)(1

13720 (210-3/LVP-88)(1

6598 (950-5/LVP-85)(1

6603 (951-4/LVP-85)(1

6614 (951-15/LVP-85)(1

7697 (096-28/LVP-85)(1

6639 (953-12/LVP-85)(1

5631 (R2/84 255.6m)(1

0.511

0.512

0.513

0.514

0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32

143 N

d/14

4 Nd

147Sm/144Nd

Dyke 210/LVP Metamorphic fractionation

Age = 1837 ± 16 Ma eps = +7

MSWD = 0.4 n=3

Sattasvaara mafic-ultramafic dykes

Fig. 36. Sm–Nd isotope data for mafic-ultramafic dykes from Sattasvaara.

4444

0.511

0.512

0.513

0.514

0.515

0.516

0.517

0.05 0.15 0.25 0.35 0.45

143 N

d/14

4 Nd

147Sm/144Nd

Sattasvaara Fm komatiites & Peuramaa picrites

Age = 2064 ± 38 Ma epsilon =+3.1

MSWD = 21 (n=44, wr & cpx)

pyroxene

picrites

Fig. 37. Sm–Nd isotope data for whole-rock samples and mineral separates from the Sattasvaara komatiites and Peuramaa picrites.

In addition to open systems, the scatter in the data (MSWD = 21) is also due to slight variation in the primary initial Nd isotope ratios (Hanski et al. 2001b).

The Linkupalo mafic volcanic rocks in Western Lapland are also assigned to the Savukoski Group

(Fig. 5). Pillow basalts are common, but primary main minerals are not preserved. The rocks are enriched in LREE and yield an average εNd(2060 Ma) value of +0.7 and TDM = 2.35 Ga (Appendix 6).

4 TAIVALKOSKI BLOCK IN THE LENTUA COMPLEX AND KUUSAMO SCHIST BELT


The Taivalkoski basement block represents the northernmost part of the more than 400-km-long Archaean basement complex (Lentua complex in Hölttä et al. 2012) in eastern Finland. On its south-ern side, the Taivalkoski block is separated from the

Kuhmo block by E–W- and SW–NE-trending shear zones (Hölttä et al. 2012). The dominant rock types are 2.7–2.8 Ga granodioritic and tonalitic gneisses that locally have largely anhydrous granulite-facies mineral assemblages (Vuollo & Huhma 2005, Lauri

_̂_̂̂__̂

_̂_̂

_̂

_̂

_̂

_̂

_̂̂__̂

_̂

_̂

_̂

_̂

_̂

_̂̂_

_̂

_̂

_̂

_̂

_̂

_̂_̂

_̂_̂

_̂_̂_̂

_̂

_̂

_̂̂__̂

_̂

_̂

Syöte

Laivajoki

Kuusamo schist belt

Taivalkoski block

Lohisärkkä

Kortejärvi

Koivuvaara

Tilsanvaara

Kallioniemi

Taivalkoski

Por�vaara

Suoperä (BD)

Uolevinlehto(UD)

Karkuvaara/Nyrhinoja

Ruukinvaara Fm/Sodankylä Gr

Petäjävaara Fm/Sodankylä Gr

Kun�järvi Fm/Kuusamo Gr

Ma�nvaara Fm/Kurkikylä Gr

A0496

A0988

A0365

A1471

A1663

A0722

A0610A0713

A1443

A0497

A1868

Kon�oluoma

Näränkävaara

MurhiniemiHirsikangas

0 30 km

¢

Finland

Russia

Sweden

Norway

Age

_̂ <1930 Ma

_̂ 1931 - 2080

_̂ 2081 - 2180

_̂ 2181 - 2280

_̂ 2281 - 2380

_̂ >2380

_̂U-Pb



Sm-Nd (±U-Pb)

Volcanic rocks


_̂

_̂

_̂


( in this paper )

Sm-Nd (age ?)

4545


Fig. 38. Geological map of the Taivalkoski block (northern part of Lentua complex) and Kuusamo schist belt showing sample localities. For symbols, see Figure 1. The Koillismaa layered intrusion suite refers to 2.44 Ga rocks in Syöte, Porttivaara and Näränkävaara near the Russian border.


et al. 2006). A prominent feature of the Taivalkoski block is the presence of 2.44 Ga mafic-ultramafic bodies assigned to the Koillismaa layered intrusion suite (Alapieti 1982, Karinen 2010) (Fig. 38). It is divided into two intrusions, the Näränkävaara lay-ered intrusion on the Finnish–Russian border and the Porttivaara layered intrusion in the western part of the Taivalkoski area. The latter intrusion is now represented by several separate intrusion blocks, which were formed by tectonic fragmentation of a larger, originally cohesive, sheet-like magmatic body.

Other manifestations of the Palaeoproterozoic mafic magmatism in the Taivalkoski basement block are mafic dykes that occur in several generations of dyke swarms. A review of the age groups, areal dis-tribution, mineralogy and geochemistry of the mafic dyke swarms occurring in the Archaean basement and Karelian schist belts in northern and eastern Finland was published by Vuollo & Huhma (2005). Before dealing with the isotope data from dyke rocks from the Taivalkoski and other basement blocks, it is worth presenting a summary of the classifica-tion of the dyke swarms by Vuollo & Huhma (2005). Based on the emplacement ages, they distinguished the following 5 dyke swarm groups: 1) 2.45 Ga dyke swarms

2) 2.32 Ga Fe-tholeiitic dyke swarm and intrusions

3) 2.2 Ga low-Al tholeiitic layered sills (karjalites, gabbro-wehrlite association)

4) ~2.1 Ga Fe-tholeiitic dyke swarm 5) ~1.98 Ga Fe-tholeiitic–tholeiitic dyke swarm

The oldest 2.45 Ga group is further divided into 5 subtypes:(1) NE–SW-trending boninite–noritic dykes

(high MgO, SiO2, Cr, Ni, and LREE, low TiO2 and Zr)

(2) NW–SE-trending gabbro-norite dykes (low TiO2, Cr, and Zr)

(3) NW–SE-trending low-Ti tholeiitic dykes(4) NW–SE-trending Fe-tholeiitic dykes,

continental type(5) E–W-trending orthopyroxene- and plagio-

clase-phyric dykes (high SiO2, LREE; calc-alkaline aff.)

In contrast to the 2.45 Ga dykes, the younger dyke swarms with ages between 2.32 and 1.98 Ga show more subtle variation in chemistry, with most of them resembling continental tholeiitic basalts in composition. However, there may be clear differ-ences in the orientation between the swarms of

4646

Fig. 39. Stratigraphic column of the Kuusamo schist belt as originally established by Silvennoinen (1972) (left) and current lithostratigraphic classification as defined by GTK (right). Note that some of the formations of Silvennoinen are now lowered to the member status, being part of a single formation in the new classification.

different age groups. As shown by a map of Vuollo & Huhma (2005, their Fig. 5.14), there is a dense network of mostly E–W-trending mafic dykes in the Taivalkoski block. Only a few age determina-tions are available for these dykes. Nevertheless, examples of the 2.45 Ga dykes have been recog-nised, such as boninite-norites with an orientation of 60o. The U–Pb ages of 2446 ± 6 Ma and 2332 ± 18 Ma mentioned by Vuollo & Huhma (2005) were based on U–Pb analyses on baddeleyite carried out in Toronto and are reported in this volume below (samples A1415 and A1471).

The Kuusamo schist belt comprises a supracrus-tal sequence that was deposited on Archaean rocks of the Taivalkoski block (Fig. 38). The Kuusamo belt continues to the Russian side of the border as the Kuolajärvi-Paanajärvi belt. In the west, the belt is cut by plutons of the Central Lapland granitoid complex, and in the north it continues as the Salla belt. The formation-level stratigraphic division of the Kuusamo schist belt, as originally established by Silvennoinen (1972), is shown in Figure 39 together with the currently used division applying a more formal lithostratigraphic nomenclature. In the latter scheme, the stratigraphic column is divided into 11 formations, which belong to the Salla, Kuusamo, Sodankylä and Savukoski Groups, i.e., the same groups that are distinguished in the stratigraphy of the Central Lapland greenstone belt (see Fig. 3). With the exception of the Salla Group, mafic extrusive magmatism is found in all groups of the Kuusamo

belt and occurs at four stratigraphic levels repre-sented by the Kuntijärvi Fm (former Greenstone Fm I), Petäjävaara Fm (former Greenstone Fm II), Ruukinvaara Fm (former Greenstone Fm III) and Liikasenvaara Fm (former Amphibole Schist Fm).

The samples studied in this work from the ca. 2.44 Ga Koillismaa layered intrusion suite represent an anorthosite dyke from the Syöte block and previ-ously studied gabbros from the Porttivaara block, which hosts the Mustavaara Fe–Ti–V ore deposit (Karinen et al. 2015). We also document abundant Sm–Nd mineral and whole-rock data for several of the mafic dyke families distinguished in Vuollo & Huhma (2005). The ca. 2.44 Ga dykes that were analysed from the Russian side of the border include five dykes from the Pääjärvi area, belonging to the gabbro-noritic, Fe-tholeiitic and orthopyroxene-phyric types, and one Fe-tholeiitic dyke from the Suoperä area (Fig. 38). In an attempt to constrain the ages and initial Nd isotope compositions of the mafic dykes occurring in a granulite-facies ter-rain, about 2.0 x 4.5 km2 in size, around the town of Taivalkoski, nine well-preserved dykes were sam-pled. One of these samples was successfully dated at ca. 2.3 Ga using the U–Pb baddeleyite method. Another representative of the dykes of this age group was studied at Karkuvaara, occurring in the SW part of the Taivalkoski block close to the Kainuu schist belt (Fig. 38). Some Nd isotope data on mafic volcanic rocks from three different stratigraphic levels of the Kuusamo schist belt are also presented.

4.2 The 2.44 Ga Koillismaa layered intrusion suite

The age of 2436 ± 5 Ma reported by Alapieti (1982) for the Koillismaa layered intrusion suite was based on sixteen U–Pb zircon analyses conducted on several gabbroic samples (Kouvo 1977). Recently, the Koillismaa intrusions were studied by Karinen (2010), who reported abundant Sm–Nd data on whole rocks, pyroxene, and plagioclase. The min-eral isochrons on two samples from the Porttivaara intrusion block (Fig. 38) yielded ages consistent with the U–Pb zircon age, but for two other samples, the Sm–Nd mineral age was slightly lower, presum-ably due to metamorphic effects (all analysed at GTK). Nevertheless, the initial εNd(2440 Ma) values were found to be negative and close to -2, which is characteristic for rocks of this family. Karinen (2010) also published Sm–Nd data on gabbroic rocks from the two PGE reefs (Rometölväs, Syöte), some of which yielded positive initial εNd(2440 Ma) val-

ues (analysed at VSEGEI, St. Petersburg). In order to confirm this result, we performed analyses on five equivalent samples from the same drill core. These data (Appendix 1) yielded initial εNd(2440 Ma) values from -1.0 to -2.8, which are in contrast to those in Karinen (2010) and typical for rocks in the Koillismaa layered intrusion suite (Fig. 40).

North of the Syöte block, mafic volcanic rocks of the Vehnäsvaara Formation are cut by an anorthositic dyke a few metres in thickness. The anorthosite was assumed to represent the 2.44 Ga magmatism, and a sample (A1663-Kaidansuvanto) was hence collected with the aim to constrain the age of the volcanic rocks intersected by the dyke. The zircon population of the rock appears hetero-geneous and yields scattered U–Pb data (Fig. 41, Appendix 7). The eleven oldest compositions are concordant and yield an age of 2.82 Ga, which prob-

4747



4848

0.5095

0.5105

0.5115

0.5125

0.02 0.06 0.10 0.14 0.18 0.22

143 N

d/14

4 Nd

147Sm/144Nd

Porttivaara & Syöte intrusions

PorttivaaraAge = 2388 75 Ma

Initial eps = -2.2MSWD = 12, n=23

plagioclase

pyroxene

Fig. 40. Sm–Nd isotope data for whole-rock samples and mineral separates from the Koillismaa layered intru-sion suite. Data from Karinen (2010, blue triangles), this study (Syöte, five red diamonds) and Sirniö felsic rocks from Lauri et al. (2006, x), all analysed at GTK, Espoo.

1200

1600

2000

2400

2800

0.15

0.25

0.35

0.45

0.55

0.65

0.75

0 4 8 12 16 20

206 P

b/23

8 U

207Pb/235U

A1663 Kaidansuvanto anorthosite dykedata-point error ellipses are 2s

Concordia Age = 2815 10 Ma

n=11

Grain 13 a & bAge = 2432 21 Ma

TIMS

Alapieti 1982:Koillismaa layered intrusion


MSWD = 12 (n=17)zircon from 6 samples

Fig. 41. Concordia plot of U–Pb zircon data obtained by LA-MC-ICPMS from the anorthosite dyke A1663. TIMS U–Pb data on zircon from the Koillismaa layered intrusion suite published by Alapieti (1982) are also shown for reference (x, +). The data with Archean age (+) are from the Soukeli sample A610.

ably dates zircons inherited from some felsic gneiss in the hosting Archaean basement complex. The results from six analyses range from 1.85 to 2.1 Ga, and three data points yield Pb/Pb ages of ca. 2.4 Ga. Two of the measurements are very likely to at least partly hit baddeleyite, yielding reversely discord-ant U–Pb results, obviously due to the use of an unsuitable standard (zircon). In spite of this, the Pb/Pb isotope ratios should still be correct, as was discussed above (in section 3.3; A136). It is tempt-ing to interpret the ca. 2.4 Ga date as representing the primary igneous age and the ages of 1.85–2.1 Ga as the variable effects of (1.8 Ga) metamorphic/hydrothermal overprinting.

Zircon grains from three earlier studied samples picked from the Porttivaara layered intrusion were recently re-analysed using LA-MC-ICPMS. Three zircon standards were employed in calibration (GJ1, A1772 and A382). Only one distinct grain from the Soukeli pyroxenite (A610) is close to 2.44 Ga in age, whereas the other data give an age of ca. 2.72 Ga, suggesting contamination from Archaean country rocks. These results are consistent with the previ-ous TIMS data acquired from this sample (Fig. 42, Alapieti 1982).

New U–Pb data obtained by LA-MC-ICPMS on zircon from the Mustavaara gabbro (A713) tend

to plot above the concordia curve, which is prob-ably due to a different matrix compared to the zir-con used as the standard. However, the average 207Pb/206Pb age of 2428 ± 6 Ma can be considered as a reliable estimate for the age of magmatic zircon (Fig. 43), consistent with the two TIMS analyses reported by Alapieti (1982), as well as with old data from the nearby Välivaara gabbro (sample A699) (Fig. 43).

The third old sample re-analysed by laser abla-tion MC-ICP-MS from the Koillismaa layered intru-sion suite represents a rock near the base of the Porttivaara intrusion. It has been called “albitite or palingenic rock” (A722 Rusamo). Earlier unpub-lished TIMS data (Appendix 5) suggested a con-tribution from the Archaean basement, which was confirmed by one spot analysis (7a). Other analyses performed on selected clean domains suggest an age of ca. 2.45 Ga (Fig. 44). The old TIMS data from sev-eral other samples further support this age (A577, A578 and A723-5, Appendix 5).

In summary, the results available on the Koillismaa layered intrusion suite confirm its age of 2.44 Ga and also show significant communica-tion of the magma with Archaean crustal material.

4949


0.38

0.42

0.46

0.50

0.54

0.58

0.62

8 10 12 14 16 207Pb/235U

206 P

b/23

8 U

2300

2400

2500

2600

2700

2800

2900



n=2, grain 4

A610 Soukeli, Koillismaa intrusion pyroxenite

Fig. 42. Concordia plot of U–Pb zircon data obtained by LA-MC-ICPMS from the Soukeli pyroxenite A610. Multigrain TIMS U–Pb data on zircon by Alapieti (1982) are also presented (red triangles).


5050

diamond - A699-Välivaara gabbro

0.40

0.44

0.48

0.52

0.56

0.60

9 10 11 12 13 207Pb/235U

206 P

b/23

8 U

2400

2500

2600

2700

Average Pb/Pb age 2428 ± 6 Ma

MSWD = 0.22, n=6


A713 Mustavaara, Koillismaa intrusion gabbro

0.32

0.36

0.40

0.44

0.48

0.52

0.56

0.60

7 9 11 13 15 207Pb/235U

206 P

b/23

8 U

2200

2300

2400

2500

2600

2700

2800



MSWD = 1.6 n=5

A722 Rusamo, Koillismaa intrusion albitite

Fig. 43. Concordia plot of U–Pb zircon data obtained by LA-MC-ICPMS from the Mustavaara gabbro A713. Old multigrain TIMS U–Pb data on zircon are also presented: A713 Mustavaara gabbro – red triangle (Alapieti 1982) and A699 Välivaara gabbro – blue diamond.

Fig. 44. Concordia plot of U–Pb zircon data obtained by LA-MC-ICPMS from the Rusamo rock A722. Multigrain TIMS U–Pb data on zircon are also presented (red triangle).

4.3 Dykes in the Lake Pääjärvi and Suoperä areas, Russia

Several generations of dykes are well exposed on Lupzenga Island in Lake Pääjärvi, Russia, where cross-cutting relationships seen in outcrops reli-ably demonstrate the relative order of emplacement of four dyke generations (Stepanov 1994, Figs. 1 and 45). The rocks are good targets for Sm–Nd stud-ies, because the primary magmatic mineralogy is generally well preserved. Isotope results from the Pääjärvi dykes were discussed by Vuollo & Huhma (2005), but the actual data have not been published until here. Palaeomagnetic studies on the dykes have been published by Mertanen et al. (1999). The 2.44 Ga layered intrusions of the Oulanga Complex (Kivakka, Tsipringa and Lukkulaisvaara) are located some 20 km NNW of the island of Lupzenga. The Sm–Nd analyses from these intrusions and two dyke rocks have earlier yielded mineral ages con-sistent with the 2.44 Ga U–Pb zircon ages (Amelin & Semenov 1996). The reported initial εNd values range between 0 and –2.

4.3.1. Gabbro-norite dyke A1412 Pääjärvi

Samples A1412 (35-VEN-94) and A1413 (36-VEN-94) represent a NW–SE-trending gabbro-noritic dyke swarm, which intrudes the Archaean granitoid basement (Vuollo & Huhma 2005). The separated

fractions of pyroxene and plagioclase from sam-ple A1412 are fresh, although plagioclase is slightly heterogeneous in colour. The analyses conducted on the whole-rock samples are identical and, together with the mineral separates, they give a Sm–Nd age of 2421 ± 32 Ma (Fig. 46, Appendix 1). The calculated initial εNd value is -1.4 (MSWD = 1).

4.3.2 “Older Fe-tholeiitic dyke” A1414 Pääjärvi

The gabbro-norite dyke discussed above is cut by a Fe-tholeiitic dyke, which was called the “Older Jatulian dyke” by Stepanov (1994). Fresh plagioclase in the sample (A1414 = 38-VEN-94) from this rock unit is fairly dark due to some pigment. The three analyses performed on this sample plot exactly on a line (MSWD = 0.01), which gives an age of 2476 ± 30 Ma (Fig. 47). The initial εNd value is +1.7. Using the age of typical mafic layered intrusions, εNd +1.4 at 2.44 Ga can be calculated for the whole-rock sample. The face age value appears to contradict the age and geological relationship of the previ-ous sample, but considering the error limits, no difference in age can be stated. Instead, the initial value is very distinct from the negative value for the gabbro-norite and layered intrusions in general.

Fig. 45. Geological map of site XD at Lake Pääjärvi after Stepanov (1994) and Mertanen et al. (1999). A1412 repre-sents XD1 gabbro-norite dyke, A1414 – XD3 (older) Fe-tholeiitic dyke, A1492 – XD4 (younger) Fe-tholeiitic dyke.

5151



A1412wr A1413wr

A1412px

A1412plag

0.5106

0.5110

0.5114

0.5118

0.5122

0.5126

0.5130

0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21

143 N

d/14

4 Nd

147Sm/144Nd

Age = 2421 ± 32 Ma eps = -1.4

MSWD = 1.5 n=4

A1412 Pääjärvi gabbronorite dyke

Fig. 46. Sm–Nd isotope data for whole rock and mineral separates from the Pääjärvi gabbro-norite dyke A1412.

A1414wr

A1414px

A1414plag

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

143 N

d/14

4 Nd

147Sm/144Nd

Age = 2476 ± 35 Ma eps = +1.7

MSWD = 0.0045 n=3

A1414 Pääjärvi Fe-tholeiitic dyke

Fig. 47. Sm–Nd isotope data for whole-rock and mineral separates from the Pääjärvi dyke A1414.

5252

4.3.3 “Younger Fe-tholeiitic dyke” A1492 Pääjärvi

According to field observations, the “Older Jatulian dyke” discussed above is cut by an E–W-trending Fe-tholeiitic dyke, which has been called the “Younger Jatulian dyke” and is considered the youngest dyke rock in the area (Stepanov 1994). Sample A1492 (39-VEN-94) from this rock was col-lected ca. 20 m NW of the location of the previous sample, A1414. The magmatic mineral paragenesis is well preserved, but plagioclase i n this rock is also relatively dark. The Sm–Nd analyses on hand-picked plagioclase and pyroxene together with the whole-rock sample give an age of 2349 ± 24 Ma (εNd = +1.0, MSWD = 1.4, Fig. 48). The age is in good agreement with the field observations and results obtained from the other rocks on Lupzenga Island.

4.3.4 Orthopyroxene-phyric dyke A1465 Pääjärvi

Orthopyroxene-phyric dykes with an east–west trend have only been discovered in a few places in Russian Karelia and near the adjacent border in Kuusamo, Finland. The colour of the dykes is almost black and the width varies from 1 to 3 m. The primary minerals include orpthopyroxene and

plagioclase. Sample A1465 (= 42-VEN-94) selected for isotopic studies was picked ca. 100 m west of the gabbro-norites on Lupzenga Island. It contains clear orthopyroxene phenocrysts (φ ca. 1 mm) in a fine-grained groundmass. Fresh plagioclase is characterised by dark pigment and, after standard Franz separation, it was recovered from the mag-netic fraction.

The three isotope analyses on minerals and whole rock reveal that the range in Sm/Nd is too limited to constrain the age of the dyke (Appendix 1). The REE level in the whole-rock sample and also in the analysed plagioclase fraction is high. The dykes have a calc-alkaline geochemical affinity, and thus they have been related to the 2.44 Ga boninitic dyke swarm. Assuming that the age is ca. 2.44 Ga, the isotope data on the whole-rock sample (and plagio-clase) suggest an initial εNd value of -2.2. However, εNd(2440 Ma) for orthopyroxene is +2.2, suggesting isotopic disequilibrium with the whole rock (Fig. 48). The Sm/Nd ratio in orthopyroxene is low for a typical igneous pyroxene. The data indicate that the origin of the pyroxene is distinct from the bulk rock. The obvious explanation would be that orthopyrox-ene crystallised early in the magma derived from depleted mantle, which was subsequently con-

5353


A1492wr

A1492px

A1492plagA1465wr

A1465opx

A1465plag0.5108

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.5136

0.5140

0.08 0.12 0.16 0.20 0.24 0.28

143 N

d/14

4 Nd

147Sm/144Nd

A1492 Pääjärvi Fe-tholeiitic dykeAge = 2349 24 Ma

eps = +1.0MSWD = 1.4 n=3

A1465 Pääjärvi opx-phyric dyke

Fig. 48. Sm–Nd isotope data for whole rock and mineral separates from the Pääjärvi Fe-tholeiitic dyke A1492 and dyke A1465.


taminated with material from the Archaean LREE-enriched lithosphere. The reason for the low Sm/Nd ratio in the orthopyroxene remains obscure.

4.3.5 Fe-tholeiitic Oulanka dyke

An altered Fe-tholeiitic dyke was sampled (113-VEN-94, 13.6 kg) near the Pääjärvi-Oulanka River road about 24 km north of Pääjärvi village. No con-tact to the country rock gneiss was visible at the sampling site. The trend of the dyke is 310º and its minimum width is 15 m. It is totally altered and no primary pyroxenes can be seen. A moderate quantity of extremely poor-quality baddeleyite grains was discovered (in Toronto). U–Pb analysis conducted on the best-quality grains is 2.3% discordant, pro-viding a minimum age of ca. 2.3 Ga (Appendix 8). The true age of the dyke cannot be determined, but an age close to that of the 2.44 Ga layered intru-sion of the Oulanka Complex (Amelin et al. 1995) is plausible (see Fig. 49 reference line 1.88–2.45 Ga).

4.3.6 Gabbro-norite dyke, A1415 Suoperä

Dykes belonging to the gabbro-norite dyke swarm trend NW and are fresh on the Russian side of the border and also in some areas on the Finnish side. These medium- to coarse-grained gabbro-norite dykes, up to 50 m in thickness, consist of clino-pyroxene (25%), calcic plagioclase (60%), and orthopyroxene (5–10%) with minor olivine (1–2%), quartz (<5%), biotite, and Fe–Ti oxides. In the same way as with boninitic-noritic dykes, they are altered in many places on the Finnish side. The areal dis-tribution of the gabbro-norite dykes is difficult to assess due to the same trend of younger dykes. Based on geochemical studies (relatively low Cr and Ni), these dykes are seen on both the Russian and Finnish side. They can be traced over a distance of some kilometres.

A fresh gabbro-noritic sample, A1415 (122-VEN-94, total 15.4 kg), was taken from the central part of a 50- to 60 m-thick gabbro-norite dyke in the

5454

A1415 (BD)

A1356 Boninite (VD)

A1410 Tholeiite (UD)

13-VEN

A1471 (AD10)

0.40

0.42

0.44

0.46

7.8 8.2 8.6 9.0 9.4 9.8 10.2 207Pb/235U

206 P

b/23

8 U

2240

2280

2320

2360

2400

2440


A1415 (BD) Pääjärvi gabbro-norite dyke

Intercepts at 837±450 & 2447 ± 10 Ma

MSWD = 0.60 (n=4)

Reference line intercepts at 1880 & 2450 Ma

13-VEN Oulanka Fe-tholeiitic dyke

Pb/Pb age 2292±4 Ma

A1471 (AD10) Taivalkoski Fe-tholeiitic dyke

Intercepts at 641±730 & 2332 ± 9 Ma

MSWD = 0.49

Baddeleyite

Fig. 49. Concordia diagram showing the U–Pb data for baddeleyite fractions analysed at the University of Toronto. The error ellipses reflect two sigma errors.

Suoperä area (site BD) located between the 2.44 Ga Näränkävaara intrusion and Lake Pääjärvi, ca. 10 km east of the Russian–Finnish border.

At site BD, there are no visible chilled margins. A large amount of relatively high-quality badde-leyite was recovered from the sample (in Toronto). All of the baddeleyite grains that were selected for analysis were rather fresh and contained no vis-ible zircon rims. Nearly all of them were broken, probably during the crushing process. The quan-tity and grain size (5–30 µm) of these brownish to pale brownish, broken baddeleyites were small. Four fractions of baddeleyite were analysed; the best baddeleyite fraction without visible inclusions, fractures or turbidity is near concordant (0.7%) with

a 207Pb/206Pb age of 2442 ± 5 Ma (Appendix 8). Three additional fractions of the second best baddeleyite grains (2–4) are slightly discordant (1.9–2.3%), with 207Pb/206Pb ages ranging from 2434 to 2436 Ma (Appendix 8). The compositions of these four fractions of baddeleyite define a discordia line (55% probability of fit) that has an upper-intercept age of 2447 ± 10 Ma (2σ) and a lower-intercept age of ca. 840 Ma (Fig. 49).

The pyroxene fraction (obtained at GTK) from sample A1415 looks good, but plagioclase is partly yellowish and cloudy. The three Sm–Nd analyses conducted on minerals and whole rock (Appendix 1) yielded an isochron age of 2420 ± 29 Ma with an initial εNd value of -2.4 (MSWD = 0.54, Fig. 50).

4.4 The 2.3 Ga Karkuvaara intrusion

The Karkuvaara (Nyrhinoja) intrusion is located within Archaean gneisses just east of the Jaurakkajärvi section of the Palaeoproterozoic Kainuu schist belt and about 6 km to the east of the major N–S-running Proterozoic fault/shear zones separating the latter from the gneisses of the Kalhamajärvi gneiss complex (Fig. 38). Despite plentiful late Proterozoic granite in the Kalhamajärvi complex and strong (1.8–1.9 Ga) mylonitic defor-mation associated with the faults in the west, rocks

of the Karkuvaara intrusion are mostly nearly unde-formed and tend to contain well-preserved primary igneous minerals. The intrusion dominantly com-prises medium- to coarse-grained metadiabasic to granular gabbroic rocks, which have a fairly Fe- and Ti-rich subalkalic basaltic composition.

Both U–Pb and Sm–Nd isotope studies were performed on three samples from the intrusion. A coarse-grained gabbro pegmatoid sample (A988 Nyrhinoja) yielded abundant zircon and some

5555


A1415 wr

A1415 px

A1415 plag

0.5102

0.5106

0.5110

0.5114

0.5118

0.5122

0.5126

0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

143 N

d/14

4 Nd

147Sm/144Nd

Age = 2420 ± 29 Ma eps = -2.4

MSWD = 0.54 n=3

A1415 Suoperä gabbronorite dyke

Fig. 50. Sm–Nd isotope data for whole rock and mineral separates from the Suoperä gabbro-norite dyke A1415.


baddeleyite, which were used for U–Pb dating. The extracted zircon grains were mostly euhedral crys-tals with sharp edges. However, the appearance of the population is somewhat heterogeneous due to variation in the colour, transparency and presence of inclusions. Much effort was put into the hand-picking of different zircon types for analyses.

Nine U–Pb analyses have earlier been conducted on zircon and baddeleyite from sample A988 under the supervision of O. Kouvo (Appendix 5). The data are technically good and provide a regression line having intercepts with the concordia curve at 2306 ± 8 and 542 ± 150 Ma (Fig. 51). An old TIMS analysis on another sample from Karkuvaara, A365, supports this age (Appendix 5). Subsequently, zircon from sample A988 was also analysed by laser ablation MC-ICP-MS. All twelve data points are concordant within error and yield an age of 2294 ± 8 Ma (Fig. 51, Appendix 7). It is notable that the ages from high-U zircon tend to be on the lower side of the

average age; rejecting four of such analyses with U >800 ppm, a concordia age of 2300 ± 10 Ma can be calculated.

The Sm–Nd mineral studies were carried out on two samples, A1456b and A1456c. The major pri-mary minerals, plagioclase and pyroxenes, appear well preserved and the mineral fractions purified by hand-picking are perfectly clean. The six analyses are of good quality and form an isochron that gives an age of 2319 ± 27 Ma (εNd= +1.8, MSWD = 1.6, Fig. 52, Appendix 1). The two samples treated separately provide ages of 2295 ± 37 Ma (A1456b) and 2345 ± 39 Ma (A1456c).

Thus, as the applied U–Pb and Sm–Nd meth-ods all produce consistent results, the nine-point multigrain TIMS age of 2306 ± 8 Ma should be a reliable age estimate for the mafic intrusion at Karkuvaara. The initial ratio (εNd = +1.8) suggests that the magma was derived from depleted mantle without major contamination from Archaean crust.

5656

2200

2300

2400

2500

0.34

0.38

0.42

0.46

0.50

0.54

0.58

7 8 9 10 11207Pb/235U

A988 Nyrhinoja gabbro

206Pb238U

Concordia Age = 2294 8 Man=12 (LA-MC-ICPMS)



MSWD = 4.6 n=9

206Pb238U




MSWD = 4.6 n=9

Fig. 51. Concordia plot of U–Pb zircon data from the Nyrhinoja gabbro A988, Karkuvaara intrusion. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red triangles.

4.5 Dykes in the Taivalkoski town area

Based on available geological and aeromagnetic anomaly maps, several dyke swarms with relatively well-preserved primary igneous mineralogy occur in an area around the town of Taivalkoski, espe-cially in the parts characterised by well-preserved Archaean granulite-facies rocks. Ten samples from the dykes were selected for preliminary Sm–Nd mineral analyses (Fig. 38). The mineral concen-trates are mostly of fairly good quality, although slight heterogeneity in the colour and brightness, especially in some plagioclase fractions, is appar-ent. No proper grain-by-grain hand-picking was applied, but the separates were only screened under the stereomicroscope for distinctly unwanted grains, which were removed (samples A1794-1802).

4.5.1 A1466 Taivalkoski

The Taivalkoski dyke (A1466, WD14) represents one of the numerous E–W-trending Fe-tholeiitic dykes observed in the Taivalkoski area. The 10-m-wide, WNW–ESE-trending, subvertical dyke is exposed in a road cut near the town of Taivalkoski. The inte-riors of the dyke are occupied by medium-grained diabase, which grades to fine-grained, glassy-looking chilled margins at the dyke contacts. The

modal composition of the dyke interior diabase is plagioclase (60%), clinopyroxene (30%), and Fe–Ti-oxides (5–10%). The chemical and mineral composition of sample A1466 is similar to that of the other well-preserved 2.3–2.0 Ga Fe-tholeiitic dykes dealt with in this work. One clear difference between this dyke swarm and 2.45 Ga dyke swarms is that plagioclase is not cloudy.

Standard separation including some hand-picking produced clean fractions of pyroxene and plagioclase. Three analyses conducted on the min-eral extracts and whole-rock powder (Appendix 1) yielded a Sm–Nd isochron with an age of 2407 ± 40 Ma (MSWD = 1.9, Fig. 53). The initial εNd value is clearly positive (+1.6).

4.5.2 A1471 Taivalkoski

Another E–W-trending Fe-tholeiitic dyke (A1471, AD10) was sampled ca. 3 km SE of the A1466 Taivalkoski site. This rock consists of plagioclase (50%), clinopyroxene (15%), secondary amphibole (20%) and quartz. A large number of good qual-ity, mostly prismatic, 5- to 70-µm-long badde-leyite grains were recovered from the large, 27.2 kg sample in Toronto. Some baddeleyite grains were

5757


A1456b A1456c

A1456cpx

A1456cplag

A1456bpx

A1456bplag

0.5110

0.5114

0.5118

0.5122

0.5126

0.5130

0.5134

0.08 0.12 0.16 0.20 0.24

143 N

d/14

4 Nd

147Sm/144Nd

A1456 Karkuvaara Age = 2319 ± 27 Ma

eps = +1.8 MSWD = 1.6 n=6

A988 Nyrhinoja U-Pb zircon age 2306 ± 8 Ma

Fig. 52. Sm–Nd isotope data for whole rock and mineral separates from the Karkuvaara gabbroic samples A1456.


observed with thin zircon rims. All four analysed fractions (Appendix 8) yielded similar slightly dis-cordant compositions (0.9 –1.7%). They define a discordia line giving an upper-intercept age of 2332 ± 9 Ma (2σ) and a lower-intercept age of 640 Ma (Fig. 49).

4.5.3 A1797 Törninkuru

A third sample from the E–W-trending Fe-tholeiitic dykes (A1797, AD13-8) was collected from a road cut ca. 10 km SW of the A1466 site. Both contacts of this dyke are exposed and show well-preserved chilled margins. Primary minerals including plagioclase and clinopyroxene are well preserved. The mineral separation showed that most of the plagioclase con-sists of darkish grains, which were collected from a slightly magnetic fraction. The Sm–Nd analysis conducted on this plagioclase, together with the data on pyroxene and whole rock (two identical analyses), provide an isochron and age of 2404 ± 53 Ma (MSWD = 1.5, n = 4). In contrast, the Sm–Nd analysis of the light plagioclase (A1797plag#1) is distinctly off the isochron (Fig. 54), which is prob-ably due to metamorphic effects.

Another dyke sample (AD13-9) from the same outcrop was also processed. Sm–Nd analysis of the whole-rock powder was roughly compatible with

the results for sample A1797, but the data on plagio-clase and pyroxene are inconsistent. The analysed plagioclase is light in colour and analogous with the previous sample; the effects of metamorphism are conceivable. The analysed pyroxene fraction may contain some amphibole.

The dyke is characterised by a positive initial εNd value, similar to that of the 2.4 Ga dyke A1466. Recently, a U–Pb age of 2339 ± 18 Ma was reported for baddeleyite from the same dyke (sample AD13; Salminen et al. 2014). The Sm–Nd and U–Pb ages from the Törninkuru dyke are thus consistent within error.

There are now two samples from the E–W-trending Fe-tholeiitic dykes that suggest a U–Pb age of ca. 2.33 Ga, whereas the Sm–Nd data tend to give marginally older age indications. Combining the Sm–Nd data on samples A1466 and A1797, an age of 2349 ± 75 Ma can be cal-culated (n = 10, MSWD = 7.9). In any case, the initial epsilon Nd value is positive (+1.6) and clearly distinct from most 2.44 Ga mafic rocks.

4.5.4 A1796 Kallioniemi

The Kallioniemi outcrop is located by the Lake Jokijärvi, ca. 20 km southeast of the town of Taivalkoski, within the farmyard of the museum

5858

A1466wr

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143 N

d/14

4 Nd

147Sm/144Nd

Age = 2407 ± 40 Ma eps = +1.6

MSWD = 1.9 n=3

A1466 Taivalkoski Fe-tholeiitic dyke

Fig. 53. Sm–Nd isotope data for whole rock and mineral separates from the Taivalkoski Fe-tholeiitic dyke A1466.

5959


A1797 A1797#2

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143 N

d/14

4 Nd

147Sm/144Nd

A1797 Törninkuru dyke Age = 2404 ± 53 Ma

eps = +1.7 MSWD = 1.5 n=4

Fig. 54. Sm–Nd isotope data for a whole-rock sample and mineral separates from the Törninkuru dyke A1797.

A1800

A1800px

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NE-trending dykes in Taivalkoski

A1798 Kontioluoma dyke (+) A1800 Murhiniemi dyke (x)

A1796 Kallioniemi dyke Age = 2352 ± 64 Ma

eps = +0.5 MSWD = 0.0007

Fig. 55. Sm–Nd isotope data for whole rock and mineral separates from the three dykes A1796, A1798 and A1800 from the Taivalkoski area.


childhood home of the renowned novelist Kalle Päätalo. What is exposed is a NE–SW trending Fe-tholeiitic diabase dyke mainly consisting of pla-gioclase and clinopyroxene that are coarse grained and well preserved in the middle parts of the dyke. The Sm–Nd analyses of clinopyroxene, plagioclase fractions and a whole-rock aliquot from sample A1799 provide an isochron with an age of 2352 ± 62 Ma and an initial εNd value of +0.5 (Fig. 55).

4.5.5 A1800 Murhiniemi

Another sample from a NE–SW-trending Fe-tholeiitic dyke was collected at Murhiniemi (A1800, AD91-3), ca. 2 km east of the sample loca-tion of the above-discussed Kallioniemi dyke. The main primary minerals are again plagioclase and clinopyroxene, which in hand samples and under the microscope appear well preserved. Sm–Nd analyses of minerals and whole rock did not, how-ever, provide any unambiguous isochron (MSWD = 5.9). We may note that the data on pyroxene and whole rock plot roughly on the isochron related to the Kallioniemi dyke (Fig. 55), but as the analysis of plagioclase is clearly above that line, the age of this rock remains unknown. Conceivable explana-tions for the heterogeneity include the following:1) Analytical errors (e.g., spike–sample disequi-

librium with the plagioclase analysis, resulting in biased Sm/Nd);

2) Metamorphic effects restricted to plagioclase. In this case, analyses of pyroxene and whole rock would give the age of crystallisation: 2295 ± 85 Ma, εNd = +0.6;

3) Crustal contamination after crystallisation of pyroxene and plagioclase (analyses of these minerals would then give the age of crystal-lisation: 2204 ± 48 Ma, εNd = +0.6);

4) Pyroxene represents an early phenocryst phase, and the rest of the rock was contaminated (not applicable for this sample).

The following comments to these alternative sce-narios can be made:1) Duplicate analysis, carried out before and dur-

ing the work presented in this paper, suggests that an analytical error is unlikely. Furthermore, many examples in this paper also suggest that ages obtained by the Sm–Nd method from well-preserved primary minerals/whole rocks

are generally consistent with the U–Pb ages for the same samples.

2) Major metamorphic effects on plagioclase may be questioned, since the analysed plagioclase appears fairly fresh.

3) The Nd contents in pyroxene and plagioclase are low compared to the amount of Nd in the whole-rock sample. Thus, as much of the REE must be in the interstitial minor phases, they also control the whole-rock Nd isotope com-position. As the country rocks are Archaean gneisses, the Nd isotope composition of these fluids very probably had a low 143Nd/144Nd ratio compared to the mafic magma, resulting in a low εNd value in the interstitial and grain bound-ary domains.

One may also note that minerals used for isotope analyses in the Taivalkoski batch (A1794-A1802) were not properly hand-picked, and even though the fractions were generally good looking, they may nevertheless have contained minor impuri-ties. Similar problems were also encountered dur-ing this work with other than Taivalkoski samples (see below), and clearly more detailed studies would be required to gain a better understanding of the factors affecting the results from these rocks.

4.5.6 A1798 Kontioluoma

A third sample (A1798, AD85-3) from the NE–SW-trending Fe-tholeiitic dykes in the Taivalkoski area was collected at Kontioluoma ca. 20 km NE of the Kallioniemi site. The major primary miner-als, plagioclase and clinopyroxene, are again well preserved, but nevertheless the analyses on these mineral concentrates and whole rock do not deter-mine any isochron (MSWD = 20!). The possible explanations for this discrepancy were discussed above. Again, the analyses on pyroxene and whole rock from sample A1798 plot very close to the 2.35 Ga isochron defined by the Kallioniemi A1796 data (Fig. 55). Considering the other option, i.e., crustal contamination in the groundmass/whole rock, the analyses of pyroxene and plagioclase give an age of 2086 ± 63 Ma (εNd +0.2). The calculated εNd(2086 Ma) value for the whole-rock sample is -0.7. We can only conclude that from the available Sm–Nd data, the emplacement age of the sampled dyke remains unknown.

6060

4.5.7 A1794, A1795 Tilsanvaara

At the Tilsanvaara site, two mafic dykes with clearly different trends are observed in the same outcrop. The samples picked for isotope work include A1794 (AD89-10) from a NW–SE-trending Fe-tholeiitic dyke and A1795 (AD89-8) from a WNW–ESE-trending tholeiitic dyke. The main minerals, pla-gioclase and clinopyroxene, are well preserved in both diabase samples. The Sm–Nd data on sample A1794 give an isochron and age of 2219 ± 37 Ma, with an initial εNd value of +0.2 (Fig. 56). The Sm–Nd isochron age obtained for the tholeiite sample A1795 is 2223 ± 45 Ma and the initial εNd value +1.0. Most of the plagioclase was found in a slightly mag-netic fraction after Frantz isodynamic separation. The analyses conducted on plagioclase from both the magnetic (plag#2) and non-magnetic fractions plot on the isochron.

4.5.8 A1802 Koivuvaara

The Koivuvaara dyke ca. 6 km NW of Tilsanvaara is associated with a NW–SE-trending narrow aero-magnetic anomaly, which suggests that the dyke is at least 10–15 km in length. In thin section, the main minerals of this Fe-tholeiitic dyke, plagioclase and

clinopyroxene, appear relatively well preserved, but the mineral concentrates obtained by separation of sample A1802 were not entirely clean. Nevertheless, the Sm–Nd compositions of these concentrates and the whole-rock powder define an isochron giving an age of 2058 ± 35 Ma and an εNd value of -0.1 (Fig. 57). The amount of Nd in the analysed plagioclase fraction is relatively high (8.7 ppm), which warrants speculation concerning whether the age obtained really dates the igneous crystallisation event. More detailed studies would be required to resolve this.

4.5.9 A1801 Hirsikangas

A third dyke sample, A1801 (MLJ40-1), from the NW–SE-trending Fe-tholeiitic swarm in the area of high-grade gneisses around the town of Taivalkoski, was collected at Hirsikangas. The main minerals in this sample are plagioclase and clinopyroxe, which appear relatively well preserved, although the plagioclase used for analysis was slightly yel-lowish. As with samples A1800 and A1798 above, the three Sm–Nd analyses conducted on the mineral separates and whole-rock powder do not yield any isochron (MSWD = 47!). The age remains unknown, but in any case, the analysis of pyroxene suggests a positive initial εNd.

6161


A1794 (AD89-10)

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A1794plag A1794plag#2

A1795 (AD89-8)

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0.5108

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0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22

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d/14

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147Sm/144Nd

Tilsanvaara dykes

A1794 NW-trending dyke Age = 2219 ± 37 Ma

eps = +0.2 MSWD = 1.4 n=4

A1795 WNW-trending dyke Age = 2233 ± 42 Ma

eps = +1.0 MSWD = 0.71 n=4

Fig. 56. Sm–Nd isotope data for whole-rock samples and mineral separates from the Tilsanvaara dykes A1794 and A1795.


4.6 Volcanic rocks in the Kuusamo schist belt

Mafic volcanic rocks from three stratigraphic lev-els in the Kuusamo belt were targets for recon-naissance Sm–Nd studies in the early 1990s (Fig. 38). The andesites from the lowermost unit, the Kuntijärvi Formation (Greenstone I by Silvennoinen 1991), are enriched in LREE and yield an average εNd(2400 Ma) value of -2.7 and TDM = 2.82 Ga (Appendix 6). In contrast, the basaltic sample from the Petäjävaara Formation (Kuusamo Greenstone II by Silvennoinen 1991) has a nearly chondritic Sm/Nd ratio and a clearly positive initial εNd value. The third unit, the Ruukinvaara Formation (Greenstone III Silvennoinen 1991), has yielded an initial εNd value close to zero.

The ages of these mafic units are constrained by intruding mafic sills dated at 2.22 Ga and by under-lying conglomerate, which contains clasts with an age of ca. 2.4 Ga (Silvennoinen 1991). We have recently obtained a new U–Pb age for a porphyry clast collected from the Kuntijärvi conglomerate (A1868). Abundant zircon extracted from sample A1868-Kuntijärvi occurs as light-coloured, fairly transparent, euhedral to subhedral prisms or frag-ments. Both TIMS and LA-MC-ICPMS were used for U–Pb dating. The U–Pb data reveal that the concen-

tration of U in zircon is anomalously low (<30 ppm). Three TIMS analyses provided concordant results and gave an average age of 2428 ± 3 Ma (Fig. 58, Appendix 5). Analyses by LA-MC-ICPMS were carried out during two different sessions with dif-ferent standards. The analyses revealed that the 206Pb/204Pb ratios are much higher than in the TIMS data (Appendix 7). The two sessions differed slightly in terms of the obtained Pb/U ratios and errors in Pb/U, which is due to differences in calibration. Nevertheless, the results are consistent within error, and the calculated 207Pb/207Pb ages are very similar from both sessions. Rejecting one analysis, the data yield an average 207Pb/207Pb age of 2428 ± 4 Ma.

It can be concluded that the age of zircon from sample A1868 is 2428 ± 3 Ma, which should also date the formation of the porphyry and constrain the maximum depositional age of the Kuntijärvi conglomerate and overlying Kuntijärvi Formation volcanic rocks. The initial εNd(2428 Ma) value for this sample is -2.7 (TDM = 2.81 Ga), and thus identical with the εNd of the andesites from the Kuntijärvi Formation.

6262

A1802 (MLJ29-1)

A1802px

A1802plag

0.5108

0.5112

0.5116

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0.5128

0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21

143 N

d/14

4 Nd

147Sm/144Nd

A1802 Koivuvaara dyke Age = 2058 ± 35 Ma

eps = -0.1 MSWD = 0.15

Fig. 57. Sm–Nd isotope data for whole rock and mineral separates from the Koivuvaara dyke A1802.

5 PUDASJÄRVI COMPLEX AND THE PERÄPOHJA SCHIST BELT


The Pudasjärvi complex consists of Archaean mig-matitic gneisses and amphibolites, the ca. 2.82 Ga Oijärvi greestone belt, mafic and felsic intru-sive rocks with ages mostly close to 2.7 Ga, and paragneisses deposited after 2.74 Ga (Lauri et al. 2011, Huhma et al. 2012a). The oldest rocks in the Fennoscandian Shield, the 3.5 Ga Siurua tonalite gneisses, are located in the Pudasjärvi complex (Mutanen & Huhma 2003).

There is evidence for the existence of mafic dyke swarms of the ~2.45 Ga, ~2.1 Ga and ~1.98 Ga age groups in the Pudasjärvi complex, but the pub-lished U–Pb and Sm–Nd isotope data are still scarce (Vuollo & Huhma 2005). The oldest dykes are rep-resented by a low-Ti tholeiitic dyke swarm trend-ing 330°. It has been dated, but the Sm–Nd age of 2461 ± 150 Ma is imprecise and the U–Pb data just give a minimum age of 2378 Ma. One of the 2.45 Ga dykes is the gabbro-noritic Loljunmaa dyke, which is located on the southeastern side of the Penikat intrusion and is considered a candidate for the

parental magma of the layered intrusions in the Tornio-Näränkävaara belt (Iljina & Hanski 2005, Yang et al. 2016). A younger ~2.1 Ga Fe-tholeiitic dyke swarm is represented by the Sipojuntti dyke close to the coast of the Bothnian Bay, which has been dated at 2118 ± 14 Ma (Perttunen & Vaasjoki 2001).

The Palaeoproterozoic Peräpohja schist belt lies unconformably on the Archaean Pudasjärvi com-plex (Fig. 59). The belt is ca. 170 km long and 80 wide and records ca. 500 Ma of geologic history in a stratigraphic succession attaining ca. 5 kilo-metres in thickness. The lithostratigraphy of the Peräpohja belt was summarised and partly revised by Kyläkoski et al. (2012) and is presented in Figure 60. The supracrustal sequence is divided into two major lithostratigraphic units, the Kivalo and Paakkola Groups. The Kivalo Group starts with basal conglomerates, which were locally deposited on partly eroded ca. 2.44 Ga mafic layered intrusions, marking the maximum sedimentation age of the

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A1868 Kuntijärvi porphyry clast in cgl

LA-MC-ICPMSIntercepts at

0 0 & 2428 4 MaMSWD = 0.88 n=25


A1868 Kuntijärvi TIMSConcordia Age = 2428 3 Ma

n=3

Fig. 58. Concordia plot of U–Pb zircon data obtained from the porphyry clast in the Kuntijärvi Formation con-glomerate. LA-MC-ICPMS analyses are presented as (large black) error ellipsoids. The three ID-TIMS analyses are coeval and concordant and shown as small error ellipsoid.


sequence. The conglomerates are overlain by suba-erially erupted basalts of the Runkaus Formation and thick orthoquartzite and arenite strata of the Palokivalo Formation. The exact depositional age of these formations is uncertain, although it is known that they are older than ca. 2.22 Ga, which is the age of cutting mafic-ultramafic sills (Hanski et al. 2010). The Palokivalo Formation is overlain by the Petäjäskoski Formation, which is composed of hematite-rich phlogopitic-sericitic and albitic schists with quartzite and dolomite interbeds. It was deposited earlier than 2.14 Ga, as indicated by the presence of mafic sills of this age (see below). The following unit is a thick suite of subaerially erupted, LREE-depleted continental flood basalts belonging to the Jouttiaapa Formation (Perttunen & Hanski 2003). Using the whole-rock Sm–Nd method, the formation was dated at 2090 ± 70 Ma by Huhma et al. (1990) and, including a few subsequent analy-ses, the age has been refined to 2105 ± 50 Ma (this paper).

The upper part of the Kivalo Group is composed of orthoquartzites of the Kvartsimaa Formation having

minor stromatolitic dolomite interbeds, followed by a cyclic repetition of formations composed of mafic volcaniclastic rocks and sedimentary carbonate rocks. The former are represented by mafic tuffs and tuffites of the Tikanmaa, Hirsimaa and Lamulehto Formations, and the latter by dolomites and phyl-lites of the Poikkimaa Formation and stromatolitic dolomites of the Rantamaa Formation (Fig. 60). The dolomitic rocks record a typical Lomagundi-Jatuli positive carbon isotope anomaly (Karhu 1993, 2005). Important age constraints for the Peräpohja belt were obtained from a tuff of the Hirsimaa Formation, for which Karhu et al. (2007) determined a U–Pb zircon age of 2106 ± 8 Ma (Fig. 59).

The second main unit, the Paakkola Group (>1–2 km), begins with the Martimo Formation (Martimo suite in Nironen et al. 2016), which comprises turbidites and mica schists with graph-ite- and Fe sulphide-bearing black schist interlay-ers. Ranta et al. (2015) determined ages of detrital zircon grains from the upper part of the Martimo Formation, showing that these rocks were depos-ited later than ca. 1.91 Ga. Pillowed basalts of the

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( in this paper )

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Fig. 59. Geological map of the Pudasjärvi complex and Peräpohja schist belt showing sample localities. For sym-bols, see Figure 1.

Väystäjä Formation are spatially associated with mica schists, but the felsic porphyries within the Väystäjä volcanic rocks have yielded a U–Pb zir-con age of 2050 ± 8 Ma (Perttunen & Vaasjoki 2001, Lahtinen et al. 2015a), suggesting that these rocks are older than the upper part of the Martimo Formation and probably represent an allochthonous unit.

Among the youngest supracrustal rocks in the Peräpohja belt are felsic and mafic tuffs of the Korkiavaara Formation (Oikarila suite in Nironen et al. 2016) and mica schists of the Pöyliövaara Formation (Oikarila suite in Nironen et al. 2016), both of which contain zircon dated at ca. 1.98 Ga (Hanski et al. 2005, Lahtinen et al. 2015a). These rock units occur in the northern part of the belt and are also in tectonic contact with associated suprac-rustal rocks of the belt. Thus, these rock units are redefined as suites instead of lithostratigraphic

formations in the recent tectonostratigraphic com-pilation (Nironen et al. 2016).

In this paper, we document U–Pb zircon and whole-rock Sm–Nd isotope data for several sam-ples from the ca. 2.44 Ga layered intrusions and one coeval “boninitic” dyke (Loljunmaa), all close to the southern margin of the Peräpohja belt. Four other dykes further to the southeast in the basement were also studied, with two of them (Uolevinlehto, Vengasvaara) being ca. 2.44 Ga in age and the other two younger (Palomaa, Tervonkangas), with an age of 2.0–2.1 Ga. From the Peräpohja schist belt, we report isotope data for two 2.13–2.14 Ga mafic dykes cutting the Petäjäskoski and Tikanmaa Formations (Figs. 59, 60) and one 2.08 Ga mafic dyke cutting the Väystäjä Formation. Mafic lava flows or tuffs were analysed for Sm–Nd isotopes from the Runkaus, Jouttiaapa, Tikanmaa and Hirsimaa Formations.

5.2 The 2.44 Ga Kemi, Penikat, Kilvenvaara and Siikakämä intrusions

The first U–Pb zircon analyses by TIMS of sample A662 from the Kemi gabbro were conducted at GTK by O. Kouvo in 1976. These analyses yielded nearly

concordant data suggesting an age of 2433 ± 4 Ma (Perttunen & Vaasjoki 2001). We have re-analysed zircon from the same sample using LA-MC-ICPMS

6565


Fig. 60. Lithostratigraphy of the Peräpohja schist belt (modified after Kyläkoski et al. 2012). Labelled boxes represent samples of this study.


(Fig. 61, Appendix 9). The U concentration in zir-con is very low (20 ppm), and a large spot size (50 µm) together with low-U standards was used in the analyses. The data tend to plot above the con-cordia curve, which is probably due to an incompat-ible standard. Nevertheless, this has only a minor effect on the Pb/Pb isotope ratios, and an average 207Pb/206Pb age of 2436 ± 8 Ma can be calculated, which is highly consistent with the TIMS result obtained 40 years earlier.

Zircon grains from two previous samples (A603, A703) from the Penikat layered intrusion were also analysed by LA-MC-ICPMS (Fig. 62, Appendix 9). The bulk of the new data on both samples are con-cordant within error and yield an average 207Pb/206Pb age of 2437 ± 6 Ma. Most data were obtained from sample A603, providing an age of 2444 ± 8 Ma (n = 16, Maier et al. in press). A few data points suggest younger ages, probably due to alteration. The strongly discordant TIMS data are consistent with this age (Fig. 62, Perttunen & Vaasjoki 2001).

The first ever Sm–Nd analyses on the 2.44 Ga mafic rocks in Finland were performed on samples from the Penikat layered intrusion, which provided an initial εNd value of –1.6 (Huhma et al. 1990). In

order to complement the isotope database of the Kemi-Penikat layered intrusions, one Sm–Nd anal-ysis was conducted on the Kemi intrusion using the U–Pb dated gabbroic sample A662 discussed above. This sample yielded a composition that is now found to be exactly on the isochron as defined by the Penikat sample with an εNd(2440 Ma) value of –1.6 (Fig. 65, Appendix 1). The same average initial value was also recently obtained by Maier et al (in press) for seven samples from the Penikat intrusion.

Two old unpublished discordant TIMS analy-ses on multigrain zircon fractions are available for sample A859 taken from the Kilvenvaara gabbro in the Portimo Complex (Narkaus layered intrusion in Nironen et al. 2016, Fig. 59). These data suggest an age about 2.4 Ga (Alapieti et al. 1989). Recent U–Pb analyses by LA-MC-ICPMS performed on grains from these same fractions provide concordant data and an age of 2430 ± 7 Ma (Fig. 63, Appendix 9).

We analysed zircon from the Siikakämä gab-bro in the Narkaus layered intrusion, which is also considered a member of the chain of 2.44 Ga mafic intrusions that flank the southeastern margin of the Peräpohja schist belt (A454, Fig. 59). The TIMS results on multigrain zircon fractions from a

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A662 Kemi intrusion gabbro

Fig. 61. Concordia plot of U–Pb zircon data for sample A662 from the Kemi intrusion gabbro. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red triangles.

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2400

2600 LA-MC-ICPMS

average 207Pb/206Pb age 2437 ± 6 Ma

(MSWD = 1.3, n=18/23)


Penikat intrusion gabbros (A603 & A703)

A603 TIMS Intercepts at 797±230 & 2428 ± 35 Ma

MSWD = 22 (n=5 baddeleyite ± zircon)

(Perttunen & Vaasjoki 2001)

A703 TIMS Intercepts at 574±110 & 2404 ± 25 Ma

MSWD = 25 n=6 (Perttunen & Vaasjoki 2001)

Fig. 62. Concordia plot of U–Pb zircon data from the Penikat intrusion gabbro samples A603 and A703. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as dots (A703 as green).

0.36

0.40

0.44

0.48

0.52

0.56

6.5 7.5 8.5 9.5 10.5 11.5 12.5

207Pb/235U

206 P

b/23

8 U

2100

2200

2300

2400

2500

2600


Concordia Age = 2430 ± 7 Ma (n=6/7)

A859 Kilvenvaara gabbro

Fig. 63. Concordia plot of U–Pb zircon data from the Kilvenvaara gabbro A859, Portimo Complex. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as triangles.


gabbro pegmatoid sample A454 are heterogene-ous and rather unusual, as the youngest apparent Pb–Pb ages were obtained for the heaviest of the analysed fractions (Mertanen et al. 1989). The 19 spot analyses by LA-MC-ICPMS also yielded het-

erogeneous results. However, it seems obvious that the most pristine domains with ages close to 2.43 Ga would give a reasonable estimate of the igneous age, whereas ages from altered domains range down to ca. 1.8 Ga (Fig. 64, Appendix 9).

5.3 Loljunmaa gabbro-noritic dyke

Geological and geophysical surveys undertaken by GTK on the Archaean basement area east of the Penikat layered intrusion have revealed a gabbro-noritic dyke at Loljunmaa, which very probably represents a magma conduit to the system that pro-duced the large 2.44 Ga layered intrusions (Alapieti et al. 1990, Iljina & Hanski 2005, Yang et al. 2016). Consequently, chemical and isotopic composi-tions of the dyke have been utilised to estimate the parental magma composition of the layered intru-sions (Iljina & Hanski 2005, Yang et al. 2016). The dyke trends NW–SE and is 20–30 m in width. The

primary magmatic mineralogy is not preserved and thus only a whole-rock powder has been used for Sm–Nd work. The Loljunmaa sample 4-LD-93 pro-vides an εNd(2440 Ma) value of -1.2 (Appendix 1), and in terms of the REE level and isotopic composi-tion, it is almost identical to the Viianki whole-rock sample considered below (Chapter 7.2). The compo-sition also plots very close to the isochron defined by the Penikat intrusion samples, being consistent with the comagmatic nature of the dyke and intru-sion (Fig. 65).

6868

1300

1500

1700

1900

2100

2300

2500

A454A +4.6 clear A454B +4.6 HF leached

A454C 4.2-4.6 A454D 4.0-4.2 >160µm

A454E 3.8-4.0 >160µm A454F 3.8-4.0 70-160µm

0.2

0.3

0.4

0.5

2 4 6 8 10 12

206 P

b/23

8 U

207Pb/235U

A454 Siikakämä gabbro

Range of Pb/Pb ages from 2.43 Ga to 1.8 Ga 19 analyses on five grains


Fig. 64. Concordia plot of U–Pb zircon data from the Siikakämä gabbro A454. LA-MC-ICPMS analyses shown as error ellipsoids and ID-TIMS analyses as black dots.

5.4 Tholeiitic dykes, A1410 Uolevinlehto, Pudasjärvi

The Uolevinlehto dyke (A1410, UD) represents the low-Ti tholeiitic NW–SE-trending dyke swarm in the Pudasjärvi complex (Fig. 59). In the out-crop, the rock is fresh and consists of plagioclase (50%), clinopyroxene (35%), and minor amounts of orthopyroxene (2%), quartz, secondary amphi-bole and Fe–Ti–Voxides. The dykes are more than 2 km long and normally 40–70 m wide. The studied dyke outcrop is about 20 m x 100 m is size, but no contacts to the country rocks have been observed. The rock displays a cumulus texture. Plagioclase is An50–60 in composition and clinopyroxene is Ca-rich augite with an average composition of Wo36En43Fs20. One feature in common with other 2.45 Ga dyke swarms is that plagioclase is faintly to strongly cloudy. A large sample (35.4 kg) was collected for U–Pb dating, but only 18 baddeleyite grains were recovered (in Toronto). Some of the grains were of good quality, but some have inclu-sions and cracks and others have a thin (1–5 µm) rim of zircon. The best 12 grains were used for anal-ysis, which yielded a 2.1% discordant U–Pb result and a minimum age of 2366 Ma (207Pb/206Pb age, Appendix 8). The exact age remains non-precise,

but ca. 2.44 Ga is still conceivable, as the data plot on a 1.88–2.45 Ga chord (see Fig. 49).

The plagioclase in the rock is slightly cloudy. The five Sm–Nd analyses available are technically good, which is supported by the duplicated analyses con-ducted on whole rock and pyroxene (Appendix 1). However, the data points do not define any well-aligned isochron, but show some scatter in excess of analytical error. For the five analyses, the ISOPLOT programme gives an age of 2447 ± 160 Ma (εNd = +0.4, MSWD = 16, Fig. 66). The age calculated for pyroxene and plagioclase alone is about the same, but the initial εNd value is slightly higher (+0.7). Excluding plagioclase, the four analyses of whole rock and pyroxene yielded an age of 2537 ± 37 Ma (MSWD = 1.8), whereas plagioclase and whole rock gave an age of 2265 ± 58 Ma. Comparing with the U–Pb data, neither of these figures can be regarded as the magmatic age estimate for the dyke. The Nd concentration in the whole-rock sample is much higher than in pyroxene and plagioclase. Thus, a significant amount of Nd is situated outside these main minerals. This, together with the obtained disequilibrium in the initial εNd values between the

6969


A1012 wr

A1012 opx

A1012 plag

HH/13

A703

A662 Elijärvi

LD-4-93 Loljunmaa

0.5102

0.5106

0.5110

0.5114

0.5118

0.5122

0.5126

0.04 0.08 0.12 0.16 0.20

143 N

d/14

4 Nd

147Sm/144Nd

Penikat intrusion Age = 2422 ± 52 Ma

eps = -1.6 MSWD = 0.95 n=5 (Huhma etal 1990)

Fig. 65. Sm–Nd isotope data for whole rock and mineral separates from the Penikat layered intrusion (Huhma et al. 1990) and whole-rock samples from Kemi intrusion (A662) and Loljunmaa dyke.


whole-rock powder and separated major miner-als, suggests that some Nd in the rock was derived from crustal sources after the crystallisation of the major minerals. Consequently, the initial isotope

composition of the magma may be best estimated using the pyroxene (and plagioclase) composition, which suggests a slightly positive εNd value of ca. +0.6 at 2.44 Ga.

5.5 The 2.44 Ga Vengasvaara intrusion

The Vengasvaara (Kärppäsuo) mafic intrusion within the Archaean Pudasjärvi complex was dis-covered by drilling during a GTK research project (Fig. 59). Sample A1744 collected from a drill core is a medium-grained gabbro, with the main miner-als being plagioclase and clino- and orthopyroxene. The pyroxenes are slightly altered to amphibole. Minor minerals include quartz, biotite, apatite and opaque minerals. Attempts were made to separate the main minerals for Sm–Nd analysis, but it turned out that the quality of the separates was poor and too difficult for hand-picking. Plagioclase is partly cloudy and the pyroxene concentrate is heteroge-neous in colour. However, a few grains of brown, transparent and anhedral zircon were obtained from the heavy (>3.6 g/cm3) fraction.

Six zircon grains were analysed using SHRIMP at the VSEGEI laboratory in St. Petersburg. The U–Pb data (Appendix 4b) reveal very high contents of U and Th in zircon. However, the compositions of all the six grains are practically concordant and yield

an age of 2444 ± 4 Ma (Fig. 67). Sm–Nd analyses of the mineral concentrates and

whole-rock powder are technically good but do not yield any isochron (Fig. 68). The age calculated from the analyses of “pyroxene” and whole rock is 2507 ± 72 Ma, which is compatible with the U–Pb age mentioned above. The date derived from plagio-clase and whole rock is ca. 2.05 Ga and probably reflects metamorphic effects. The calculated ini-tial Nd isotope ratio for the whole-rock sample at 2.44 Ga is very unradiogenic (εNd = -5.4) and much lower than the ratios measured for other 2.4 Ga rocks in the shield. This suggests a significant con-tribution of old LREE-enriched lithosphere in the genesis of this intrusion. It is tempting to relate this with the 3.5 Ga old Siurua gneisses, which out-crop only 10 km NE of Vengasvaara (Mutanen & Huhma 2003). The εNd(2440 Ma) value for the Siurua gneisses is still much lower (ca. -15, Mutanen & Huhma 2003, Huhma et al. 2012b).

7070

A1410

A1410 px

A1410 plag

A1410#2

A1410 px#2

0.5108

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.5136

0.08 0.12 0.16 0.20 0.24

143 N

d/14

4 Nd

147Sm/144Nd

Age = 2447 160 Maeps = +0.4

MSWD = 16 n=5

A1410 Uolevinlehto tholeiitic dyke

Fig. 66. Sm–Nd isotope data for whole rock and mineral separates from the Uolevinlehto tholeiitic dyke A1410.

5.6 Palomaa dyke, A1743

The NW–SE-trending Palomaa dyke forms an aero-magnetic anomaly cutting the Archaean gneisses ca. 5 km NW of the Vengasvaara intrusion discussed above. The main minerals of the 30-m-wide dyke are plagioclase and clino- and orthopyroxene, which appear slightly altered in thin section. Brown amphibole and biotite are common and carbonate, apatite and opaque are among the accessory min-

erals. The mineral concentrates from the drill core sample A1743 used for Sm–Nd analyses are fairly fresh, although pyroxene appears slightly hetero-geneous. Plagioclase is relatively dark. The Sm–Nd data yield an isochron that gives an age of 2077 ± 34 Ma (Fig. 68). The calculated initial εNd value of +1.1 suggests an origin from depleted mantle for the magma.

5.7 Tervonkangas dyke A1808

Drilling by GTK of an aeromagnetic anomaly at Tervonkangas, Ranua, revealed a mafic–ultra-mafic dyke complex within the Archaean gneisses, having a length of ca. 3 km and a width of a few hundred metres. Sample A1808 taken from a NE–SW-trending dyke in the complex contains extremely well-preserved primary igneous min-erals: large (>5 mm), zoned augite phenocrysts, large biotite oikocrysts, olivine and plagioclase. The Sm–Nd analyses conducted on good quality mineral separates and a whole-rock powder defined an isochron with a date of 2623 ± 34 Ma and initial εNd value of +1.0 (MSWD = 1.7, Fig. 69, Appendix 1). Taking into account the geological setting, the

analytical results are technically good, but it is still questionable whether a rock with such a well-preserved primary texture and minerals is indeed Archaean in age. In order for this date to represent the real magmatic age, these two basic assumptions of isochron dating should be valid: 1) all miner-als crystallised at the same time and had the same initial Nd isotope composition and 2) the Sm–Nd system remained closed after the formation of these minerals. In this particular case, it is possible that large augite phenocrysts crystallised in the magma already prior to its final emplacement and solidi-fication. Assuming further that significant crustal contamination took place between these two stages,

7171


2410 2430

2450 2470

2490 2510

2530

0.43

0.45

0.47

0.49

0.51

9.6 10.0 10.4 10.8 11.2

206 P

b/23

8 U

207Pb/235U

A1744 Vengasvaara gabbro

Concordia Age = 2441 ± 4 Ma (n=3)

SHRIMP data-point error ellipses are 2s


MSWD = 1.5 (n=6)

Fig. 67. Concordia plot of U–Pb zircon data obtained by SIMS from the Vengasvaara gabbro A1744.


7272

A1743 wr

A1743 px

A1743 plag A1744 wr

A1744 "px"

A1744 plag

0.510

0.511

0.512

0.513

0.06 0.10 0.14 0.18 0.22

143 N

d/14

4 Nd

147Sm/144Nd

A1743 Palomaa dyke Age = 2077 ± 34 Ma

epsilon= +1.1 MSWD = 0.77

A1744 Vengasvaara wr & "px":

Age = 2507 ± 72 Ma epsilon = -4.6

Fig. 68. Sm–Nd isotope data for whole-rock samples and mineral separates from the Vengasvaara gabbro A1744 and Palomaa dyke A1743.

A1808

A1808px


A1787 Heinisuo

A1687 Soidinmaa

0.5098

0.5102

0.5106

0.5110

0.5114

0.5118

0.5122

0.5126

0.5130

0.04 0.08 0.12 0.16 0.20 147Sm/144Nd

143Nd 144Nd A1808:

The analyses define likely a mixing line and the age

has no meaning. A1808 Tervonkangas dyke

Age = 2623 ± 34 Ma epsilon = +1.0

MSWD = 1.7 n=4

Reference "isochron" 2100 Ma Eps = -0.7

A1808 Tervonkangas mafic dyke

Fig. 69. Sm–Nd isotope data for whole rock and mineral separates from the Tervonkangas dyke A1808.

plagioclase and whole rock would have had lower 143Nd/144Nd ratios by the time of crystallisation. In this case, the first assumption of meaningful isoch-ron dating would not be valid and the isochron steepened by contamination would yield an age that is too old. We are inclined to accept this interpreta-tion. We recall that initial isotopic disequilibrium between pyroxene and whole rock was also inferred in the case of the Pääjärvi/Lupzenga dyke (A1465) treated above. Assuming a Palaeoproterozoic age for the Tervonkangas dyke, the calculated initial

εNd value becomes slightly negative, being -1.9 at 2.0 Ga, for example.

Attempts to obtain zircon or baddeleyite for dating were not successful with the Tervonkangas sample. The same problem was also faced with the NW–SE-trending Heinisuo mafic dyke A1787, occurring 15 km NE of the site of sample A1808. A Sm–Nd analysis conducted on whole-rock pow-der yielded an initial εNd value of -1.2, if the age is assumed to be 2 Ga (Appendix 1).

5.8 The 2.13–2.14 Ga dykes in the Peräpohja schist belt, A1214 Koppakumpu and A2087 Kuusivaara

According to Perttunen & Hanski (2003), mafic tuffs of the Tikanmaa Formation in the Peräpohja schist belt are cut by a diabase dyke at Koppakumpu. The Tikanmaa tuffs overlie the metabasalts of the Jouttiaapa Formation in the Peräpohja stratigraphy (Fig. 59). Sample A1214 was already collected and processed for zircon in 1990, but due to the very small number of recovered zircon grains, it was not analysed until the access to in situ analytical techniques. The U–Pb data obtained by SHRIMP (St Petersburg) are technically of good quality, and ten analyses on nine grains provide a chord that has

intercepts with the condordia curve at 75 ± 110 and 2129.5 ± 4.2 Ma (MSWD = 1.19, Fig. 70, Appendix 4b). Most of the data points are concordant despite the relatively high concentrations of U. A special feature of the zircon is its high Th/U ratio, although this is not uncommon for zircon in mafic dykes. The high levels of U and Th are consistent with the dark and turbid appearance of the zircon grains. Despite of the radiation damage, magmatic zon-ing is still clearly visible in the CL images of these zircon grains (Fig. 70).

1920 1960

2000 2040

2080 2120

2160 2200

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0.44

5.4 5.8 6.2 6.6 7.0 7.4 7.8

206 P

b/23

8 U

207Pb/235U

A1214 Koppakumpu diabase

Intercepts at 2130 ± 4 & 75 ± 110 Ma

MSWD = 1.19 n=10

SHRIMP data-point error ellipses are 2s

lenght of grain 5: 120µm

R4F2t.jpg:A1214 grains 6,4,5,7

7373


Fig. 70. Concordia plot of U–Pb zircon data obtained by SIMS from the Koppakumpu diabase A1214.


The magmatic zoning preserved in Kuoppakumpu zircon grains backs the interpretation that the obtained U–Pb age of 2130 ± 5 Ma can be con-sidered the igneous crystallization age of the Koppakumpu diabase. This gives age constraints for the supracrustal rocks. Accordingly, the basalts of the Jouttiaapa Formation should be older than 2130 ± 5 Ma, which is (within error) in agreement with the Sm–Nd age of 2103 ± 50 Ma available for the Jouttiaapa metabasalts (Chapter 5.10).

A similar U–Pb zircon age of 2140 ± 11 Ma has recently been published for a mafic sill intruding

the Petäjäskoski Formation, which lies directly below the Jouttiaapa Formation in the Peräpohja lithostratigraphy (Kyläkoski et al. 2012). This age was based on U–Pb isotope analyses using the LA-MC-ICPMS technique on zircon grains from a gabbroic sample (A2087) picked from a drill core intersecting the dated sill at Kuusivaara (Fig. 59). Both the Kuoppakumpu and Kuusivaara mafic dykes have a nearly chondritic Sm/Nd ratio and give clearly positive initial εNd values of +3.2 and +3.5 (Appendix 1).

5.9 Rytijänkkä dyke A854

The uppermost allochthonous stratigraphic unit of the Peräpohja belt, the Väystäjä Formation, is cut by a metadiabase dyke at Rytijänkkä. The dyke locates within the mafic volcanic rocks of the Väystäjä Formation, ca. 600 m north of its contact to the underlying turbiditic Martimo Formation meta-sediments. Sample A854 was taken from a coarse-grained, light-coloured diabase and processed for zircon in the 1970s. The separation yielded a small amount of fairly turbid zircon, which was recently studied by laser ablation MC-ICP-MS. The obtained spot data are scattered, but an igneous age of 2084 ± 11 Ma can be constrained based on data from the

most pristine-looking domains (Fig. 71, Appendix 9). Few measurements of altered zircon yielded ages of ca. 1.8 Ga. Based on CL images, these grains may have baddeleyite cores. Two distinct grains are Archaean in age, representing either inher-ited Archaean zircon crystals or grains obtained by contamination during sample processing (the latter could have happened in the 1970s, when a large, difficult-to-clean jaw crusher, roller mill, and Wilfley table were used in the processing). The Sm–Nd analysis of sample A854 indicated a chondritic Sm/Nd ratio and an initial εNd of +1.0 (Appendix 1).

7474

Fig. 71. Concordia plot of U–Pb zircon data obtained by LA-MC-ICPMS from the Rytijänkkä diabase A854.

1500

1700

1900

2100

2300

0.24

0.28

0.32

0.36

0.40

0.44

0.48

2 4 6 8 10

206 P

b/23

8 U

207Pb/235U

A854 Rytijänkkä diabase data-point error ellipses are 2s

Concordia Age = 2089 11 Man=11 (/20)


MSWD = 0.33 n=11

5.10 Volcanic rocks

The age of the lowermost volcanic unit of the Peräpohja schist belt, the Runkaus Formation is between 2.25 Ga (metamorphic titanite) and 2.44 Ga (Penikat layered intrusion). Sm–Nd isotope data on these rocks published by Huhma et al. (1990) are presented in Figure 72, in order to emphasise the slight variation in the initial εNd values between the lower and higher lava flows in the Runkaus Formation. For reference, estimates of the timing of the regional metamorphic effects are also shown. The samples from the top part of the first recognised lava flow (14C, 14C2) display strong LREE depletion at ca. 1.7–1.8 Ga, and some secondary fractionation may have also influenced other samples. As a whole, the isotope and chemical data suggest that the first flow had primarily a slight LREE enrichment and initial epsilon value close to zero, whereas later flows show a significantly higher REE level, more LREE enrichment and slightly negative initial εNd values, suggesting stronger crustal contamination (Huhma et al. 1990).

So far, the Jouttiaapa Formation basalts have not been dated precisely. The best available age

estimate is based on whole-rock Sm–Nd isotope analyses. Combining the analytical data published by Huhma et al. (1990) with three subsequent anal-yses on strongly LREE-depleted samples, an age of 2105 ± 50 Ma can be calculated (Fig. 73, Appendix 6). As was discussed above, a mafic dyke (A1214-Koppakumpu) cutting the Tikanmaa Formation above the Jouttiaapa Formation in the stratigraphy has yielded a U–Pb age of 2130 ± 5 Ma. Accordingly, the basalts of the Jouttiaapa Formation should be older than 2130 ± 5 Ma, which is (within error) in agreement with the Sm–Nd age derived above for the formation. Using an age of 2130 Ma, an average epsilon of +3.9 can be calculated for the analysed 16 basalt samples. Highly positive initial εNd values exceeding +3 were also obtained for the mafic dykes at Koppakumpu (A1214) and Kuusivaara (A2087, age 2140 ± 11 Ma) discussed above.

A recent Sm–Nd analysis conducted on a sam-ple from mafic tuffs in the Tikanmaa Formation revealed that the rock has a chondritic Sm/Nd ratio and a highly positive εNd(2130 Ma) value of +4.1 (Appendix 6). A sample from mafic tuffites

7575


14A epidote

14A albite 14A amphibole

14A amphibole

14C albite

14A

0.5104

0.5108

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.06 0.10 0.14 0.18 0.22 0.26 147Sm/144Nd

143 N

d/14

4 Nd

Runkaus metabasalts 1st flow (triangles)

Reference line 2.3 Ga epsilon = 0

Metamorphic LREE depletion at ca. 1.7-1.8 Ga

flow top 14C, 14C2 Epidote whole rock

1571±52 Ma

Flows 2-5 (squares) Reference line 2.3 Ga

epsilon = -1

14C, C2

Fig. 72. Sm–Nd isotope data for whole rock and mineral separates from the Runkaus basalts. Flow top samples with secondary REE fractionation are presented as open symbols (data from Huhma et al. 1990).


of the Hirsimaa Formation above the Tikanmaa Formation has also been analysed. This sample (A1788) yielded a U–Pb zircon age of 2106 ± 8 Ma

(Karhu et al. 2007), has a high REE abundance, low Sm/Nd, and gives an initial εNd(2106 Ma) value of +0.9 (Appendix 6).

6 KUHMO BLOCK IN THE LENTUA COMPLEX


The Kuhmo basement block forms the central part of the large Archaean basement complex (Lentua com-plex) in eastern Finland, being located between the Palaeoproterozoic Kainuu schist belt and the east-ern border of Finland. As mentioned earlier, there are shear zones at the contact between the Kuhmo and Taivalkoski blocks. That these two blocks have undergone partly different post-Archaean/Proterozoic geological histories is evidenced by the obvious differences between the orientations of the Palaeoproterozoic dyke swarms occurring in these regions. Moreover, observations for the youngest, 1.98 Ga NW–SE-trending swarm, which is well-presented in the Kuhmo block (see below), are so far lacking from the Taivalkoski block (see Fig. 5.21 in Vuollo & Huhma 2005).

The Kuhmo block is divided into two parts by a narrow, approximately 220-km-long N–S-trending

chain of Archaean greenstone belts (Fig. 74), which is collectively called the Tipasjärvi-Kuhmo-Suomussalmi greenstone complex (Papunen et al. 2009). The surrounding rocks are composed of migmatitic tonalitic gneisses, migmatised gneisses of sedimentary origin (paragneisses), and various plutonic rocks, such as tonalite-trondhjemite-granodiorite series and granodiorite-granite-monzogranite series rocks, and sanukitoids. More information on the Archaean geology and geochro-nology of the Kuhmo block is presented in Papunen et al. (2009), Huhma et al. (2012a, b), Mikkola et al. (2011) and references therein.

The Archaean basement of the Kuhmo block is abundantly cut by Palaeoproterozoic diabase dykes (Vuollo & Huhma 2005). Most of the dykes are geochemically similar Fe-tholeiites, but in terms of age and orientation, two distinct swarms can

7676

CHUR

Tikanmaa tuff

0.5125

0.5135

0.5145

0.5155

0.16 0.20 0.24 0.28 0.32 0.36 0.40

143 N

d/14

4 Nd

147Sm/144Nd

Jouttiaapa metabasalts

Age = 2105 ± 50 Ma eps=+4.2

MSWD = 3.9 n=16

lower part

upper part

Fig. 73. Sm–Nd isotope data for whole-rock samples from the Jouttiaapa basalts and Tikanmaa tuff (data from Huhma et al. 1990 and this study). CHUR = chondritic uniform reservoir (De Paolo & Wasserburg 1976)

be recognised. The older ~2.1 Ga group comprises dykes with a trend of 280° and the younger 1.98 Ga dykes show a trend of 320°. Also, representatives of the ca. 2.45 Ga age group can be found in the

Kuhmo block, belonging to the NE–SW-trending boninite-noritic, E–W-trending orthopyroxene-plagioclase-phyric and NW–SE-trending low-Ti tholeiitic subtypes (Vuollo & Huhma 2005). The

7777


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_̂

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_̂

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_̂

_̂

_̂

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_̂

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_̂

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_̂

_̂

_̂̂_

_̂

_̂_̂

_̂

_̂

_̂

Lapinlah�

Koli

Arola

Humppi

Otanmäki

Nieminen

Kylylah�

Kapea-aho

Panjavaara

Lohisärkkä

Koirakoski

Honkaniemi

Raatelampi

Veitsivaara

Ke�ukallio

Siilinjärvi

Viianki (VD)

Petäiskangas

Paukkajavaara

Ha�uselkonen

Mus�kkarinne

Syväri

Jun�lanniemi

Paha Kapustasuo

Kaavi

Kivikevä� (KD)

Vuotjärvi

PeräahoIlomantsi

Oravaara

Siunaussalmi/Tulisaari

Varisniemi Fm

Kiihtelysvaara

Siilinjärvi ParkkilaSiilinjärvi Vuorimäki

Ma�nvaara Fm/ Kurkikylä Gr

A1231

A0376

A1368

A1460

A1838

A0261

A0729A0196

A1672

A1363

A1361A1673

A0769

A0496

Romuvaara

0 30 km¢

Russia

Sweden

Norway

Finland

Age

_̂ <1930 Ma

_̂ 1931 - 2080

_̂ 2081 - 2180

_̂ 2181 - 2280

_̂ 2281 - 2380

_̂ >2380

_̂U-Pb



Sm-Nd (±U-Pb)

Volcanic rocks


_̂

_̂

_̂


( in this paper )

Sm-Nd (age ?)

Fig. 74. Geological map of eastern Finland showing the sample locations. For symbols, see Figure 1.


boninite-norites include the Viianki dyke, which potentially gives important geochemical evidence for the parental magma composition of the 2.44 Ga layered intrusions in Finland (Vogel et al. 1998, Yang et al. 2016). Hanski et al. (2010) documented the occurrences of 2.22 Ga sills and dykes (GWA association) within the Archaean Kuhmo green-stone belt, but no signs of dykes of this age group have been discovered from the Archaean granitoid basement in the Kuhmo block, except at the eastern margin of the Kainuu schist belt. In general, the mafic dykes are better preserved, containing pri-mary plagioclase and clinopyroxene, in the eastern part of the Kuhmo block, close to the Russian border (see Fig. 5.23 in Vuollo & Huhma 2005).

In this paper, we present isotope data for dykes of the age groups mentioned above. In addition, we provide new isotopic evidence for the presence of ca. 2.3 Ga dykes in a WNW–ESE-trending dyke swarm close to the Suomussalmi greenstone belt and in a SE–NW-trending dyke swarm in an area between the Kuhmo greenstone belt and Kainuu schist belt. Furthermore, we have confirmed an age of ca. 2.2 Ga for a dyke that cuts the Saari-Kiekki belt, a small Palaeoproterozoic schist belt crossing the Finnish–Russian border (Luukkonen 1989), and an age of 2.15 Ga for a dyke representing a NW–SE-trending swarm that occurs just east of the Kainuu schist belt.

6.2 The 2.4 Ga boninite-norite Viianki dyke, A1356

The Viianki dyke (A1356, VD) occurs on the Finnish–Russian border (Fig. 74) and represents the group of noritic dykes with boninitic geochemical affin-ity (Vuollo & Huhma 2005). Dykes of this type are encountered in many places in the Kuhmo block, near the 2.44 Ga layered intrusions from Näränkävaara through Koillismaa to Peräpohja, and also in Russian Karelia (Stepanov 1994, Vuollo & Fedotov 2005). In the Kuhmo block, they trend

NE, are 30–60 m thick, and some can be traced along strike for more than 40 km. They consist typically of coarse plagioclase (35%), orthopyrox-ene (30%), clinopyroxene (20%), with minor oli-vine (5%), chromite and Fe–Ti oxides. These dykes are found well preserved only near the Finnish–Russian border and further east on the Russian side. Elsewhere, pyroxenes are altered to amphiboles and olivine to serpentines. The dykes tend to lack well-

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A1356 Viianki boninite-norite dykeAge = 2409 46 Ma

eps = -1.3MSWD = 1.6 n=5

Fig. 75. Sm–Nd isotope data for whole rock and mineral separates from the Viianki dyke A1356.

developed chilled margins. The boninite-norite dykes are characterised by high contents of MgO, SiO2, Cr and Ni, whereas TiO2 and Zr are low. Chondrite-normalised REE patterns are fairly steep, with enrichment in LREE.

The width of the Viianki dyke is 30 m and it can be traced in the NNE direction for about 200 m in the field (Kilpelä 1991). The major minerals are plagioclase (An 52%), clinopyroxene, and olivine, which are well preserved in places. A large sample (29 kg), labelled A1356, was collected from the dyke for U–Pb dating, but only 11 grains of baddeleyite were obtained (in Toronto). Some of the grains were rather good, but some had inclusions, cracks, and a thin (1–5 µm) rim of zircon. The measured U–Pb

isotope composition of these grains is 1.7% discor-dant (Fig. 49, Appendix 8) and gives a minimum age of 2385 Ma (207Pb/206Pb age). As many of the baddeleyite grains have a thin rim of polycrystalline metamorphic zircon (cf. Heaman & LeCheminant 1993), the primary age of the magmatic baddeleyite should be older than that obtained from the bulk material, conceivably close to 2.44 Ga (see Fig. 49).

Five Sm–Nd analyses were performed on a whole-rock powder and plagioclase and pyroxene separates from Viianki (Appendix 1). These data plot along a chord with an age estimate of 2409 ± 46 Ma and an initial εNd of -1.3 (MSWD = 1.6, Fig. 75), thus being consistent with the age obtained by U–Pb.

6.3 The 2.3 Ga dykes, Lohisärkkä A1914, Kovavaara A1361, Karhuvaara A1672

The WNW–ESE-trending, 200-m-wide Lohisärkkä dyke swarm extends more than 30 km, cutting Archaean rocks in the Suomussalmi area (Bernelius 2009, Fig. 74). The dykes are usually only slightly strained, coarsely plagioclase-phyric metadiabases of Fe-tholeiitic composition. The main minerals are typically labradoritic plagioclase, tschermakitic hornblende–tschermakite and epidote. A small amount of fine-grained baddeleyite was recov-ered from the sample (A1914) collected for isotopic studies. The two multigrain TIMS analyses revealed fairly high common lead and provided discordant data with 207Pb/206Pb ages of ca. 2.2 Ga (Fig. 76, Appendix 5). A closer look at the grains mounted on epoxy revealed that many baddeleyite grains are mantled by zircon. For LA-MC-ICPMS analy-sis, we have used an in-house baddeleyite standard, A974, which has a TIMS U–Pb age of 1256 ± 2 Ma (Söderlund et al. 2004). Forty spots were analysed using laser ablation (Fig. 76, Appendix 10); most of these were targeted at baddeleyite, but some measurements may also have hit zircon deeper in the sample. Many analyses, especially those of zir-con, showed a high amount of common lead. Zircon domains were analysed using the baddelyite stand-ard calibration, and the reported Pb/U ratios are not therefore correct, but Pb–Pb ages should be rel-evant. This conclusion is based on the finding that the measurements on our in-house zircon standard A382 during the same session closely produced the expected age of 1877 ± 2 Ma (Appendix 10).

Rejecting data with high common lead (206Pb/204Pb<600), the analyses on baddeley-ite give an age of 2321 ± 21 Ma (n = 27). The

206Pb/207Pb ages on zircon are younger, down to 1.8 Ga, which well explains the age from the bulk TIMS data. We may conclude that the Lohisärkkä dyke swarm belongs to the 2.3 Ga dyke group and records 1.8 Ga metamorphic effects, which are common throughout Finland. Sm–Nd whole-rock analysis yielded an εNd(2321 Ma) value of +1.5, which is similar to those from the 2.3 Ga dykes in the Iisalmi complex (Chapter 8, Appendix 1).

Samples from several Fe-tholeiitic dykes occur-ring in the Archaean migmatite gneiss-dominated area between the Kainuu schist and Kuhmo schist belts were collected for isotope dating in 1993 in connection with the project “The age and character of the Proterozoic mafic magmatism” (Department of Geology, University of Oulu). These originally diabasic rocks have been variably sheared/foli-ated and metamorphosed under amphibolite facies conditions to more or less schistose metadiabases usually containing amphibole and oligoclase as their main minerals. Because of the metamorphic effects, many of the samples produced discordant multigrain zircon TIMS U–Pb data, often difficult to interpret reliably and, in the best cases, yielding imprecise ages.

Sample A1361-Kovavaara was picked from the coarse-grained inner part of a SE–NW-trending dyke with a thickness of 60–80 m (Fig. 74). In contrast to many mafic dykes occurring in the area between the Kuhmo greenstone belt and Kainuu schist belt, this rock is only mildly deformed and preserves its primary diabasic texture fairly well, although without any traces of primary main min-erals. A small amount of translucent, brownish

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Fig. 76. Concordia plot of U–Pb zircon data obtained from the Lohisärkkä dyke A1914. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as dots.

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A1672 Karhuvaara Intercepts at 76 ± 75 & 2314 ± 6 Ma

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A1361 Kovavaara reference line intercepts

700 & 2280 Ma (3 TIMS analyses)

Fig. 77. Concordia plot of U–Pb zircon data obtained from the Kovavaara (A1361) and Karhuvaara (A1672) dykes. LA-MC-ICPMS analyses for A1672 are shown as error ellipsoids and ID-TIMS analyses for A1361 as red dots.

zircon was obtained from the sample. Three multi-grain TIMS analyses yielded discordant results and do not define an unambiguous straight-line chord. However, the average age of the three fractions, ca. 2.28 Ga, could well be close to the magmatic age of the dyke (Fig. 77, Appendix 5), which is supported by the fact that the zircon has an unusually high Th/U ratio, similarly to the zircon grains in the 2.3 Ga Karkuvaara dyke occurring in the Taivalkoski block (Chapter 4.4).

Sample A1672-Karhuvaara was collected 16 km south of the Kovavaara dyke, from a similar though more strongly strained/foliated, SE–NW-trending Fe-tholeiitic metadiabase dyke 60–80 metres in

thickness. The zircon population extracted from sample A1672 looks heterogeneous, and many grains appear turbid and hence altered. The 18 analyses by LA-MC-ICPMS conducted on 15 grains were tech-nically good and the data from the best-preserved domains yield Pb–Pb ages close to 2.3 Ga (Fig. 77, Appendix 10). The analyses of altered domains tended to give significantly younger Pb–Pb ages, and two Archaean ages were obviously measured from xenocrystic grains. The obtained isotope data suggest a magmatic age of 2314 ± 6 Ma for this Fe-tholeiitic dyke, being similar to the age that was determined for the Kovavaara dyke (A1361) discussed above.

6.4 The 2.2 Ga Rasiaho dyke A261

The Rasiaho dyke is a major metadiabase dyke in the Kuhmo area close to the Russian border, where it intrudes the supracrustal rocks of the Saari-Kiekki schist belt (A261, Fig. 74). The Rasiaho trunk dyke, which has several sizeable apophyses (pajonettes), is up to 240 m wide and can be traced in the NW–SE direction for more than 14 km. The dyke rocks are mostly only very weakly foliated to not foliated at all, preserving their primary diabasic textures, although they mostly contain pervasively hydrated mineral assemblages of lower amphibolite facies, including calcic amphibole, sodic plagioclase and epidote as their main minerals.

A sample from a coarse-grained segregation from Rasiaho has earlier been studied by Luukkonen (1989), who published multigrain TIMS U–Pb data on zircon (with baddeleyite inclusions) and titanite. The studied zircon grains extracted from sample A261 were mostly turbid, altered-looking and con-tained some baddeleyite cores. The obtained TIMS data were discordant with Pb–Pb ages mostly at 1.9–1.97 Ga. Interestingly, an analysis of low-den-sity material (3.8–4.0) gave the highest Pb–Pb age of 2.14 Ga. The emplacement age of the rock was estimated from two analyses of titanite at 2.2 Ga. We observed that many zircon grains have a badde-lyite core, which is clearly visible in the BSE images.

In situ LA-MC-ICPMS analyses revealed different ages for zircon and baddeleyite, with the badde-leyite in the zircon cores giving ages of ca. 2.2 Ga and the zircon rims recording ages of ca. 1.8 Ga. It is obvious that the multigrain TIMS data rep-resent mixtures between these two components. Unfortunately, the analytical session that included the baddelyite measurements was short and with only a few measurements of the standard, resulting in large errors in the analyses (Fig. 78a, Appendix 10). Subsequently, the age of ca. 2.2 Ga has been confirmed for baddeleyite, using SC-ICP-MS Attom (Fig. 78b, Appendix 10).

It is to be noted that the 2.2 Ga age and weakly to non-deformed nature of the Rasiaho dyke raise doubts over the presently assumed Sumi-Sariolan (2.5–2.3 Ga) age of the Saari-Kiekki belt (Luukkonen 1989). Namely, if the belt was indeed younger than 2.5 Ga, then the 2.2 Ga age of the Rasiaho dyke would constrain the timing of the complex lower amphibolite facies deformation of the Saari-Kiekki belt (Luukkonen 1989) to a pre-2.2 Ga, i.e., Sumi-Sariolan or Jatulian event. A problem with this is that no such dynamothermal event has been reported from other parts of the Karelian craton.

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A261 Rasiaho diabasebaddeleyite (+-zircon)

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Fig. 78. A) Concordia plot of U–Pb data obtained from the Rasiaho diabase A261. LA-MC-ICPMS analyses con-ducted on zircon and baddeleyite are presented as error ellipsoids and ID-TIMS analyses of zircon (±baddeleyite) as black dots. B) Concordia plot of U–Pb data obtained by Attom on baddeleyite from the Rasiaho diabase A261. In the back-scattered electron image, baddeleyite forms distinct cores mantled by zircon.

6.5 The 2.15 Ga Petronjärvi dyke A1363

The Petronjärvi dyke (A1363) is one of the many Fe-tholeiitic metadiabase dykes that cut the Archaean gneissic terrain immediately to the east of the Kainuu schist belt as NW–SE-trending, 10- to 50-m-wide magmatic bodies (Fig. 74). Most of these dykes are metamorphosed to amphibolite facies mineral assemblages and deformed to show at least some obvious foliation and lineation. Zircon grains obtained from sample A1363 are turbid and appear to be extensively altered. Five multigrain fractions were initially analysed using TIMS. The U–Pb data are discordant and plot in a cluster with

Pb–Pb ages of 1.93–1.97 Ga. Subsequent to the com-pletion of the U–Pb zircon dating, zircon grains from the original mineral separate were analysed by laser ablation MC-ICP-MS. Five analyses of the best-preserved domains yielded a concordia age of 2148 ± 8 Ma, which may be considered as the igne-ous age of the rock. Several altered zircon domains gave ages of ca. 1.87 Ga (Fig. 79, Appendix 10). We point out that the appearance and chemical com-position of this dyke are very similar to those of the 2.3 Ga dykes discussed above.

6.6 The 2.1 Ga Kapea-aho dyke A1212

The Kapea-aho dyke represents a local swarm of E–W-trending diabase dykes, which are common in the area east of the northern Kuhmo green-stone belt and where they tend to be nonfoliated and often comprise only slightly altered primary mineral assemblages (Fig. 74). The Kapea-aho dyke has been studied by Kilpelä (1991), who states in his Master’s thesis that the dyke is 50–60 m wide and can be traced for 6–7 km along strike in out-

crops and as a positive aeromagnetic anomaly. At the Kapea-aho sampling site, where the dyke has a width of approximately 50 metres, the central part of the dyke is typically hydrated to metadiabase containing mainly actinolitic hornblende, horn-blende, andesine and muscovite. However, closer to the margins, zones of unaltered rock are found, where the major minerals are labradorite (An 51%) and clinopyroxene. The chemical characteristics of

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A1363 Petronjärvi metadiabase (Fe-tholeiitic) data-point error ellipses are 2s

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n=5

Average Pb/Pb age 1868 ± 31 Ma 4 youngest

TIMS

Fig. 79. Concordia plot of U–Pb zircon data obtained from the Petronjärvi dyke A1363. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red triangles.


the Kapea-aho dyke indicate an evolved, Cr- and Ni-poor composition with Fe-tholeiitic affinity (Kilpelä 1991, Vuollo & Huhma 2005).

The main minerals, plagioclase and pyroxene, were separated from sample A1212 for Sm–Nd iso-tope measurements. Hand-picking was used as the final step of the extraction process, yielding fresh

and clean-looking fractions of both minerals. The performed five analyses, including two for the whole-rock powder, yielded an isochron with an age of 2133 ± 29 Ma (εNd = +0.7, MSWD = 0.2, Fig. 80, Appendix 1), which we interpret to represent the most probable emplacement age of the Kapea-aho dyke.

6.7 The 2.0–1.95 Ga dykes, Kivikevätti A1409, Puuropuro A1673, Peräaho A1519, Kivimäki A1460

The Kivikevätti dyke is one of the numerous NW–SE-trending dykes that occur in the Kuhmo block, in this case in the area east of the Kuhmo green-stone belt (Fig. 74). The Fe-tholeiitic dyke swarm represented by the A1409 Kivikevätti sample is in its area of occurrence the youngest known and least deformed mafic swarm. In well-preserved samples, the mostly coarse- to medium-grained dykes con-sist of subhedral coarse plagioclase (40%), set in a matrix of fine-grained, anhedral-subhedral clino-pyroxene (25%), Fe–Ti oxide (10%) with quartz (3%), biotite (4%) and uralitic amphibole (10%). Plagioclase laths are characterised by tea-coloured clouding just as plagioclase, e.g., in well-preserved 2.45 Ga dykes. The observed maximum width of the Kivikevätti dyke is ca. 60 m, and along strike, it can be traced in the field for more than 10 km (Kilpelä

1991). At the A1409 sampling site, both margins of the dyke are exposed and show chilling against the country gneiss. The primary magmatic minerals in the sample are well preserved.

Hundreds of relatively high-quality baddeleyite grains were recovered from the sample (in Toronto, sample wt 16.8 kg). Most baddeleyites were brown-ish needles without inclusions. The four U–Pb anal-yses conducted on baddeleyite by TIMS were slightly discordant (1.4–2.3%) and yielded 207Pb/206Pb ages of 1977–1979 Ma (Appendix 8). A regression line through the data gives intercepts of 1980 ± 4 Ma (2σ) and 329 Ma (Fig. 81).

Well-preserved primary minerals, plagioclase and clinopyroxene, were used for Sm–Nd dating. The analysis of plagioclase (#1) involved problems related to the dissolution and subsequent spike-

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A1212 Kapea-aho dykeAge = 2133 29 Ma

eps = +0.7MSWD = 0.20 n=5

Fig. 80. Sm–Nd isotope data for whole rock and mineral separates from the Kapea-aho dyke A1212.

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Fig. 81. Concordia diagram showing U–Pb TIMS data for baddeleyite fractions analysed in Toronto from the Kivikevätti dyke. The error ellipses reflect two sigma errors.

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A1409 Kivikevätti dyke

Fig. 82. Sm–Nd isotope data for whole rock and mineral separates from the Kivikevätti dyke A1409.


sample homogenisation. These problems were avoided in the duplicated analysis (plag#2), and due to uncertainty in Sm/Nd, the analysis plag#1 should be omitted. The data on the whole-rock sample and plagioclase (#2) and pyroxene separates give an age estimate of 2014 ± 33 Ma (εNd = +0.3, MSWD = 0.9, Fig. 82, Appendix 1), which is consistent with the baddeleyite U–Pb age within error.

The NW–SE-trending Fe-tholeiitic Puuropuro dyke (A1673) is located about 8 km to the east of the Kainuu schist belt, about 5 km west of the 2.3 Ga Kovavaara dyke (Fig. 74). The width of the dyke is 60–80 metres and along strike, it can be fol-lowed in outcrops for at least 4 km. The rock in the dyke varies from nearly non-foliated to conspicu-ously foliated metadiabase that contains actinolitic hornblende, plagioclase, and in places also garnet as the main minerals. Compared with many other metadiabase dykes of this study, the zircon grains extracted from sample A1673 appear exceptionally fresh and good in their preservation quality. The LA-MC-ICPMS data (n = 14) are concordant and suggest an age of 1979 ± 6 Ma, which is probably the igneous age for this dyke. A multigrain TIMS analysis is consistent with this result (Fig. 83).

The Koidanvaara dyke is a large tholeiitic diabase dyke close to the Russian border in the Ilomantsi area (Fig. 74; Kärenlampi 2015). The dyke persists along strike for more than ten kilometres and has a width of 0.5–1 kilometres. Signs of deformation are mostly slight and, despite amphibolite facies metamoprphism, primary minerals including pla-gioclase, clinopyroxene, Fe-Ti oxides, quartz and potassic feldspar with minor subsolidus ferrohorn-blende and fayalite are widely preserved, but usu-ally with at least some secondary amphibole and plagioclase.

The sample for zircon dating (A1519), which was taken at the Peräaho locality from a pegmatoid seg-regation in the diabase, produced abundant light-coloured, although mostly fairly turbid, acicular zircon grains. Several conventional U–Pb TIMS analyses yielded discordant and slightly scattered data with apparent 207Pb/206Pb ages of 1.91–1.96 Ga (Fig. 84, Appendix 5). The common-lead content in these data is also fairly high. Instead, a more recent analysis using the chemical abrasion technique gave a concordant result at 1956 ± 3 Ma. Nevertheless, due to the slightly heterogenous population, it was still questionable whether this could be considered

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A1673 Puuropuro metadiabase (Fe-tholeiitic) data-point error ellipses are 2s

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Fig. 83. Concordia plot of U–Pb zircon data obtained from the Puuropuro dyke A1673. LA-MC-ICPMS analyses are presented as error ellipsoids and an ID-TIMS analysis as a red dot.

as an unambiguous igneous age. Therefore, zir-con from sample A1519 was also hit by laser and analysed by MC-ICP-MS. Rejecting a few analyses with high common lead, an age of 1962 ± 12 Ma (n = 23) can be calculated. Using only the spots (n = 7) on domains of apparently best preservation, the age becomes 1973 ± 13 Ma (Fig. 84, Appendix 10). This may be considered as the igneous age of this dyke. The Sm–Nd analysis of whole rock yielded an εNdvalue of -0.7 (Appendix 1).

The Kivimäki dyke (A1460) is one of the several N–S-trending, mostly conspicuously foliated meta-diabase dykes that cut Archaean gneisses south of the Archaean Tipasjärvi greenstone belt. Towards the south and southeast from Kivimäki, the dykes become less foliated and gradually turn to the NW-SE direction. The rock in the sampled dyke is strongly foliated, and one reason for the dating was to constrain the age of the deformation, including the map-scale northwards (clockwise) rotation of the Kivimäki dykes. Zircon grains from the sample are mostly turbid, appearing poor in their preserva-tion quality. Accordingly, most LA-MC-ICPMS data give ages of ca. 1.83 Ga, which may be considered an approximate age of the metamorphism and proba-

bly also foliation and rotation of the dyke. One of the studied grains appears more pristine. Two analyses of this grain yielded an age of 1954 ± 19 Ma (grain 10, Fig. 85, Appendix 10). It is tempting to consider this as an igneous age, but the data are too scarse for a solid conclusion. The three multigrain TIMS analyses conducted before the laser work are dis-cordant and consistent with the laser data (Fig. 85, Appendix 5). However, a closer inspection reveals an unusual pattern, with the analyses of heavier zircon giving slightly younger Pb–Pb ages and lower Th/U compared to the analysis of the 4.0–4.2 g/cm3 den-sity fraction, and also to the main ICP-MS data (the mount was very likely made on heavy zircon). This suggests that a lot of material must be older than 1.83 Ga, the age that was obtained for the dominant grain type by LA-ICP-MS. The low Th/U ratio of the heaviest zircon fraction is a characteristic of metamorphic zircon (e.g., Rubatto 2002).

Recently, another diabase dyke of this age group was recognised by Mikkola et al. (2013) at Hattuselkonen, in the northern Lieksa area (Fig. 74). They reported a U–Pb age of 1989 ± 9 Ma and an initial εNd of -0.7 for this diabase (A2071, Fig. 86).

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Fig. 84. Concordia plot of U–Pb zircon data obtained from the Peräaho dyke A1519. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.


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Fig. 85. Concordia plot of U–Pb zircon data obtained from the Kivimäki dyke A1460. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.

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0.42

5.5 5.7 5.9 6.1 6.3 6.5 6.7 6.9

206 P

b/23

8 U

207Pb/235U

A2071 Hattuselkonen diabase

Concordia Age = 1989 9 Ma(2s, decay-const. errs ignored)

n=10 (all)


Fig. 86. Concordia plot of U–Pb zircon data obtained by LA-MC-ICPMS from the Hattuselkonen dyke A2071 (Mikkola et al. 2013).

6.8 Dykes in the Veitsivaara area, A1489b & A1489c

Studies in the Veitsivaara area, a former candidate area for a nuclear waste repository, have given valu-able information on the trends and age relation-ships of mafic dykes in the Kuhmo block (material from Posiva Ltd). Two main swarms have been rec-ognised. The E–W-trending (280o) swarm is con-sidered older than the other principal dyke swarm, which has a trend to NW (320o). In the Veitsivaara area, both diabase dyke swarms contain rock varie-ties which still, to a variable extent, preserve their primary magmatic textures and major minerals plagioclase and clinopyroxene. Fresh samples for isotopic studies were taken from drill cores, with sample A1489b representing the NW–SE-trending dykes and sample A1489c the E–W-trending dykes.

The four analyses conducted on whole rock and primary minerals from sample A1489b gave an age of 2005 ± 40 Ma (εNd = +0.3, MSWD = 0.5). The three Sm–Nd results from the other sample A1489c yielded an age of 2054 ± 40 Ma (εNd = +0.3, MSWD =1, Fig. 87, Appendix 1). Considering the error limits, the Sm–Nd ages derived for the two dyke swarms at Veitsivaara overlap. Neither can the swarms be distinguished based on their initial Nd isotope compositions.

In a broader view, the isotope data available from the Kuhmo block, however, show some consistent

age grouping with the dyke orientation. Namely, two E–W-trending (280o) dykes have yielded Sm–Nd mineral ages of 2133 ± 29 Ma (A1212 Kapea-aho, which is located 20 km east from Veitsivaara) and 2054 ± 40 Ma (A1489c Veitsivaara), whereas three NW–SE-trending dykes have given a U–Pb age of 1981 ± 4 Ma (Kivikevätti) and Sm–Nd ages of 2014 ± 33 Ma (A1409 Kivikevätti) and 2005 ± 40 Ma (A1489b Veitsivaara). A slight difference is also apparent in the chemical compositions, as the NW–SE-trending dykes seem to have higher REE concentrations (Appendix 1).

Provided that the two E–W-trending dykes (A1212 and A1489c) are coeval with an identical intial Nd isotope ratio, one can calculate a regression line using eight analyses. This gives an age of 2106 ± 40 Ma (εNd = +0.6, MSWD = 1.9, Fig. 88). Similarly, the seven analyses available from the NW–SE-trending dykes yield an age of 2010 ± 26 Ma (εNd = +0.3, MSWD = 0.6). The small MSWD is well consistent with a common origin for these geographically dis-tant rocks. Instead, if all 15 analyses from the two dyke groups discussed here are regressed together, the MSWD rises to 5, which probably means that the samples are unrelated.

Fig. 87. Sm–Nd isotope data for whole rock and mineral separates from two Veitsivaara dykes.

8989


A1489c

A1489cpx

A1489cplag

A1489b

A1489bpx

A1489bplagA1489bplag#2

0.5114

0.5116

0.5118

0.5120

0.5122

0.5124

0.5126

0.5128

0.10 0.12 0.14 0.16 0.18 0.20 0.22

143 N

d/14

4 Nd

147Sm/144Nd

E-trending dykeA1489c

Age = 2054 40 Maeps = +0.3

MSWD = 1.0 n=3

Veitsivaara

NW-trending dykeA1489b

Age = 2005 40 Maeps = +0.3

MSWD = 0.5 n=4


6.9 Dykes in the Romuvaara area

In the Romuvaara area, both boninitic and Fe-tholeiitic dykes have been recognised. The Sm–Nd data on four whole-rock samples from the Fe-tholeiitic dykes are scattered and do not provide any ages. However, the data are roughly consistent with the data acquired for the 2.1 Ga dykes, yielding εNd(2100 Ma) values from -0.5 to +0.8. The eight analyses conducted on four boninitic whole-rock

samples instead gave a rather strange pattern, sug-gesting an age of ca. 1.0 Ga (Fig. 88). Two of the samples have a Sm/Nd ratio fairly typical for 2.44 Ga rocks, whereas the two other samples have a much higher Sm/Nd ratio, suggesting hydrother-mal alteration and associated depletion of LREE at ca. 1.0 Ga.

7 KAINUU SCHIST BELT


The Palaeoproterozoic Kainuu schist belt (KSB) forms a N–S-trending, folded and faulted inlier between the Archaean Iisalmi–Pudasjärvi and Lentua complexes (Fig. 89, Laajoki 2005). Tapering to the north and south, the belt is about 220 km long and has a maximum width of 40 km in its middle part. It was deformed and metamorphosed during the Svecofennian orogenic tectonism, with the conditions reaching upper greenschist facies in the north and amphibolite facies in the south.

The KSB is dominantly metasedimentary in composition. The lowermost sedimentary rocks along with the contacts with the Lentua and Iisalmi complexes lie unconformably on the mig-matitic-plutonic gneisses of the named blocks. The Archaean-Palaeoproterozoic contact, where not faulted, is generally defined by Jatulian (2.30–2.10 Ga) quartz-pebbly arenites sitting on the Archaean gneisses. Remnants of older Sumi-Sariolan (2.50–2.30 Ga) volcanic and metasedimentary rocks are

9090

A1212 & #2

A1212px A1212px#2

A1212plag

A1489c

A1489c_px

A1489cplag

A1409

A1409px

A1409plag#2 A1489b

A1489bpx

A1489bplag A1489bplag#2

RO-KR3

KR-25 189-SSP

RO-KR-5

1.1-ROM

2-ROM & #2 & #3

3-ROM & #2

4-ROM & #2

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.09 0.11 0.13 0.15 0.17 0.19 0.21 0.23

143 N

d/14

4 Nd

147Sm/144Nd

Mafic dykes in Kuhmo

E-trending dykes Age = 2106 ± 40 Ma

eps = +0.6 MSWD = 1.9 n=8

NW-trending dykes Age = 2010 ± 26 Ma

eps = +0.3 MSWD = 0.58 n=7

Romuvaara boninitic dykes Age = 914 ± 200 Ma

eps = -12 MSWD = 5.3 2.44 Ga rocks

Fig. 88. Sm–Nd isotope data for whole-rock samples from Romuvaara compared to results from other dykes in the Kuhmo area. Green triangle – Fe-tholeiitic dyke, red dot – boninitic dyke.

locally met, but only in any significant volume in the north. These volcanic rocks of the Kurkikylä Group are chemically similar to and probably cor-relate with the Kuntijärvi Formation metabasalts in the Kuusamo schist belt (Fig. 39). The quartz-pebble conglomerates and quartzites of the immediately overlying Korvuanjoki Group have been correlated with the Vesivaara and Koli Formations in the North Karelia schist belt (Kohonen & Marmo 1992).

At several locations, the Jatulian sequence attains a thickness of more than 2 km (up to 3.5 km). As in the prototypical Eastern Puolanka Group in the northern part of the belt (Laajoki 1991), it comprises mainly shallow-marine platformal feldspathic and quartz arenites, with the latter commonly having a supermature, extremely quartz-rich character (Kontinen 1986; Laajoki 1991). The upper part in the sequence has been proposed (Nironen et al. 2016) to correlate, for instance, with the Rukatunturi Formation (Silvennoinen 1972) and Puso Formation (Kohonen & Marmo 1992) in the Kuusamo and North Karelia schist belts, respectively. The Jatuli sequence is topped by dolomite intercalated with mafic volcanic rocks. These rocks probably corre-late with ca. 2.10 Ga upper Jatulian dolomite–vol-canite sequences elsewhere, such as the Tikanmaa Formation (upper part of the Kivalo Group) in the Peräpohja area (Fig. 60) (Perttunen & Vaasjoki 2001, Kyläkoski et al. 2012).

For a length of about 100 km, the western mar-gin of the KSB against the Pudasjärvi complex is defined by a mostly narrow (<1–5 km), folded–folded stripe of the dominantly metasedimen-tary Central Puolanka Group (CPG). The Central Puolanka Group is a tripartite sequence of inter-bedded feldspathic wackes–aluminous pelites (Puolankajärvi Formation), fluvial-shallow marine feldspathic arenites (Akanvaara Formation), and shallow marine interbedded quartzite–pelite with mafic and intermediate-felsic tuff–tuffite interca-lations (Pärekangas Formation). The age of deposi-tion of the Central Puolanka Group has so far only tentatively been defined as Archaean at the latest or Palaeoproterozoic at the earliest (Laajoki 2005, Kontinen et al. 2014). An age of >2.20 Ga is obvious, as over its whole extent the sequence is intruded by differentiated sills of the 2.22 Ga gabbro-wehrlite (karjalite) event. The presence of 2.22 Ga sills within the CPG means that this unit was buried at that time less than 4–5 km below the surface. The 2.22 Ga sills are, all over the KSB, also abundant in the Jatuli strata and their immediately underlying Archaean

basement. Extensive detrital zircon dating for the Central Puolanka Group in the Oikarila area/struc-ture has shown that the metasediments, including assumed felsic tuffites of the CPG, exclusively con-tain Archaean detrital zircon grains, mostly aged ca. 2.72 Ga, but for a small part up to 3.7 Ga (Kontinen et al. 2014).

At Varisniemi, a basaltic metalava (Pitkälika metabasalt), probably correlating with the Central Puolanka Group, is intruded by the 2.44 Ga (Chapter 7.2 of this work) granophyre of the Juntti - lanniemi layered gabbro intrusion (Kontinen et al. 2014). This basalt (Pitkälika metabasalt) is overlain by a 50-m-thick conglomerate-wacke layer with lithic clasts and zircon grains from the granophyre, which in turn is topped by sev-eral hundreds of metres of metabasalt (Varisniemi metabasalt). The volcanic rocks are physically (dominantly richly amygdalous) and chemically similar to Sumi-Sariolan metabasalts in north-ern Kainuu (Kurkikylä Group), Posio (Karkuvaara Formation) and Kuusamo (Kuntijärvi Formation). These relationships suggest that the Oikarila structure contains both <2.44 Ga and >2.44 Ga supracrustal units. Assuming a correlation of the Pitkälika basalt with the CPG, Kontinen et al. (2014) proposed an age of >2.44 Ga for the Central Puolanka Group.

According Laajoki (2005), in the Puolanka area, the Central Puolanka Group schists grade at the western margin of the KSB to gneisses of the Kalpio Complex, with the latter making up the SE corner of the Pudasjärvi complex. However, in the Kivesvaara area, some 40 km to the south, there is clearly a fault between these units, as there is an abrupt change in the metamorphic grade from upper greenschist facies (garnet-biotite) in the CPG schists to middle amphibolite facies (garnet-staurolite-sillimanite) in the Kalpio Complex gneisses just at their con-tact. Also, there are abundant late Palaeoproterozoic (1.86–1.80 Ga, Vaasjoki et al. 2001) granites in the Kalpio Complex gneisses immediately on the west-ern side of the contact fault, but none in the Central Puolanka Group schists on its eastern side. This stark contrast introduced by post-1800 Ma tec-tonism/faulting characterises the entire length of the Pudasjärvi complex (Kalpio-Kalhamajärvi)–KSB contact.

The middle parts of the KSB are occupied by Kalevian (2100–1900 Ma) deep-water gravity flow metasediments. A two-part succession has been distinguished (Kontinen 1987, Laajoki 2005,

9191



9292

A1838Jäkäläkangas

Otanmäki

Jormua

A1596Varisniemi

A0198Mustikkarinne

A1457Liminpuro

A1362Petäiskangas

A0769Niskansuo

A0759Kettukallio

A0988 Nyrhinoja

A1456 Karkuvaara

Kurkikylä

A1373 Paha Kapustasuo

A1595Parvialankangas

A1673Puuropuro

A1363Petronjärvi

A0496Jokijyrkkä

A0199Raatelampi

A0651Honkaniemi

A0613Hukkavaara

A0437Latolan- vaara

PUDASJÄRVI COMPLEX

MANAMANSALO COMPLEX

KUHMOCOMPLEX

IISALMICOMPLEX

10 km7070000

35

00

00

0

35

70

00

0

7220000

Otanmäki suite, A-typegneissic granitoids (2.05 Ga)

Otanmäki gabbros (2.06 Ga)

Sumi-Sariola

Late Svecofennian granitoids (1.86-1.80 Ga)

Kaarakkala gabbro-diorite (1.86 Ga)

Upper Kaleva

Central Puolanka Group

Marine Jatuli

Archaean complexes

Jormua Ophiolite Complex(magmatic units 1.95 Ga)

Junttilanniemi intrusion(2.44 Ga)

2.43 Ga gneissic A-typegranitoids

Lower Kaleva

Kalpio & KalhamajärviComplexes

Jatuli

Kapustakangas suite,ultramafic-mafic intrusions(2.3 Ga?)

Fig. 89. Geological map of the Kainuu schist belt with the sample locations.

Kontinen & Hanski 2015). The Lower Kaleva records a lithologically heterogeneous package with geo-graphical variation in its internal stratigraphy. It consists of mass-flow conglomerates, interbedded metagreywackes, metepelites, and quartz-rich metawackes with iron formations and sulphide-graphite-rich metashales in its upper part. The provenance of the detrital material was in the Archaean basement and its pre-Kalevian sedimen-tary cover (Kontinen & Hanski 2015).

The Upper Kaleva, in turn, is a relatively monoto-nous unit mainly consisting of turbidite-type inter-bedded metagreywacke and pelite with infrequent black metashale interbeds. Additionally, ophi-olitic fragments are locally found as fault-bound lenses, of which the largest, in the middle of the KSB, belong to the Jormua ophiolite (Kontinen 1987, Peltonen 2005a). In marked contrast to the lower Kaleva and older Palaeoproterozoic units, which mostly only contain Archaean detrital zircon grains, Upper Kaleva metasediments contain a large Palaeoproterozoic detrital component, as revealed by zircon grain populations having ages in the range of 2.00–1.92 Ga (Lahtinen et al. 2010). It is noteworthy that the 1.95 Ga magmatic age of the gabbros and plagiogranites in the ophiolite fragments (Kontinen 1987, Peltonen et al. 1998) is by a significant margin older than the ca. 1920 Ma maximum age indicated by detrital zircon data for the Upper Kaleva depo-sition. The Upper Kaleva, with its exotic ophiolite fragments, is interpreted as an allochthonous unit thrust on the Archaean-Karelian basement-cover structure at ca. 1900 Ma (Peltonen et al. 2008).

Other important rock units within the KSB include the ca. 2060–2050 Ma gabbros and A-type

granites of the Otanmäki suite (Talvitie & Paarma 1980, Kontinen et al. 2013), restricted in the area south of Lake Oulunjärvi in a 1- to 6-km-wide, 60-km-long, E–W-trending fault-bound sliver (Fig. 89). These granites locally contain Jatuli-type rocks (orthoquartzite-dolomite) as inclusions. In the Ristijärvi area, two roundish (<10 km) grano-diorite–granite plutons (Fig. 89) dated at ca. 1860 Ma (GTK, unpublished data) are emplaced in Upper Kaleva metasediments.

In Finland, the ca. 2.44 Ga mafic-ultramafic layered intrusions are concentrated in the Tornio-Näränkävaara belt supplemented with a few occur-rences further to the north in Lapland (Alapieti et al. 1990). Recently, petrological evidence for the pres-ence of intrusive magmatism of that age has been found in the Kainuu schist belt at Junttilanniemi, near Paltamo (Halkoaho & Niskanen 2013), repre-senting the southernmost discovery of these intru-sions in Finland. In this work, the granophyre of the Junttilanniemi layered gabbro was dated to confirm the age of the intrusion, but also to place constraints on the age of the closely associated pre-Jatulian metavolcanic rocks. We have made an attempt to constrain the depositional age of Central Puolanka Group by dating a sill of the Kapustakangas intru-sive suite intruding the Puolankajärvi Formation. The dated mafic sills also include those cutting the Archaean basement and its quartzitic cover at the eastern contact of the KSB and those intruding the Kalpio Complex on the western side of the KSB. A couple of samples from the Jormua ophiolite pro-viding updated results are also included.

7.2 The 2.44 Ga Junttilanniemi plutonic-volcanic complex (A1595-6)

The Junttilanniemi layered intrusion is found at the western margin of the Kainuu schist belt in a stratigraphically and structurally complex setting including both Archaean and Palaeoproterozoic rocks (Fig. 74, Fig. 89). According to Kontinen et al. (2014), the Junttilanniemi gabbro intruded the Pitkälika metavolcanic rocks, which possibly cor-relate with the basalts of the Pärekangas Formation, the uppermost unit of the Central Puolanka Group (cf. Laajoki 1991). The Pitkälika metavolcanic rocks and the apparent roof granophyre of the Junttilanniemi gabbro are unconformably over-lain by a thin veneer of conglomerate succeeded by metabasalts of the Varisniemi Formation. The

Soidinsuu conglomerate contains pebbles and gran-ules of granophyre, obviously from the underlying Junttilanniemi granophyre.

Two geochemically similar felsic granophyre samples (A1595 Parvialankangas granophyre, A1596 Varisniemi “rhyodacite”) were used for age deter-mination. A small amount of mostly light-coloured, turbid zircon was obtained from both samples. The mineral separate from sample A1596 also contains a few brown, translucent crystals. The results of multigrain TIMS analyses are discordant, with 207Pb/206Pb ages of ca. 2.35–2.38 Ga (sample A1595) and ca. 2.08 Ga (A1596, Fig. 90, Appendix 5).

9393



In search for more concordant data, zircon grains from sample A1596 were analysed by SHRIMP at VSEGEI in St. Petersburg. Despite the high U con-tent in zircon of sample A1596, the SIMS results for brown, translucent crystals (image in Appendix 4b) are concordant, good in quality and provide an age of 2444 ± 4 Ma (Fig. 90, Appendix 4b). Instead, an analysis of a turbid, dark grain (A1596.7.1, marked in CL-light grain without label) revealed a high abundance of common lead and gave an imprecise date of ca. 1.7 Ga. The discordant TIMS data are consistent with the SHRIMP results. The Sm–Nd analyses conducted on these two samples give simi-lar negative initial εNd(2444 Ma) values of -1.8 and -1.9, which are within the range of εNd values typical for the 2.44 Ga intrusions (Appendix 1).

Sample A2050, picked ca. 400 metres west of the granophyre sample A1596, represents the massive-schistose, pink-grey weathering feldspathic wacke that dominates the Soidensuu conglomerate-wacke member at the base of the dominantly basaltic to

andesibasaltic Varisniemi Formation. In thin sec-tion, the sample resembles arkosic wacke up to ca. 1.2 mm in its grain size. Some of the larger clasts are clearly granophyre, similar to that in the nearby 2.44 Ga Junttilanniemi differentiated intru-sion. There are a few, up to 60-μm-sized zircon grains visible in the thin section, most of them dull, brownish prisms.

Fifty U–Pb analyses on zircon performed using LA-MC-ICPMS provided a range of mostly Archaean ages (Appendix 11, Fig. 91A). However, a few grains constrain the depositional time as early Palaeoproterozoic. Sm–Nd analysis on whole rock gave a T-DM model age of 2.92 Ga (Appendix 1).

Sample A2105 represents the trondhjemitic xeno-liths that are found in abundance in the Pitkälika Formation meta-dacite, which is cut by the grano-phyre of the Junttilanniemi intrusion. All zircon grains analysed from this sample were found to be Archaean, mostly between 2.6 and 2.8 Ga in age (Appendix 11, Fig. 91B).

9494

1600

1800

2000

2200

2400

2600

A1596A

A1595A

A1595B Parvialankangas granophyre

A1373-n652 Paha KapustasuoNORDSIM (green error ell)

A1373A & B Paha Kapustasuo

0.26

0.30

0.34

0.38

0.42

0.46

0.50

2 4 6 8 10 12

206 P

b/23

8 U

207Pb/235U

Junttilanniemi granophyredata-point error ellipses are 2s

A1596 Varisniemi rhyodaciteSHRIMP Concordia Age =

2444 4 Ma, n=6

Fig. 90. Concordia plot of U–Pb zircon data obtained from the Junttilanniemi layered intrusion (A1595, A1596) and Kapustakangas suite gabbro A1373. SIMS analyses presented as error ellipsoids and ID-TIMS analyses as dots.

9595


0

2

4

6

8

10

12

14

2300 2400 2500 2600 2700 2800 2900 3000 3100 3200

Relative probabilityNu

mbe

r

Pb/Pb age

A2050 Soidensuu arkosic wacke

A)

0

1

2

3

4

5

6

7

2550 2650 2750 2850 2950 3050

Relative probability

Num

ber

Pb/Pb Age

A2105 (165-AVL-89) Pitkälika trondhjemite xenolith

B)

Fig. 91. Distribution of 207Pb/206Pb ages of concordant U–Pb data for A) Soidensuu arkosic wacke (A2050) and B) Pitkälika trondhjemite xenoliths (A2105).


7.3 The Kapustakangas intrusive suite (A1373)

The Kapustakangas suite refers to mafic-ultra-mafic intrusions within the Central Puolanka Group flanking the western margin of the Kainuu schist belt (Laajoki 1991). In an attempt to date these intrusions, sample A1373 was collected from a coarse-grained pegmatoid segregation in the upper gabbroic part of a gabbro-peridotite sill at Paha Kapustasuo (Fig. 89). The lower peridotite part of the intrusion is now serpentinite, and an intrusive contact of the upper metagabbro against staurolite schist of the Central Puolanka Group can be seen at the sampling site. The effects of pervasive amphibolite facies metamorphism and subsequent retrograde alteration of the Paha Kapustasuo rocks are also reflected in the zircon grains, which are reddish and turbid. The two multigrain analyses by TIMS yielded discordant results, with Pb/Pb ages of ca. 1.95 Ga (Fig. 92, Appendix 5).

Subsequent to the TIMS analyses, zircon grains from sample A1373 were analysed using NORDSIM in Stockholm (n652_1999, Fig. 92, Appendix 4a). The SIMS U–Pb data on nine zircon grains are scat-tered and discordant. Two analyses on the appar-ently most pristine, slightly translucent domains

yielded 207Pb/206Pb ages of ca. 2.33–2.36 Ga, whereas

the data obtained from turbid zircon plot on a loose-fitting chord with an upper intercept age of ca. 1.8 Ga. The Th/U ratio in the older grains is rela-tively high, as is often typical for igneous zircon of mafic rocks. In contrast, in the 1.8 Ga zircons, the Th/U ratio is low, as it is in many cases for meta-morphic zircon grains.

Recently, zircon from sample A1373 was also analysed using laser ablation MC-ICP-MS. The fourteen data points analysed were all from turbid zircon grains. Seven of them were concordant at 1794 ± 15 Ma. Six analyses yielded Pb–Pb ages of ca. 1.86–1.90 Ga and one spot on a distinctly more pristine domain had a Pb–Pb age of ca. 2.36 Ga (Fig. 92, Appendix 11). The U–Pb results suggest that zircon in A1373 was badly altered by high-temperature fluid activity at ca. 1.8 Ga, which was probably related to the extensive late granite vein-ing west of the Kainuu schist belt approximately at the same time. The three analyses on the apparently best-preserved zircon domains suggested that the emplacement age of the Kapustakangas intrusions was at least 2.36 Ga. Of particular note is that no

9696

1300

1500

1700

1900

2100

2300

2500

0.15

0.25

0.35

0.45

0.55

2 4 6 8 10 12

206 P

b/23

8 U

207Pb/235U

A1373 Paha Kapustasuo metagabbro data-point error ellipses are 2s


(grain A1373-12a)

Concordia Age = 1794 ±15 Ma n=7 youngest

TIMS

SIMS

Fig. 92. Concordia plot of U–Pb zircon data obtained from the Kapustakangas suite gabbro A1373. SIMS analyses are presented as blue triangles (also shown in Figure 90 as green error ellipsoids), ID-TIMS analyses as red dots and LA-MC-ICPMS data as error ellipsoids.

ages close to 2.2 Ga, which would suggest a linkage to the 2.22 Ga gabbro-wehrlite (karjalite) sills, were obtained. Sm–Nd analysis indicated that sample

A1373 is strongly enriched in LREE and yielded an εNd value of -0.6 at 2.4 Ga.

7.4 The 2.22 Ga intrusions in the Kainuu schist belt

In the past years, many samples have been taken from mafic sills and dykes in the Kainuu schist belt, but magmatic ages have often remained unresolved because of problems with metamorphic overprint-ing and typically a high degree of discordance of zircon. Recent analyses by LA-MC-ICPMS have confirmed that several of these dyke samples come from differentiated intrusions of the ca. 2.22 Ga gabbro-wehrlite association (GWA), which are characteristic of Karelian schist belts throughout Finland (Hanski et al. 2010).

Two of these 2.22 Ga intrusions, Mustikkarinne (A198) and Raatelampi (A199), are located in the southern part of the Kainuu schist belt. The sam-ples were collected by Matti Havola in the 1980s. Sample A198 Mustikkarinne is from a major mafic dyke east of the central part of the Kainuu schist belt in the Sotkamo area. Owing to its high mag-netite content, the Mustikkarinne dyke is traceable on geophysical maps as a distinct aeromagnetic anomaly. Based on outcrop and magnetic maps, the

dyke forms a ca. 10-km-long, mosty 200-m-wide intrusion within the Archaean rocks and seems to turn into a sill within the Jatulian quartzites when entering the eastern boundary of the Kainuu schist belt (Fig. 89). Sample A198 is from a medium-grained, only slightly deformed metadiabase, with actinolitic hornblende and sodic plagioclase as its main constituents.

Most zircon grains in sample A198 are turbid and appear to be strongly altered. The recently acquired LA-MC-ICPMS data on better preserved domains give concordant U–Pb results, which suggest an age of 2210 ± 10 Ma (Fig. 93, Appendix 11). Some of the analysed zircon spots have high U (>1000 ppm) and tend to give younger Pb/Pb ages. The three multigrain TIMS analyses performed before the laser experiment yielded discordant results and were consistent with the interpretation based on the LA-MC-ICPMS data (Fig. 93, Appendix 5).

Sample A199 Raatelampi is from a leucogabbroic part of a mafic sill intruding the interface of narrow

9797


1800

1900

2000

2100

2200

2300

2400

0.26

0.30

0.34

0.38

0.42

0.46

0.50

4.5 5.5 6.5 7.5 8.5 9.5

206 P

b/23

8 U

207Pb/235U

A198 Mustikkarinne metadiabase

Intercepts at 992 250 & 2192 19 MaMSWD = 0.43 n=15 (all)


Concordia Age = 2210 10 MaMSWD=2 (n=12/15)

TIMS

Fig. 93. Concordia plot of U–Pb zircon data obtained from the Mustikkarinne diabase A198. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.


stripes of Archaean gneisses and Jatulian quartzites in a fault-bound (thrust) sliver within the Kainuu schist belt, ca. 5 km to the north of the Talvivaara mine (Fig. 89). In contrast to the grains in sample A198, most zircon grains in sample A199 are fairly pristine. The LA-MC-ICPMS data on good zircon domains yield a concordant age of 2222 ± 7 Ma (Fig. 94, Appendix 11). Analyses on altered domains yielded much younger apparent ages and explain the previous discordant multigrain TIMS results. The initial εNd(2220 Ma) values for these dykes are close to zero (A198 +0.7, A199 -1.0, Appendix 1), which is typical for rocks of this family (Hanski et al. 2010).

Three of the samples with an age of ca. 2.2 Ga, A769 Niskansuo, A 759 Kettukallio and A651 Honkaniemi, are from mafic intrusions in the mostly metasedimentary schists and gneisses of the Prejatulian Central Puolanka Group and the Kalpio Complex, its presumed lithodemic correlative. According to Laajoki (1991), the Niskansuo metadi-abase (A769) has intruded into the contact between the Pärekangas and Mäntykangas Formations, i.e. between metasediments of the Central Puolanka Group and Jatulian Vihajärvi Group (Fig. 89). Zircon grains obtained from sample A769 show good pres-

ervation and the U–Pb data obtained by LA-MC-ICPMS provide concordant results with an age of 2214 ± 7 Ma (Fig. 95, Appendix 11). Three earlier discordant multigrain TIMS analyses have yielded an upper intercept age of 2189 ± 8 Ma.

Sample A759 Kettukallio is from a mafic dyke in the Kettukallio quartzites of the Kalpio complex (Laajoki 1991), collected close to the sediment con-tact. Most zircon grains in this sample are turbid and appear altered, but there are also more pristine-looking grains. The LA-MC-ICPMS data on these grains are concordant and give an age of 2202 ± 5 Ma (n = 26, Fig. 96, Appendix 11). The results of analyses on altered domains (10b) are discordant with younger Pb/Pb ages, explaining well the scat-ter in the previously obtained TIMS data. A TIMS analysis on monazite yielded a concordant age of 1792 ± 3 Ma, recording the timing of metamor-phism, implying growth or blocking of the monazite during the 1800 Ma “Kajaani event” that produced voluminous Himalaya-type leucogranite-pegmatite granite west of the KSB. The initial εNd value of the whole-rock powder of this sample is -0.4 (Appendix 1), typical for the 2.22 Ga GWA intrusions (Hanski et al. 2010).

9898

1600

1800

2000

2200

2400

0.24

0.28

0.32

0.36

0.40

0.44

0.48

3 5 7 9

206 P

b/23

8 U

207Pb/235U

A199 Raatelampi metadiabasedata-point error ellipses are 2s

Concordia Age = 2222 7 MaMSWD= 0.3 (n=23/25)

TIMS

Fig. 94. Concordia plot of U–Pb zircon data obtained from the Raatelampi mafic sill (sample A199). LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.

9999


2000

2100

2200

2300

0.32

0.36

0.40

0.44

0.48

5.5 6.5 7.5 8.5 9.5

206 P

b/23

8 U

207Pb/235U

A769 Niskansuo metadiabasedata-point error ellipses are 2s

Concordia Age = 2214 7 Man=11/13


n=3 (2 outside figure)

1600

1800

2000

2200

2400

0.22

0.26

0.30

0.34

0.38

0.42

0.46

0.50

3 5 7 9

206 P

b/23

8 U

207Pb/235U

Kettukallio data-point error ellipses are 2s

A759 Kettukallio metadiabaseLA- MC-ICPMSConcordia Age

2202 5 Ma (n=26)

TIMS A759 Monazite 1792 3 MaA760 Titanite 1784 5 Ma

TIMS A759

A760 zr TIMS

Fig. 95. Concordia plot of U–Pb zircon data obtained from the Niskansuo diabase A769. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.

Fig. 96. Concordia plot of U–Pb data on zircon, monazite and titanite obtained from the Kettukallio diabase A759 and amphibolite A760. LA-MC-ICPMS analyses on zircon are presented as error ellipsoids and ID-TIMS analyses as dots.


A sample (A760) from a banded amphibolite interlayer within the Kettukallio quartzite was also collected for isotope studies. This sample has only been studied for multigrain fractions by TIMS. Both of the performed two TIMS analyses yielded discordant results with Pb–Pb ages of ca. 2.5 Ga (Laajoki 1991, Fig. 96, Appendix 5). Titanite from the amphibolite gave a concordant TIMS U–Pb age of 1784 ± 5 Ma, providing further support for the age of metamorphism as discussed above. Ages of ca. 1.8 Ga have previously been obtained for mona-zites from the Kalpio complex and also from the correlative Kalhamajärvi complex further north (Vaasjoki et al. 2001). Sm–Nd analysis indicated that the Kettukallio amphibolite is strongly enriched in LREE and provided a TDM model age of 2.85 Ga (Appendix 1), indicating an Archaean age or a large component of inherited Archaean material.

Sample A651-Honkaniemi (Vihajärventie) is from a layered mafic–ultramafic intrusion found within staurolite mica schists of the Puolankajärvi Formation, the lowermost main unit of the Central Puolanka Group (Laajoki 1991, Fig. 89). It was col-lected from the upper gabbroic part of the intrusion ca. 3 km west of the Niskansuo (A769) site discussed above. The extracted zircon grains appear mostly

poor in their preservation. Three TIMS analyses on multigrain zircon fractions were performed, resulting in scattered data with Pb–Pb ages of ca. 2.1–2.2 Ga. The data subsequently obtained by LA-MC-ICPMS are also strongly scattered (Fig. 97, Appendix 11). Analyses of the domains that appear best preserved have tended to give ages approach-ing 2.2 Ga. Some technically good data give clearly younger ages, obviously due to metamorphic effects (15a, 21a), which can be expected based on the con-cordant TIMS age of ca. 1.84 obtained for titanite. Part of the analyses produced discordant data with Pb–Pb ages exceeding 2.2 Ga. For these analyses, the relatively low 206Pb/204Pb ratios imply possible problems with common-lead correction. According to Laajoki (1991), the sampled intrusion is part of the Kapustakangas igneous suite, as is the Paha Kapustasuo sample A1373 discussed above, for which an age exceeding 2.36 Ga could be specu-lated. The data available for sample A651 do not allow strict conclusions on the magmatic age of the Honkaniemi intrusion, although an age of ca. 2.2 Ga could be suggested as the most probable one. Sm–Nd analysis on sample A651 gave an εNd(2200 Ma) value of -2.7, which is much lower than those measured for typical GWA intrusions.

100100

1600

1800

2000

2200

2400

0.24

0.28

0.32

0.36

0.40

0.44

0.48

0.52

3 5 7 9 11207Pb/235U

A651 Vihajärventie gabbro

206Pb238U


LA-MCICPMS data with 206/204Pb > 4000 (n=21/33)

TIMS A651D titaniteConcordia Age = 1838 13 Ma

"7 best preserved domains"Pb/Pb ages ca. 2.16-2.19 Ga

206Pb/204Pb<4000

TIMS

Fig. 97. Concordia plot of U–Pb zircon and titanite data obtained from the Honkaniemi gabbro A651. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as dots.

Over the past decades, several other samples from the Puolanka area have been collected for dat-ing purposes. Three of these, A496 Jokijyrkkä, A613 Hukkavaara and A437 Latolanvaara, are from three separate mafic intrusions, all intruding Jatulian quartzites of the East Puolanka Group, which is

the dominant rock unit in the eastern part of the Kainuu schist belt (Laajoki 1991). Only discordant multigrain TIMS data on zircon (and baddeleyite) are available, but emplacement ages close to 2.2 Ga seem most probable for the three sampled intru-sions (Fig. 98, Appendix 5).

7.5 The 1.95 Ga Jormua ophiolite

Kontinen (1987) published an age of ca. 1.95–1.96 Ga for the rocks of the gabbro-Fe-gabbro-plagiogran-ite suite located in the upper part of the Jormua ophiolite. This age estimate was based on the U–Pb zircon data produced by O. Kouvo on trondhjemite (A196) and gabbro samples (A729). Given the some-what discordant and heterogeneous data on which this age estimate was based, we re-analysed zircon from these two samples using the CA-TIMS method by Mattinson (2005). The chemically abraded zir-con fractions yielded concordant data with ages of 1950 ± 3 Ma for the trondhjemite sample A196 and 1952 ± 2 Ma for the gabbro sample A729 (Fig. 99, Appendix 5). These results are identical to the pre-cise TIMS age of 1953 ± 3 Ma previously obtained

for a Jormua gabbro pegmatoid (A1402; Peltonen et al. 1998).

Peltonen et al. (2003) published U–Pb SIMS anal-yses conducted on zircon from two clinopyroxene-rich dyke samples, A1528 and JCX-23B, from the central (Lehmivaara) and western (Hannusranta) mantle blocks of the Jormua ophiolite, respectively. They regarded sample A1528 as belonging to their “OIB dykes” and interpreted sample JCX-23B and other similar dykes in the Hannusranta block as coeval-cogenetic with the cumulate dykes. In addi-tion to zircon grains with ages of ca. 2 Ga, several xenocrystic Archaean zircon grains were discovered in both samples. We note here that a previously unpublished multigrain TIMS result from sample

101101


400

800

1200

1600

2000

0.0

0.1

0.2

0.3

0.4

0 2 4 6 8

206 P

b/23

8 U

207Pb/235U

Gabbroic rocks within Jatulian quartzites

A496 JokijyrkkäIntercepts at

269 150 & 2192 37 MaMSWD = 47 n=6(/8)

A496 Jokijyrkkä (red circle)A437 Latolanvaara (green square)A613 Hukkavaara (blue triangle)

2200

Fig. 98. Concordia plot of TIMS U–Pb zircon data obtained from four mafic samples from the Kainuu schist belt.


102102

18501870

18901910

19301950

19701990

A196A +4.5

A196B 4.45-4.55A196C 4.3-4.55

A196D 4.2-4.3

A196E 4.0-4.2 HFA196F 4.0-4.2 CRU

A196G 4.3-4.5 +200 CA

0.31

0.32

0.33

0.34

0.35

0.36

0.37

5.1 5.3 5.5 5.7 5.9 6.1207Pb/235U

206Pb238U

Intercepts at -33 230 & 1948.9 6.9 Ma

MSWD = 5.1 n=7


A196 Jormua trondhjemiteCA-TIMS analysis A196G


Kontinen 1987: 1954 11 Ma

A)

1870

1890

1910

1930

1950

1970

1990

A729A GABBRO +4.2 Jormua

A729B 4.0-4.2

A729C +4.3

A729D 4.2-4.3

A729E 4.2-4.4 +125 CA

0.315

0.325

0.335

0.345

0.355

0.365

5.2 5.4 5.6 5.8 6.0

206 P

b/23

8 U

207Pb/235U


MSWD = 0.5, n=5


A729 Jormua gabbroCA-TIMS analysis E


Kontinen 1987: 1960 12 Ma

B)

Fig. 99. A) Concordia plot of U–Pb TIMS data on zircon obtained from the Jormua trondhjemite A196 (data from Kontinen 1987 and this study). B) Concordia plot of U–Pb TIMS data on zircon obtained from the Jormua gabbro A729 (data from Kontinen 1987 and this study).

A1528 is nearly concordant and shows a Pb–Pb age of 2.01 Ga, which implies that zircon in this sample must be dominantly Palaeoproterozoic in age (Fig. 100, Appendix 5).

Zircon from two other samples of the Jormua central and western block dykes has also been ana-lysed (Appendix 4a). Sample A1529 picked from the central block represents a heavily altered (chlori-tised), apatite-bearing OIB dyke, which yielded a small quantity of zircon grains. Five concordant analyses by SIMS on these grains indicated ages from 1.96 to 2.02 Ga (Fig. 100, Appendix 4a). A TIMS analysis on a multigrain fraction was slightly dis-cordant, with a Pb–Pb age of 1974 Ma, which is consistent with the results of in situ analyses (Fig. 100, Appendix 5).

The other sample (60L-ATK) represents carbon-atitic veins occurring at the southern margin of the Hannusranta block of the Jormua ophiolite. It is to be noted that the sample was collected from a boul-der. However, there are several serpentinite boul-ders with carbonatitic veins in the sampled boulder field, attesting for a local source in the Hannusranta sliver. Also, a drill core at the site has intersected thin carbonatitic veins in serpentinite. Several of these carbonate-rich veins occurring in a very

Zr- and P-poor serpentinite (peridotite) host are rich in zircon and apatite, which favours a magmatic rather than metasomatic-hydrothermal origin. Two U–Pb analyses by SIMS on dark zircon cores yielded dates of ca. 2.08 Ga, but the other data on fairly clear homogeneous zircon are concordant at ca. 1.95 Ga. These clear zircon grains have very low concentra-tions of U and Pb, and consequently larger than typical analytical errors (Fig. 101, Appendix 4a). In contrast to Peltonen et al. (2003), we are inclined to accept these low-U zircon grains as primary igneous phases from the carbonatitic magma and interpret the older grains as xenocrystic.

The earlier Sm–Nd results from the Jormua ophiolite suggested that the E-MORB-type basaltic, gabbroic and plagiogranitic rocks have εNd(1950 Ma) values of ca. +2, whereas the at least somewhat older OIB-like dykes tend to give εNd(1950 Ma) values close to zero (Huhma 1986, Peltonen et al. 1996, 1998). The dykes discussed here also yield εNd values at 1.95 Ga close to zero, except the carbonatitic (boulder) sample 125-JCB, which gives εNd(1950 Ma) of +1.9 (Appendix 1). In addition to whole-rock powder, we have also ana-lysed apatite from this sample, which together with the whole-rock powder gave a Sm–Nd age

103103


1940

1980

2020

2060

2100

A1529A TIMS

A1528 TIMS

0.33

0.35

0.37

0.39

0.41

5.5 5.7 5.9 6.1 6.3 6.5 6.7 6.9

206 P

b/23

8 U

207Pb/235U

A1529 Jormua OIB dyke data-point error ellipses are 2s

A1529 OIB dyke Concordia Age = 1970 ± 8 Ma

n=4

Fig. 100. Concordia plot of U–Pb zircon data obtained from the Jormua OIB dyke A1529. SIMS analyses are pre-sented as error ellipsoids and an ID-TIMS analysis as a red dot. An ID-TIMS analysis on a clinopyroxenite dyke A1528 is also shown.


104104

1400

1600

1800

2000

2200

0.15

0.25

0.35

0.45

0.55

2.5 3.5 4.5 5.5 6.5 7.5 8.5 207Pb/235U

60L-ATK, Jormua carbonatitic vein

206Pb 238U


60L-ATK, carbonatitic vein Concordia Age = 1948 ±30 Ma

n=6

Concordia Age = 2084 ±10 Ma, n=2

Fig. 101. Concordia plot of U–Pb zircon SIMS data obtained from the Jormua carbonatitic vein.

97-JCX

97-JCX zirconolite

125-JCB 125-JCB apatite

OIB dykes

clinopyroxenite dyke carbonatitic vein 60L-ATK

0.511

0.513

0.515

0.517

0.05 0.15 0.25 0.35 0.45 0.55 0.65 147Sm/144Nd

143 N

d/14

4 Nd 97-JCX zirconolite-whole rock

Age = 1782±16 Ma

125-JCB carbonatite boulder apatite-whole rock Age = 1765±74 Ma

Jormua

Fig. 102. Sm–Nd isotope data for whole rock and mineral separates from two Jormua samples.

estimate of 1765 ± 74 Ma (Fig. 102). We have also analysed a zirconolite (CaZrTi2O7) concentrate from a clinopyroxenite-ilmenite-magnetite cumulate (97-JCX). This analysis indicated that the zircono-lite has a high concentration of REE and a strongly subchondritic LREE/HREE ratio. Sm–Nd analysis of zirconolite combined with whole-rock isotope data suggests an age of 1782 ± 16 Ma, which prob-

ably records the time of formation of zirconolite. This is consistent with the discordant results of a poor U–Pb analysis, giving a Pb–Pb age of 1751 ± 69 Ma (unpublished). Secondary REE mobility was also involved in the Jormua metabasalts, which have yielded a Sm–Nd isochron age of 1.72 ± 0.12 Ga (Peltonen et al. 1996).

7.6 Volcanic rocks

In addition to the Jormua metabasalts, Sm–Nd isotope data are available from two other mafic volcanic formations in the Kainuu schist belt, the Matinvaara and Varisniemi Formations. The Matinvaara Formation represents the lowermost Palaeoproterozoic volcanic unit on the Archaean basement in the northern part of the Kainuu schist belt in Kurkikylä (Fig. 89). The Varisniemi Formation in the central part of the Kainuu belt

is underlain by a conglomerate, which contains granophyre clasts from the 2.44 Ga Junttilanniemi intrusion. The volcanic rocks in both formations are enriched in LREE and have yielded clearly nega-tive εNd(2400 Ma) values and Archaean TDM ages (Appendix 6). These results compare well with the data obtained from the Kuntijärvi Formation, the lowest unit in the Kuusamo schist belt (Chapter 4.6).

8 IISALMI COMPLEX


The Archaean Iisalmi complex is located between the Kainuu schist belt in the NE and the Svecofennian domain in the WSW. It is separated from the Lentua complex on its eastern side by a Palaeoproterozoic SE–NW-trending shear zone. The rocks in the east-ern and western parts of the Iisalmi complex dif-fer significantly in their lithology and ages. In the east (Rautavaara complex of Hölttä et al. 2012), the rocks are dominated by TTG gneisses with vari-able amounts of amphibolite and biotite-plagioclase paragneiss enclaves with strong signs of chemical alteration, and the obtained ages fall in the range of 2.66–2.75 Ga. In the west (Iisalmi complex of Hölttä et al. 2012), the oldest rocks are Mesoarchaean 3.2–3.1 Ga migmatitic gneisses (Mänttäri & Hölttä 2002), which were intruded by 2.70 Ga quartz diorites. At ca. 2.68–2.62 Ga, the rocks were metamorphosed up to medium-pressure granulite facies conditions producing garnetiferous two-pyroxene mafic and intermediate granulites. Archaean granulites occur in an area of approximately 20 by 70 km in extent, having been studied in detail in the Varpaisjärvi area (Hölttä et al. 2000).

A large number of NW–SE-trending Palaeo-proterozoic diabase dykes cut Archaean rocks in the

Iisalmi complex and have yielded ages of ca. 2.3 and 2.1 Ga (Paavola 1988, Toivola et al. 1991, Hölttä et al. 2000). These rocks have been utilised to study the effects of the Palaeoproterozoic deformation and metamorphism on Archaean rocks in the area. In the areas retaining granulite-facies mineralogy, the dykes are mostly undeformed and unmetamor-phosed, whereas in the areas retrogressed during the ca. 1.9 Ga Svecofennian metamorphism, they are strongly deformed and display mineral assemblages of hydrated metadiabases (Paavola 1988, Toivola et al. 1991). The western areas of the Iisalmi com-plex also contain, in addition to felsic plutons, a few mafic intrusions related to the Svecofennian magmatism (Paavola 1988, Peltonen 2005b), and the southern areas, such as Kuopio and Siilinjärvi, contain small belts of Palaeoproterozoic supracrus-tal rocks (Lukkarinen 2008).

In the southern part of the Iisalmi complex, Palaeoproterozoic mafic volcanic rocks erupted on Archaean basement are found in small supracrus-tal belts, e.g., in the Siilinjärvi and Kuopio areas. In the Siilinjärvi belt, they form bulk of the Koivusaari Formation, occurring as pillowed and massive lavas with minor pyroclastic interbeds (Lukkarinen 2008).

105105



An intervening felsic member has been dated at 2062 ± 2 Ma using the U–Pb zircon method (Lukkarinen 2008). Mafic intrusive magmatism of approxi-mately the same age is represented by the Otanmäki Fe–Ti–V oxide-bearing gabbroic intrusion, which is located in the NE part of the Iisalmi complex, close to the Kainuu schist belt (Lindholm & Anttonen 1980, Talvitie & Paarma 1980, Nykänen 1995).

In this paper, we report U–Pb and Sm–Nd iso-tope data on several mafic dykes with ages of ca.

2.32 Ga, 2.13 Ga and 2.0 Ga, two gabbro–anorthosite plutons, the 2.06 Ga Otanmäki Fe–Ti–V ore-bear-ing intrusion, and the 1.89 Ga Lapinlahti intrusion, with the last-mentioned intrusion representing the Svecofennian synorogenic magmatism. In addition, we document Nd isotope characteristics of Jatulian 2.06 Ga volcanic rocks that occur in the Siilinjärvi area (Fig. 74).

8.2 The 2.3 Ga dykes, Humppi A135, Siunaussalmi A1369, Petäiskangas A1362

The NW–SE-trending Humppi dyke (A135) cuts NW–SE-trending Archaean gneisses at Lapinlahti in the Iisalmi complex (Fig. 74). Paavola (1988) reported an age estimate of 2331 ± 33 Ma for the dyke. This was based on slightly heterogeneous U–Pb data on zircon. Recently, zircon grains from this sample were re-analysed using laser ablation MC-ICP-MS. Nine data points gave a concordia age of 2323 ± 13 Ma (Fig. 103, Appendix 11), being consistent within error with the date obtained by Paavola (1988).

We analysed Sm–Nd isotope ratios for the pyroxene and plagioclase concentrates from sam-

ple A135. Under the microscope, the plagioclase in this coarse-grained rock looks fairly fresh, with only some patchy clouding, whereas the pyroxene is partly altered to amphibole. The three analyses yielded an age estimate of 2270 ± 40 Ma (εNd = +1.3, MSWD = 0.8, Fig. 105). However, the overall REE level in the analysed pyroxene is relatively high (Appendix 1) and some metamorphic effects on the Sm–Nd system are likely, as also witnessed by the amphibole growth on pyroxene grains.

Another example of the ca. 2.3 Ga mafic dykes is provided by the Tulisaari dyke, which is one of the relatively wide and long, E–W-trending diabases

106106

Fig. 103. Concordia plot of U–Pb zircon data obtained from the Humppi diabase A135. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red triangles. TIMS data on Tulisaari dyke A1333 are shown for reference (x).

2000

2200

2400

2600

0.32

0.36

0.40

0.44

0.48

0.52

0.56

0.60

5 7 9 11 13 207Pb/235U

A135 Humppi diabase

206Pb 238U


A135 Concordia Age =

2323 ±13 Ma n=9

TIMS A135 A1333 Tulisaari Intercepts at

801±320 & 2307±21 Ma MSWD = 6.5 (n=8, x)

in the Varpaisjärvi area (Fig. 74). It has a width of ca. 200 m and a length of at least several kilo-metres. Based on discordant multigrain TIMS data, Hölttä et al. (2000) reported a U–Pb age of 2295 ± 5 Ma for zircon from a coarse-grained gabbroic sample A1333. The data are slightly heterogeneous, and including all eight data points, a somewhat less precise upper intercept age of 2307 ± 21 Ma is obtained (Fig. 103).

A new sample, A1369 Siunaussalmi, was taken from the Tulisaari dyke for Sm–Nd isotope analy-sis. The plagioclase in this sample appears fresh in thin section, but the margins of pyroxene grains are slightly altered to amphibole. However, after separation, including hand-picking as its final step, the pyroxene fraction was clean looking. Due to slight technical problems, the Sm/Nd error in the plagioclase analysis is estimated larger (1%) than usual. Three analyses on the mineral separates and one whole-rock sample gave an age of 2350 ± 40 Ma (εNd = +1.8, MSWD = 0.2, Fig. 104). It should be noted that the Sm–Nd age estimates based on so few analyses are sensitive to slight variation in the 143Nd/144Nd ratio, e.g., the age here would be 50 Ma younger if the 143Nd/144Nd ratio in pyroxene was 0.005% smaller than the measured value.

The age and initial 143Nd/144Nd ratio of the Tulisaari dyke (A1333 & A1369) are thus roughly similar to those of the Humppi dyke (A135). The samples from these dykes have similar trace element characteris-tics; however, the samples are rather course grained and thus may not precisely represent the composi-tion of the injected Fe-tholeiitic magma. Provided that the dykes are coeval, the best estimate for the intrusion age is 2323 ± 13 Ma. With this age, the initial εNd values for the two samples from the dyke are +1.3 and +1.6 (Appendix 1). Using all Sm–Nd data gathered from these two samples in the regression yields an age of 2322 ± 67 Ma (εNd = +1.6, MSWD = 3.5, n = 6).

The Petäiskangas mafic dyke (A1362) intrudes Archaean rocks in the Manamansalo complex west of the Kainuu schist belt (Fig. 89). The Archaean gneisses in the region are variably affected by the 1.8–1.9 Ga Svecofennian tectonic and metamorphic overprinting, and cross-cutting Palaeoproterozoic diabase dykes are often foliated and even show fold-ing. In spite of this, in several locations, the dykes have surprisingly well-preserved parts with little strain and partially preserved primary minerals. The ophitic texture of the dyke at the Petäiskangas sam-pling site is well preserved and the rock contains

107107


A1369wr

A1369px

A1369plag

A135 wr

A135px

A135 plag

0.5110

0.5114

0.5118

0.5122

0.5126

0.5130

0.5134

0.5138

0.08 0.12 0.16 0.20 0.24 0.28

143 N

d/14

4 Nd

147Sm/144Nd

A1369 SiunaussalmiAge = 2350 40 Ma

eps = +1.8MSWD = 0.19 n=3

A135 HumppiAge = 2270 38 Ma

eps = +1.3MSWD = 0.66 n=3

A135 Humppi zircon U-Pb age 2323 ± 13 Ma

Fig. 104. Sm–Nd isotope data for whole rock and mineral separates from the Humppi (A135) and Siunaussalmi (A1369) dykes.


mainly clinopyroxene and plagioclase. However, garnet replacing plagioclase surrounding ilmeno-magnetite grains and partial replacement of clino-pyroxene grains by secondary amphiboles are evidence that the mineral assemblage is not purely primary magmatic in origin. In the sampled out-crop, the diabase is crossed by many decimetre- to metre-wide shear zones with pervasive foliation and purely amphibolite facies secondary mineral composition.

A small number of light turbid zircon grains were obtained from sample A1362. The four multigrain TIMS analyses were all discordant and yielded a chord with an upper intercept with the concordia curve at 1964 ± 32 Ma (Fig. 105, Appendix 5). As the multigrain data were quite scattered, this sample was also recently studied using the laser ablation MC-ICP-MS. Seventeen of the total of 22 analyses gave an age of 1842 ± 10 Ma. Two of the analysed spots suggest ages of 2.0–2.1 Ga, but must be con-sidered suspect because of the considerable amount of common lead, whereas two analyses on a single grain (18 in Fig. 105) were concordant at 2322 ± 28 Ma (Appendix 11, Fig. 105). The grain (18) is dis-tinct in its BSE image, appearing better preserved than the main population. As the TIMS data suggest

an age of ca. 1.96 Ga, a lot of the zircon in A1362 must be older than 1.84 Ga. All matters considered, it apprears conceivable that 2322 ± 28 Ma could be the igneous age of this dyke, although more data are needed for a definite conclusion.

The mineral fractions from sample A1362 that were used for Sm–Nd studies appear fairly clean, although plagioclase is slightly turbid. The three Sm–Nd analyses yielded an isochron with an age of 1932 ± 42 Ma (εNd = -1.4, MSWD = 0.8, Fig. 106), which may well register metamorphic effects (see below).

In order to date the garnet-producing metamor-phic event, another NW–SE-trending dyke was sampled at Liminpuro, ca. 1 km south of the A1362 locality. Sample A1457 Liminpuro contains abun-dant garnet replacing ilmeno-magnetite and plagi-oclase. The stepwise dissolution method by DeWolf et al. (1996), involving powdering in a boron carbide mortar as the first step and leaching in 6N HCl as the second step, was used for Sm–Nd analysis of garnet (Fig. 107). Combining the garnet data with the analytical results from two whole-rock pow-ders, 1794 ± 15 Ma can be calculated for the clo-sure timing of garnet. This is close to the age of the dominant zircon phase in the previously considered

108108

1500

1700

1900

2100

2300

2500

A1362-18a, 18b

0.24

0.28

0.32

0.36

0.40

0.44

0.48

0.52

2 4 6 8 10

206 P

b/23

8 U

207Pb/235U

A1362 Petäiskangas Vaala metadiabase

LA-MC-ICPMS Intercepts at 1800 65 & 2337 160 Ma

MSWD = 0.76 (n=22)


Primary igneous ?Concordia Age = 2322 28 Ma

n=2 (only grain 18)

Strong metamorphic stageConcordia Age =

1842 10 Ma (n=17)


MSWD = 9.1 (n=4)

grain length ca. 100µm

Fig. 105. Concordia plot of U–Pb zircon data obtained from the Petäiskangas dyke A1362. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.

sample A1362, as well as the age of the latest major metamorphic-magmatic event in the area (e.g., Vaasjoki et al. 2001). It seems conceivable that the Sm–Nd age of ca. 1.93 Ga for the Petäiskangas dyke also reflects major metamorphic effects, and the

igneous age could well be close to 2.3 Ga. The initial εNd(2320 Ma) values for the whole-rock samples are +1.1 (A1362) and +1.5 (A1457), and are thus similar to the results obtained for the 2.3 Ga dykes discussed above.

109109


A1362wr

A1362px

A1362plag

0.5110

0.5112

0.5114

0.5116

0.5118

0.5120

0.5122

0.5124

0.5126

0.07 0.09 0.11 0.13 0.15 0.17 0.19

143 N

d/14

4 Nd

147Sm/144Nd

A1362 Petäiskangas dyke Age = 1930 ± 42 Ma

epsilon = -1.3 MSWD = 0.77

Fig. 106. Sm–Nd isotope data for whole rock and mineral separates from the Petäiskangas dyke A1362.

A1457 wr

A1457 grt

0.510

0.512

0.514

0.516

0.518

0.520

0.0 0.2 0.4 0.6 0.8

143 N

d/14

4 Nd

147Sm/144Nd

Garnet-whole rock Age 1794 ± 15 Ma

A1457 Liminpuro dyke (cf. A1362)

Fig. 107. Sm–Nd isotope data for whole-rock sample and garnet from the Liminpuro dyke A1457.


8.3 The 2.13 Ga Nieminen dyke, A1223 and A1368

The E–W-trending, 25- to 30-m-wide Nieminen dyke is one of the well-preserved diabase dykes intruding Archaean granulites in the Iisalmi com-plex (Fig. 74). The dyke is nearly undeformed, shows a homogeneous diabasic texture, and is well exposed in a dimension stone quarry, where the sampling for isotope studies was carried out. The main silicate constituents are plagioclase and clino-pyroxene accompanied by minor orthopyroxene. Secondary minerals, such as light-green amphibole, biotite, and epidote, occur only sporadically along grain boundaries. Geochemically, the Nieminen dyke is tholeiitic basalt with 1.2 wt% TiO2 and 6.2 wt% MgO and is in this respect similar in com-position to many other diabase dykes in the area (Toivola 1988).

The diabase in the Nieminen quarry contains patches where the rock is more coarse-grained and plagioclase-rich. Sample A1368 was taken from such a pocket, which after crushing and separation yielded a small amount of dominantly clear, col-ourless, long and acicular zircon grains. The eight multigrain U–Pb TIMS analyses performed on vari-ous fractions of the zircon separate were, however,

heterogeneous (Fig. 108, Appendix 5). Especially analysis C on a few brownish long crystals with a low density was very distinct from the other data, providing a slightly discordant result with a 207Pb/206Pb age of 1866 Ma. The other seven analyses conducted on heavy zircon were variably discord-ant, but all with 207Pb/206Pb ages of ca. 2.1 Ga (Fig. 108). Rejecting one technically poor analysis (E), the six data points provide a chord with intercepts at 2100 ± 3 Ma and -256 ± 150 Ma (MSWD = 0.8). The negative lower intercept age is obviously impossi-ble, questioning the value of the data in determining the magmatic age of the Nieminen dyke.

Subsequently, zircon was also re-analysed using laser ablation and MC-ICP-MS. The obtained data are all concordant within analytical error and yield a Concordia age of 2121 ± 7 Ma (Fig. 108, Appendix 11). As a whole, the TIMS data are roughly consistent with the in situ analyses.

The Nieminen dyke was also sampled for Sm–Nd isotopic studies. The diabase sample A1223 consists of fresh and clean plagioclase, clinopyroxene and minor orthopyroxene as the main minerals. Early dating results on this sample were published by

110110

1900

2000

2100

2200

0.32

0.34

0.36

0.38

0.40

0.42

0.44

5.0 5.4 5.8 6.2 6.6 7.0 7.4 7.8 207Pb/235U

A1368 Nieminen diabase

206Pb 238U



TIMS

TIMS minimum age 2111 ± 5 Ma

Fig. 108. Concordia plot of U–Pb zircon data obtained from the Nieminen diabase A1368. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.

Toivola et al. (1991), who reported an age estimate of 2085 ± 95 Ma (εNd = +2.1), which was based on Sm–Nd mineral data analysed by a non-commer-cial, in-house-built mass spectrometer at GTK. These old analyses also involved aliquoting the HCl solution prior to the addition of the Sm–Nd spike. Subsequently, five more analyses have been con-ducted using the currently employed techniques. Due to the low concentrations of Sm and Nd (less

than 1 ppm in plagioclase), the error in the old analyses is relatively large. There is, however, no systematic error in the old data, and using all the 12 analyses available, the obtained isochron gives an age of 2127 ± 42 Ma (εNd = +2.5, MSWD = 2.7, Fig. 109, Appendix 1), which is consistent with the U–Pb zircon dating results. The positive initial εNd sug-gests that the magma was derived from depleted mantle without significant crustal contamination.

8.4 The 2.06 Ga Otanmäki intrusion, A1381

The Fe-Ti oxide-bearing Otanmäki intrusion is located SE of Lake Oulunjärvi, west of the Kainuu schist belt (Fig. 89). Together with another mafic intrusion, Vuorokas, it was mined for iron, tita-nium and vanadinium from 1953 to 1985 (30 Mt). The southern contacts of the intrusions show that they were emplaced into Archaean amphibolite-banded TTG migmatites. At their northern mar-gins, the bodies have sharp, fault-defined contacts to ca. 2050 Ma syenites and peralkaline granites (Kontinen et al. 2013).

In earlier studies on the Otanmäki intrusion, zir-con occurring in mafic pegmatoids was used for U–Pb dating. Based on the data provided by O. Kouvo on two samples (A671, A672), an age of 2065 ± 4 Ma has been published by Talvitie and Paarma (1980).

The reported eleven analyses, which are discord-ant with a relatively limited spread in Pb/U, yield 207Pb/206Pb ages from 2020 to 2045 Ma. Using all these data and the current regression algorithm (Ludwig 2003), an upper intercept age of 2058 ± 13 Ma can be calculated (lower intercept at 809 ± 220 Ma, MSWD = 2).

The Otanmäki and Vuorokas intrusions are for large parts strongly deformed and pervasively met-amorphosed. Nevertheless, especially the southern parts of the Vuorokas intrusion also contain gabbro varieties in which primary igneous phases, pyrox-enes and plagioclase, are surprisingly well preserved (Nykänen 1995). Three samples of such well-pre-served gabbros from the middle and lower parts of the Vuorokas block were collected by V. Nykänen

111111


A1223 px#1

A1223px#2, #3,#4, #5, #6

A1223plagA1223plag#4A1223plag#5

A1223wr#1, #2, #3

0.5105

0.5115

0.5125

0.5135

0.5145

0.06 0.10 0.14 0.18 0.22 0.26 0.30

143 N

d/14

4 Nd

147Sm/144Nd

A1223 Nieminen dykeAge = 2127 42 Ma

eps = +2.5MSWD = 2.7 n=12

U-Pb zircon age 2121 ± 7 Ma

Fig. 109. Sm–Nd isotope data for whole rock and mineral separates from the Nieminen dyke A1223.


for mineral separation. They are the clinopyrox-ene-orthopyroxene-olivine-plagioclase cumulate A1381a, clinopyroxene-orthopyroxene-plagioclase-olivine cumulate A1381b, and plagioclase-clinopy-roxene-orthopyroxene cumulate A1381c.

As usual, the Sm–Nd analyses were conducted on high-purity hand-picked mineral fractions. The nine analyses available define an isochron, which gives an age of 2043 ± 19 Ma (εNd = -0.8, MSWD = 1.5, Fig. 110, Appendix 1), corresponding to the U–Pb age given by the pegmatoid zircons. The data indi-

cate that there is no significant variation in the ini-tial ratios between the different samples. The REE level is relatively low and the most mafic cumulates have nearly chondritic relative REE concentrations. The initial 143Nd/144Nd ratio obtained from the isoch-ron is not far from the chondritic ratio, either, and shows that the source for the magma was not in the convective/depleted mantle, which is considered to have been characterised by positive time-integrated εNd values throughout the Earth’s history.

8.5 The ca. 2.0 Ga dykes Koirakoski A1875, Jäkäläkangas A1838

Traditionally, it was commonly assumed that the dominantly Fe-tholeiitic NW–SE-trending diabase dykes occurring abundantly in the Archaean terrains in eastern Finland are mostly ca. 2.1 Ga in age, as with the Nieminen dyke discussed above (e.g., Vuollo et al. 1992). Subsequently, it has become apparent that this is not true, but this group of dykes com-prises both significantly older and younger dykes. The Koirakoski and Jäkäläkangas below are exam-ples of the latter.

The Koirakoski dyke, which is located ca. 26 km north of the Nieminen dyke, is one of the abun-dant NW–SE-trending dykes cutting Archaean high-grade gneisses in the Varpaisjärvi-Iisalmi-

Sonkajärvi area (Fig. 74). The dyke is ca. 30 m wide and contains primary igneous pyroxene and pla-gioclase in places. However, in the sampled outc-rop, the NE margin of the dyke is metamorphosed to hornblende-bearing metadiabase with zones of schistose and folded amphibolite. Sample A1875 collected for Sm–Nd studies represents the well-preserved SW part of the dyke. A thin section pre-pared from the separated mineral fractions revealed that both the plagioclase and pyroxene fractions are fresh; however, a few grains of amphibole were observed in the pyroxene fraction. Sm–Nd analyses conducted on minerals and whole rock are techni-cally of good quality and provide an isochron that

112112

wrwr

wr

plag

px

plag

px

plag

px

0.5104

0.5108

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.04 0.08 0.12 0.16 0.20 0.24

143 N

d/14

4 Nd

147Sm/144Nd

Otanmäki intrusion A1381

Age = 2043 19 MaEpsilon = -0.8

MSWD = 1.5 n=9

A671& A672 U-Pb zircon age 2058 ± 15 Ma

Fig. 110. Sm–Nd isotope data for whole-rock samples and mineral separates from the Otanmäki mafic intrusion (A1381).

gives an age of 1968 ± 38 Ma (MSWD = 1.5, εNd = +0.5, Fig. 111, Appendix 1). Using analyses of the whole-rock powder and plagioclase only, the result is 2000 ± 63 Ma, while whole rock and pyroxene give a date of 1913 ± 95 Ma. These dates are not

distinct within error, but it is possible that met-amorphic amphibole may have some effect on the analytical results of the pyroxene fraction.

The Jäkäläkangas dyke is an at least 60-m-wide, SW–NE-trending mafic dyke cutting Archaean

113113


A1875

A1875plag

A1875px

0.5110

0.5114

0.5118

0.5122

0.5126

0.5130

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22

143 N

d/14

4 Nd

147Sm/144Nd

A1875 Koirakoski dyke Age = 1968 ± 38 Ma

epsilon = +0.5 MSWD = 1.5

Fig. 111. Sm–Nd isotope data for whole rock and mineral separates from the Koirakoski dyke A1875.

1880

1900

1920

1940

A1838A

A1838B .

0.326

0.330

0.334

0.338

0.342

0.346

0.350

0.354

5.3 5.4 5.5 5.6 5.7 5.8

206 P

b/23

8 U

207Pb/235U

A1838 Jäkäläkangas metadiabase Intercepts at

289 ± 150 & 1946 ± 4 Ma


zircon grains euhedral long turbid

Fig. 112. Concordia plot of U–Pb zircon TIMS data obtained from the Jäkäläkangas diabase A1838.


migmatitic gneisses west of the central Kainuu schist belt (Fig. 74). The dyke is only weakly deformed to non-deformed and, in the sampled outcrop, contains many irregular, metres-across patches, which preserve primary minerals, the fore-most being clinopyroxene and plagioclase, although the rock is mostly hydrated to amphibolite-facies metadiabase. Sample A1838 Jäkäläkangas was collected from a decimetre-wide, coarse-grained band in a relatively well-preserved part of the dyke. Mineral separation yielded a small amount of light-coloured, slightly turbid, needle-shaped (>100 µm) zircon grains.

Only two multigrain TIMS U–Pb analyses have been performed, but the one on a zircon fraction purified by mechanical abrasion overnight was nearly concordant (Fig. 112, Appendix 5). The two fractions yielded an upper intercept age of 1946 ± 4 Ma, which is also proposed to be the magmatic age of the Jäkäläkangas dyke. The previous discor-dant U–Pb zircon analyses of some diabase samples (A256-8, A991; collected by M. Havola) from the Kainuu schist belt, east of the site of sample A1838, are roughly compatible with the results from the Jäkäläkangas dyke.

8.6 The 1.89 Ga Lapinlahti intrusion

The Lapinlahti gabbro refers to a concentrically zoned, circular (6–7 km) gabbro-anorthosite intrusion and is among the rare examples of the synorogenic Svecofennian mafic intrusions that were emplaced into the Archaean crust close to the margin of the Karelian craton (Peltonen 2005b). Paavola (1988) published an age of ca. 1.89 Ga for the Lapinlahti gabbro. The main gabbro and anorthosite phases of the Lapinlahti intrusion are low in Zr and do not produce zircon grains in sepa-ration. However, locally, the gabbros have pegma-toid segregations up to several metres in thickness

and several tens of metres in length that contain some zircon. Sample A1688 was taken from such a segregation in a gabbro exposed in a macadam quarry at the Taskilanmäki locality. A small amount of dominantly transparent, pale zircon was recov-ered and analysed using LA-MC-ICPMS. All data are concordant and yield an age of 1888 ± 5 Ma, which is identical to the age obtained in an earlier multigrain TIMS analysis (Fig. 113, Appendix 11, 5).

The Sm–Nd data on the two dated pegmatoid gabbro samples (A708, A1688) give distinctly nega-tive initial εNd values of -6.5 and -5.3, suggesting a

114114

Fig. 113. Concordia plot of U–Pb zircon data obtained from the Lapinlahti gabbro (A1688). LA-MC—ICP-MS analyses are presented as error ellipsoids and an ID-TIMS analysis as a red dot.

significant contribution of Archaean LREE-enriched lithosphere (crust, Appendix 1). In this respect, a sharp contrast is obvious to the roughly coeval

gabbros from the Svecofennian domain, having εNd values from +3 to 0 (Huhma 1986, Makkonen & Huhma 2007).

8.7 Volcanic rocks in the Siilinjärvi area

The Koivusaari Formation refers to a 20-km-long and 3- to 4-km-wide, fault-bounded block of Palaeoproterozoic metavolcanic rocks lying on Archaean basement gneisses in the Siilinjärvi area, 20 km north of Kuopio (Lukkarinen 2008). The for-mation consists of three mafic units, the Parkkila, Vehkasuo and Vuorimäki Members, and one felsic unit, the Kirjoniemi Member. The mafic amphib-olite-facies metavolcanic rocks comprise massive and pillowed lavas with intercalated tuffs. The rocks in the Parkkila and Vuorimäki Members clas-sify mainly as tholeiitic and those in the Vehkasuo Member alkaline basalts. The Kirjoniemi Member felsic rocks are amygdaloidal, quartz-phyric A-type rhyolites.

The age of the Koivusaari Formation is based on old U–Pb TIMS analyses conducted on zircon grains from two samples, A242 and A481, both represent-ing metarhyolites of the Kirjoniemi Member. The U content of zircon is relatively low and the data

obtained for 8 multigrain fractions are partly con-cordant, providing an age of 2062 ± 6 Ma (Fig. 114, Appendix 5).

Whole-rock Sm–Nd isotope analyses conducted in the late 1980s on samples of mafic and felsic vol-canic rocks from the Koivusaari Formation yielded somewhat scattered results. Although the analytical errors are slightly larger than in recent analyses, the scatter is mostly due to variation in the initial Nd isotope composition of the samples (Appendix 6). The tholeiitic pillow lavas of the Parkkila Member (the lowest stratigraphic unit) and the Kirjoniemi Member felsic rocks show clearly negative initial εNd values, suggesting a major involvement of older LREE-enriched lithosphere in their genesis. The other mafic rocks, including the Vehkasuo Member alkali basalts, have yielded a positive εNd(2062 Ma) value of about +2. Using the whole dataset, the range in Sm/Nd allows an age of 2090 ± 62 Ma to be calculated (Fig. 115). Combining these isotope

115115


Fig. 114. Concordia plot of U–Pb zircon data obtained from the Siilinjärvi felsic volcanic rocks (analyses by Kouvo/GTK).

1990

2010

2030

2050

2070

2090

2110

A242A +4.6/HF

A242B 4.2-4.6/HF

A481A +4.6/HF/crushed A481B +4.6/HF/uncr.

A481C +4.6/abr 5 h A481D +4.6/abr 3 h

A481E +4.6/<70/abr 3 h A481F +4.6/>70/abr 5 h

0.35

0.36

0.37

0.38

0.39

6.0 6.2 6.4 6.6 6.8 7.0 207Pb/235U

Siilinjärvi felsic volcanics

206Pb 238U

Intercepts at 2062 ± 6 & 770 ± 260 Ma

MSWD = 1.7 n=8

TIMS data-point error ellipses are 2s


data with geochemical data, Lahtinen et al. (2015b) recently concluded that the Siilinjärvi volcanic rocks were derived from both plume (OIB) and sub-conti-

nental lithospheric mantle (SCLM) magma sources and were also contaminated by Archaean crust.

9 THE OUTOKUMPU AREA

The Outokumpu allochthon in the North Karelia schist belt comprises numerous fault-bound ophi-olite fragments dominated by serpentinised metape-ridotites, but many also with metagabbro and basalt as small stocks and dykes (Peltonen et al. 2008). Based on TIMS U–Pb data on zircon, Huhma (1986) determined an age of 1972 ± 18 Ma for a coarse-grained metagabbro occurring in such a fragment at Horsmanaho. The relevance of this result was con-firmed by Peltonen et al. (2008) by TIMS analyses conducted on another gabbro (A1029 Huutokoski) from another ophiolite fragment, which yielded an age of 1959 ± 5 Ma. They also reported a SIMS age of 1971 ± 15 Ma for a trondhjemite gneiss (A1754) that is associated with metagabbroic rocks in the Kylylahti ophiolite fragment. In addition, SIMS (SHRIMP) and TIMS data are available for two drill core samples from the Kylylahti ophiolitic ultramafic body. These are the skarnoid metabasite sample

OKU794B/490.80–482.70 and chlorititised alka-line dyke sample R393 (M52/4224/R393/422.40–427.40), both from the southern part of the Kylylahti body. Sample OKU794B/490.80–482.70 represents a pervasively carbonate-altered and subsequently metamorphosed (low amphibolite facies) mafic rock, probably originally gabbro. Zircon grains extracted from the sample are mostly euhedral, small and low in U. The U–Pb data obtained by TIMS and SIMS yield results that are consistent with the earlier data from much less altered metagabbros, also suggesting an age of ca. 1.96 Ga.

Sample R393 comes from a several-metre-wide, pervasively chloritised and subsequently metamor-phosed, probably basaltic dyke occurring in a talc-carbonate rock-serpentinite environment. High TiO2 and P2O5 suggest an alkaline affinity for the basaltic protolith. Most data on the dyke give simi-lar ages to the above-mentioned gabbros, but there

116116

Kirjoniemi felsic tuff

Kirjoniemi felsic tuff Parkkila pillow lava

Parkkila pillow lava

0.5113

0.5115

0.5117

0.5119

0.5121

0.5123

0.5125

0.5127

0.5129

0.10 0.12 0.14 0.16 0.18 0.20 147Sm/144Nd

Siilinjärvi volcanics

143Nd 144Nd

Age = 2090 ± 62 Ma Nd-epsilon = +2.2

MSWD = 1.1 n=5

Fig. 115. Sm–Nd isotope data for whole-rock samples of volcanic rocks from the Siilinjärvi area.

are also a few significantly older grains suggesting a xenocrystic origin (Fig. 116, Appendix 4b, 5). The three earlier-performed TIMS analyses resulted in discordant Pb–Pb dates of ca. 2.1, probably repre-senting analyses of mixed zircon populations.

Sm–Nd analysis conducted on sample R393 indicated a strong LREE enrichment and gave an εNd(1950 Ma) value of -1.3 (Appendix 1). Peltonen et al. (2008) published an εNd(1960 Ma) value of +1.7 for the Kylylahti plagiogranite sample A1754, and an

even higher value (~ +2.5) has been obtained from the Horsmanaho gabbro (Huhma 1986). However, the Huutokoski gabbro sample A1029 has given an εNd(1960 Ma) value of ca. +0.5 (Appendix 1). The large Sm–Nd database available from the ophiolitic gabbroic-basaltic rocks of the Outokumpu region do not suggest a significant quantity of rocks with depleted mantle-like Nd isotope signatures, as the initial εNd values are mostly close to zero (A. Kontinen & H. Huhma, unpublished).

117117


1300

1500

1700

1900

2100

2300

0.18

0.22

0.26

0.30

0.34

0.38

0.42

0.46

2 4 6 8

206 P

b/23

8 U

207Pb/235U

Skarn, Outokumpu ophiolitedata-point error ellipses are 2s

OKU794BAverage 207Pb/206Pb age

1963 21 MaMSWD = 0.46

n=7 (nearly concordant data)

OKU794Bnearly concordant data Intercepts at 300 300 (forced) & 1955 26 Ma

MSWD = 0.29 n=7

triangle - TIMS data

A)

1000

1400

1800

2200

2600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 4 8 12 16

207Pb/235U

Alkaline dyke, Outokumpu ophiolite

206Pb238U

data-point error ellipses are 2sR393 KylylahtiIntercepts at

417 240 & 1959 14 MaMSWD = 1.20 n=10


n=6


B)

1300

1500

1700

1900

2100

2300

0.18

0.22

0.26

0.30

0.34

0.38

0.42

0.46

2 4 6 8

206 P

b/23

8 U

207Pb/235U

Skarn, Outokumpu ophiolitedata-point error ellipses are 2s

OKU794BAverage 207Pb/206Pb age

1963 21 MaMSWD = 0.46

n=7 (nearly concordant data)

OKU794Bnearly concordant data Intercepts at 300 300 (forced) & 1955 26 Ma

MSWD = 0.29 n=7


A)

1000

1400

1800

2200

2600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 4 8 12 16

207Pb/235U

Alkaline dyke, Outokumpu ophiolite

206Pb238U

data-point error ellipses are 2sR393 KylylahtiIntercepts at

417 240 & 1959 14 MaMSWD = 1.20 n=10


n=6


B)

Fig. 116. A) Concordia plot of U–Pb zircon data obtained for the metabasaltic skarn rock sample OKU794B from Kylylahti. SIMS analyses are presented as error ellipsoids and ID-TIMS analyses as red triangles. B) Concordia plot of U–Pb zircon data obtained for the alkaline dyke R393 from Kylylahti. SIMS analyses are presented as error ellipsoids and ID-TIMS analyses as red triangles.


10 TOHMAJÄRVI VOLCANIC COMPLEX AND A BASEMENT DYKE

10.1 Oravaara gabbro A398

The Tohmajärvi volcanic complex is a 20-km-long and 2-km-wide sliver of mafic sills and metavol-canic rocks located in the SE corner of the North Karelia schist belt close to the Finnish–Russian bor-der (Nykänen 1971) (Fig. 74). Occurring in an anti-clinal structure together with Jatulian quartzites, conglomerates and dolomites, the volcanic complex is surrounded by younger Kalevian metawackes. It is composed of pillow lavas, pyroclastic deposits and subvolcanic intrusions varying in composi-tion from magnesian basalts to Fe-rich andesites with non-fractionated or slightly LREE-enriched chondrite-normalised REE patterns (Nykänen et al. 1994). Pekkarinen & Lukkarinen (1991) correlated the metavolcanics of the Tohmajärvi area with the Koljola Formation in the Kiintelysvaara area, which is found 20–30 km north of Tohmajärvi in a well-studied Jatulian supracrustal succession lying on Archaean basement gneisses. The Koljola Formation mafic lava flows and pyroclastic rocks are sand-wiched between two Jatulian quartzite formations

and are thought to be genetically related to a WNW-trending swarm of mafic diabase dykes cutting the Archaean basement. Pekkarinen & Lukkarinen (1991) published a joint U–Pb zircon age of 2113 ± 4 Ma for two samples from such a dyke close to the contact to the Palaeoproterozoic cover sequence (Kiihtelysvaara in Fig. 74). Although the Tohmajärvi volcanic rocks differ in chemistry from the Koljola Formation rocks, they appear to be more or less coeval. Namely, Huhma (1986) obtained an age of 2105 ± 15 Ma for the Tohmajärvi mafic magmatism based on slightly discordant TIMS analyses con-ducted on zircon separated from a gabbroic rock (sample A398), probably a sill, occurring in meta-volcanic rocks at Oravaara.

The age from the Oravaara gabbroic sample was recently tested by utilising the original zircon extract from sample A398 and subjecting it to laser ablation MC-ICP-MS analysis. Sixteen isotope measurements were carried out on grain domains with the best apparent preservation quality. All the compositions

118118

Fig. 117. Concordia plot of U–Pb zircon data obtained from the Oravaara gabbro A398. LA-MC-ICPMS analyses are presented as black error ellipsoids and ID-TIMS analyses by Huhma (1986) as red dots. The blue error ellipse denotes the final age calculated by the ISOPLOT program (Ludwig 2003) from the LA-MC-ICPMS data.

19802020

20602100

2140

21802220

TIMS

0.33

0.35

0.37

0.39

0.41

0.43

5.8 6.2 6.6 7.0 7.4 7.8207Pb/235U

A398 Oravaara gabbro Tohmajärvi complex

206Pb238U



overlap with the concordia curve and provide an age of 2103 ± 8 Ma, thus confirming the previously obtained TIMS age (Fig. 117, Appendix 10).

Huhma (1986) analysed a whole-rock powder of sample A398 for Sm–Nd isotopes, obtaining an initial εNd value of +2.6 ± 1.0. We re-analysed the same sample in order to assess the reliability of the earlier Sm–Nd results, which were obtained using aliquoting and a home-made mass spec-

trometer. The new analyses yielded a slightly lower, although within errors similar, εNd value of +1.6 ± 0.5 (Appendix 1), which should better characterise the sample because of the newer improved tech-nology involved. Compared to the Jatulian magma-tism at Kiihtelysvaara, as represented by the Hyypiä sill, which has yielded an εNd(2100Ma) value of -1.7 (Huhma 1987), the Tohmajärvi rocks are clearly more depleted in their Nd isotope signature.

10.2 Purola dyke A1231

The southernmost part of the Archaean basement complex in eastern Finland, called the Ilomantsi complex by Hölttä et al. (2012), is cut by tholeiitic mafic dykes with their thickness falling commonly in the range of 10–30 m, sometimes exceeding 100 m, and their orientation varying mostly between 275o and 325o (Pekkarinen & Lukkarinen 1991). The Purola metadiabase dyke (A1231) represents a NW–SE-trending dyke swarm. The sampling site is located 20 km northeast of Kiihtelysvaara (Fig. 74). An old TIMS U–Pb analysis on turbid zircon was

discordant, with a Pb–Pb age of 1.97 Ga (Appendix 5). The new results obtained using laser ablation MC-ICP-MS are concordant within error for the most pristine zircon domains and provide an age of 2106 ± 11 Ma, whereas the data on altered domains give Pb–Pb ages of ca. 1.87 Ga (Fig. 118). The older age may be considered as the igneous age of the rock. Consequently, the Purola dyke appears coeval with the Oravaara gabbro in the Tohmajärvi area and the basement dyke dated from the Kiintelysvaara area in Pekkarinen & Lukkarinen (1991).

119119


1700

1900

2100

2300

TIMS

0.26

0.30

0.34

0.38

0.42

0.46

0.50

3.5 4.5 5.5 6.5 7.5 8.5 9.5

206 P

b/23

8 U

207Pb/235U

A1231 Purola metadiabase data-point error ellipses are 2s


n=7


n=7

Fig. 118. Concordia plot of U–Pb zircon data obtained from the Purola dyke A1231. LA-MC-ICPMS analyses are presented as error ellipsoids and an ID-TIMS analysis as a red triangle.


11 CARBONATITES AND LAMPROPHYRES


There are around ten known carbonatite occur-rences in Finland. The most prominent of them are the Archaean ca. 2.6 Ga Siilinjärvi carbonatite in the Iisalmi complex (O’Brien et al. 2015), the host rock of an active apatite mine, and the Sokli carbonatite in eastern Lapland (O’Brien and Hyvönen 2015), which is part of the Devonian Kola alkaline province (Fig. 119) and contains large regolithic phosphorite resources. Smaller Palaeoproterozoic occurrences include the ~2.0 Ga Laivajoki and Kortejärvi carbon-atites (O’Brien 2015), which are located in a shear zone between the Archaean Lentua and Pudasjärvi complexes. In this paper, we provide refined zircon

age data from the Siilinjärvi, Laivajoki and Kortejärvi carbonatites, obtained using the ID-TIMS, LA-MC-ICPMS and SIMS methods. The same samples were also used to measure the Nd isotope composition of these carbonatites.

Like carbonatites, lamprophyres also provide useful probes of mantle sources, as due to their usually high concentrations of REE, they are rela-tively insensitive to crustal contamination. In this paper, we deal with Palaeoproterozoic lamprophyres occurring at Niinivaara, North Karelia, and Kuotko and Palovaara in the Central Lapland greenstone belt (Fig. 119).

120120

Fig. 119. Location of the studied alkaline rocks. The dashed line shows the boundaries of the Devonian Kola alka-line province after O’Brien (2015).

11.2 The 2.61 Ga Siilinjärvi carbonatite

The first U–Pb analyses of zircon from the Siilinjärvi carbonatite were carried out in the late 1970s by Olavi Kouvo. The results suggested an age close to 2.6 Ga. These data on large (several cm) single zircon crystals indicated an unusually low abun-dance of uranium (5 ppm). Later, in the 1980s, U–Pb analyses were conducted on three other carbonatite samples at GTK. The analytical results are listed in Appendix 5 and plotted in the concordia diagram in Figure 120. These data suggest an age of 2610 ± 4 Ma. This result is supported by the 207Pb/206Pb age of subsequent CA-TIMS analysis on a zircon fraction from one (A187) of the samples (No Pb/U available, Appendix 5).

Recently, zircon grains from the Siilinjärvi carbonatite were also analysed by LA-MC-ICPMS

using a 50 µm laser spot and utilising three low-U zircon standards having a range of ages up to Archaean (GJ1, A1912, A2024). These analyses yielded a concordia age of 2608 ± 6 Ma (Fig. 120, Appendix 11).

Sm–Nd analyses on two of the U–Pb-dated sam-ples gave initial εNd values close to zero and model ages TDM of ca. 2.75 Ga (Appendix 1, analysed in 2001). These values are supported by the Nd isotope composition of the apatite concentrate collected from the production line of the Siilinjärvi mine (Nd = 700 ppm, εNd(2610 Ma) = -0.3). Based on a mineral isochron with an age of 2615 ± 57 Ma, a similar εNd value (+0.4 ± 0.2) has been reported by Zozulya et al. (2007).

11.3 The Sokli carbonatite

The Sokli carbonatite complex is one of the 22 alka-line complexes that constitute the Devonian Kola alkaline province (O’Brien & Hyvönen 2015). Based

on several methods, a Devonian age was already established in the 1970s (Vartiainen & Woolley 1974), and a Rb–Sr age of 365 ± 3 Ma was later

121121


0.45

0.47

0.49

0.51

0.53

0.55

0.57

11.0 11.4 11.8 12.2 12.6 13.0 13.4 13.8 207Pb/235U

206 P

b/23

8 U

2560

2600

2640

2680

2720

LA-MC-ICPMS Concordia Age = 2608 ± 6 Ma (2s, decay-const. errs ignored)

MSWD (of concordance) = 0.68, Probability (of concordance) = 0.41


Siilinjärvi carbonatite

TIMS Intercepts at 686±170 & 2610 ± 4 Ma

MSWD = 1.6 (n=6, A187, A300, A376)

Fig. 120. Concordia plot of U–Pb zircon data obtained by ID-TIMS (red error ellipse) and LA-MC-ICPMS from the Siilinjärvi carbonatite.


reported by Kramm et al. (1993). Our contribution here is to present an unpublished U–Pb analysis on zircon carried out by O. Kouvo (in 1980), which gave a concordant age of 366 ± 3 Ma (Appendix 5). Zircon from Sokli was also used in the Lu-Hf studies by Patchett et al. (1981), who reported an

initial εHf value of +8.2 suggesting a primary origin from a depleted mantle-type reservoir. We have also included a U–Pb analysis of pyrochlore (conducted in 1969 by Kouvo), which suggested a U–Pb age of ca. 350 Ma (Appendix 5).

11.4 The 2.0 Ga Laivajoki and Kortejärvi carbonatites

The Laivajoki and Kortejärvi carbonatites are located in the Koillismaa area between the Archaean Lentua and Pudasjärvi complexes (Nykänen et al. 1997, Fig. 38). The first U–Pb analysis conducted on Laivajoki zircon that was already obtained in 1973 suggested an age of ca. 2.0 Ga, which was confirmed by sub-sequent U–Pb TIMS data on the same sample (A497 Laivajoki). The data are slightly heterogeneous and discordant, providing an average 207Pb/206Pb age of 1980 Ma (Fig. 121, Appendix 5). Recently, more U–Pb analyses were performed using laser abla-tion MC-ICP-MS. Excluding one analysis, the data yielded a concordant age of 2001 ± 7 Ma, which is considered the best estimate for the igneous age of this rock (Fig. 121, Appendix 7). One analysis suggested a younger age, which was probably due to a crack hit by the laser beam. The crack effect

provides an explanation for the slightly younger multigrain TIMS result. A closer inspection of the ICP-MS data reveals large variation in the U (and Pb) concentrations. Three analyses of BSE-darker domains yielded less than 8 ppm U (analysis 6a, Fig. 121) whereas the rest had up to 400 ppm U, but the ages from both domains are indistinguishable.

Two other samples from these carbonatites were collected for dating during the research project on alkaline magmatism in the 1990s. Zircon from the Laivajoki sample A1443 mainly consists of large crystals. In BSE images, the mega-grains often show lighter inner domains surrounded by wide darker outer domains. The U–Pb analyses revealed low U in these darker domains, as was also obtained in sample A497 above. TIMS and SIMS methods were applied, with both producing slightly scattered

122122

1800

1900

2000

2100

0.28

0.32

0.36

0.40

0.44

4.6 5.0 5.4 5.8 6.2 6.6 7.0 7.4

206 P

b/23

8 U

207Pb/235U

A497 Laivajoki carbonatitedata-point error ellipses are 2s



Fig. 121. Concordia plot of U–Pb zircon data obtained from the Laivajoki carbonatite A497. LA-MC-ICPMS analyses are presented as error ellipsoids and ID-TIMS analyses as red dots.

data that were roughly consistent with an age of 2.0 Ga (Fig. 122, Appendix 4a & 5). A small amount of monazite was also found from this sample. A multigrain TIMS analysis dated the monazite at ca. 1.7 Ga. This supports the assumption that meta-morphic effects are the reason for the scatter in the zircon U–Pb data. Based on six concordant SIMS analyses on inner zircon domains, an age of 1999 ± 9 Ma can be calculated, which may be considered as the best estimate for the age of this rock.

Sample A1444 collected from the Kortejärvi car-bonatite contains zircon, which is bright and clear up to a gem quality. The U–Pb analyses using TIMS demonstrated that the U concentrations are low, but the data still tend to be discordant. A small amount of baddeleyite found from this sample was also ana-lysed by TIMS. These data are also discordant, but

consistent with the zircon U–Pb data indicating an age of ca. 2.0 Ga (Fig. 122, Appendix 5). Zircon was also analysed by SIMS, but due to extremely low concentrations of U and Pb, the errors are very large and the data useless (Appendix 4a).

The C and O isotope compositions reported by Nykänen et al. (1997) for the Laivajoki and Kortejärvi carbonatites are within the range obtained for car-bonatite melts in equilibrium with mantle miner-als (δ13C ~ -4.2, δ18O ~ +7). We have used the same samples for Sm–Nd isotope analyses. The REE level in these rocks is high (Nd ~350 x chondrites) and the obtained initial εNd(2000 Ma) values are system-atically positive, providing an average value of +2.4 (Appendix 1). This suggests that convective depleted mantle was the ultimate source for these rocks.

11.5 Lamprophyres

A Palaeoproterozoic U–Pb age of ca. 1.8 Ga was obtained for the Kaavi lamprophyres in the early 1980s (Huhma 1981). Recently, zircon from the same sample (A159 Niinivaara, Fig. 74) was re-analysed by SIMS (NORDSIM facility), yielding good concordant U–Pb data and an age of 1784 ±

4 Ma (Woodard et al. 2014). The initial εNd(1784 Ma) value and TDM for this sample are +0.4 and 1.98 Ga, respectively (Woodard & Huhma 2015, Appendix 1). Near chondritic initial isotope ratios are also evident in other high-REE dykes in the north-ern Savo region, such as the Syväri lamprophyres

123123


1700

1800

1900

2000

2100

2200

0.24

0.28

0.32

0.36

0.40

0.44

4 5 6 7 8 207Pb/235U

Laivajoki (A1443) and Kortejärvi (A1444) carbonatites

206Pb 238U


A1443G monazite TIMS Pb/Pb age

1687 ± 20 Ma

TIMS data diamond - A1443 zircon triangle - A1444 zircon

square - A1444 baddeleyite

A1443 NORDSIM Average Pb/Pb age

1990 ± 10 Ma (all data)

Fig. 122. Concordia plot of U–Pb data obtained from the Laivajoki and Kortejärvi carbonatites. SIMS analyses conducted on zircon A1443 are presented as error ellipsoids and ID-TIMS analyses as dots.


(average εNd(1784 Ma) = -0.5, Woodard & Huhma 2015, Appendix 1) and the Panjavaara carbonatite (average εNd(1800 Ma) = -0.5, unpublished, Torppa & Karhu 2007). The roughly coeval dykes of this rock family in the Svecofennian and NW Ladoga regions also share this feature. These include the Naantali and Halpanen carbonatites (average εNd(1800 Ma) = -0.1 and -0.3, respectively) and NW Ladoga lam-prophyres (average εNd(1800 Ma) = -0.3, Woodard & Huhma 2015, Appendix 1). This rock suite is also characterised by distinctively low δ13C values from -12 to -16 (Tyni et al. 2003, Torppa & Karhu 2007, Woodard & Huhma 2015). The lamprophyres and related 1.8 Ga rocks have been interpreted to origi-nate from a metasomatically enriched lithospheric mantle (Eklund et al. 1998, Andersson et al. 2006, Woodard et al. 2014). Furthermore, Torppa & Karhu (2007) considered that the depletion in 13C relative to the average mantle value could be related to sub-duction of organic-rich crustal material.

This might suggest that the chondritic initial εNd values are a result of mixing of various components rather than a signature of the primary unfraction-

ated mantle material. It may be of interest to note that the Pb isotope ratios obtained for galena from the Panjavaara and related Petäiskoski carbon-atites plot exactly on the terrestrial evolution line of Stacey & Kramers (1975) at 1.8 Ga (Tyni et al. 2003).

Two lamprophyres from Central Lapland have also been analysed by the Sm–Nd method (Appendix 1). The Kuotko lamprophyre (A1168), which occurs within the 2.0 Ga Kittilä Group volcanic rocks about 45 km NE of Kittilä, just west of the Ruoppapalo granodiorite intrusion (Fig. 2), gave an εNd(2000 Ma) value of -3.1 (TDM = 2.43 Ga). A dyke at Palovaara (A1439), 15 km east of Kittilä, was found to have a εNd(2000 Ma) value of +2.3 (TDM = 2.07 Ga). The Palovaara sample was obtained from a drill core penetrating sedimentary rocks of the Sodankylä Group close to the contact of the Savukoski Group metavolcanics. Another lamprophyre sample (A1441) from the same locality has yielded a U–Pb monazite age of 1771 ± 8 Ma, which was thought to record the latest major stage of alteration in Central Lapland (Rastas et al. 2001).

12 DISCUSSION

12.1 Episodic rifting stages of the Archaean lithosphere

The U–Pb and Sm–Nd ages that are available on mafic rocks in the Fennoscandian Shield manifest several stages of rifting of the Archaean lithosphere before its major breakup ca. 2 Ga ago, which is the age of ophiolites (Kontinen 1987). The results from the Finnish Karelia province are summarised in Figure 123. Most of the analytical results provide geochronological information on the emplacement of gabbroic rocks in intrusions and dykes, but also constrain the timing of mafic volcanism and the overall geological evolution. From northern Finland, many of these ages were already reported in a spe-cial volume containing U–Pb isotope data (Vaasjoki 2001), and together with new results suggest a range of ages, mostly at ca. 2.44 Ga, 2.22 Ga, 2.15 Ga, 2.12 Ga, 2.05 Ga and 2.0 Ga (Hanski et al. 2001a), providing cornerstones for chronostratigraphy over the Karelian craton (Fig. 3).

The isotopic results reported from northern and eastern Finland in this work confirmed new localities for ca. 2.3 Ga and 2.44 Ga mafic rocks

and introduced a totally new discovery of 2.5 Ga rocks. Obviously, the new data are largely based on U–Pb spot analyses on zircon, but the Sm–Nd mineral ages determined for well-preserved rocks are generally consistent with the U–Pb zircon ages. There are a few exceptions, such as the Rantavaara intrusion case, in which the apparent old Sm–Nd mineral age is explained by slight crustal contami-nation in the intercumulus plagioclase occurring as a minor phase in an ultramafic rock. Problems in dating often relate to secondary open systems, but can also be due to isotopic disequilibrium between phenocrysts and their host rock, as was observed, for example, in the Suisarian picrites in the Onega region, Russian Karelia (Puchtel et al. 1998, Huhma et al. unpublished), or in some dykes in this work (A1465, A1808).

An overview of the ages within each area is pre-sented in Figures 124a–f, which also give individual sample numbers.

124124

125125


1700 1800 1900 2000 2100 2200 2300 2400 2500 2600

Rel

ativ

e pr

obab

ility

Age (Ma)

Age of mafic rocks in the Karelia province/ Finland Blue - Lapland (n=76) Red - Pudasjärvi & Taivalkoski (n= 56) Green - Kuhmo & Iisalmi (n= 57)

Fig. 123. U–Pb and Sm–Nd age determinations for the Palaeoproterozoic mafic rocks in the Karelia province of Finland.


126126

Fig. 124. Ages of mafic rocks in different areas, with two-sigma error bars.

A036

0 Ts

hokk

oaivi

i A1

475

A131

7 A1

318

A147

4 A0

604 K

oite

laine

n A1

204

A131

0 Aka

nvaa

ra

A058

0 A0

666

A131

2 A1

525 L

ehto

maa

A1

311

A140

5 A1

337

A143

1 A0

281

A089

2 To

kka

A047

0 A0

825

A013

6 A0

379

A091

6 A0

915

A170

0 A1

408

A143

0 A0

900

A167

4 Kan

nusv

aara

A0

962

A047

2 A0

817

A084

1 A0

066

A007

7 A0

078

A144

6 2000

2100

2200

2300

2400

2500

2600

Age

(Ma)

data-point error symbols are 2s

Lapland mafic rocks

A)

A041

2 A0

818

Keivi

tsa N

I-PGE

A1

316

A081

6 A0

863

A045

0 A1

390

Jees

iörov

a A0

820

A086

1 A1

445

A122

6 A0

469

A040

1 A0

655

A143

6 A2

288

A074

2 A0

741

A228

9 A0

964

A229

0 A1

563

A094

4 A0

946

A127

2 A1

273

Vesm

ajärvi

L-

132

A136

6 A0

142

A144

8 A0

414

A191

6 A1

665

A144

1 A0

959

1700

1800

1900

2000

2100

Age

(Ma)


B)

A141

4 A0

722

A070

9 A0

700

A091

9 A0

698

A166

3 A0

713

A186

8 A1

412

A141

5 22

B-TT

K-01

A1

466

A179

7 A1

796

A149

2 A1

456

A098

8 A1

795

A179

4 A0

847

A049

6 A1

802

A049

7 A1

443

1900

2000

2100

2200

2300

2400

2500

2600

Age

(Ma)


Taivalkoski block, mafic rocks (n=25)

C)

A141

0 A1

744

A060

3 A0

662

A085

9 A0

703

A101

2 A0

999

A047

6 A0

408

A040

7 A0

305

A059

2 A0

477

A030

4 A0

409

A047

4 A0

706

A086

5 A0

475

A208

7 A1

214

A098

6 A0

480

A041

0 A1

788

A100

9 A0

854

A174

3 A0

755

A090

7

1700

1900

2100

2300

2500

2700 data-point error symbols are 2s data-point error symbols are 2s

Age

(Ma)

Pudasjärvi complex & Peräpohja belt, mafic rock ages (n=31)

D)

A135

6 A1

914

A167

2 A1

361

A058

6 A1

182

A019

8 A0

261

A122

0 A1

096

A136

3 A1

212

A046

5 A0

427

A123

1 A0

398

A148

9 A1

409

A207

1 A1

673

A151

9 A0

149

A175

4 A1

144

OKU-

794B

A1

029

R393

A1

460

A015

9 Ka

lto 2A

1600

1800

2000

2200

2400

2600

Age

(Ma)


Kuhmo block mafic rock ages (n=30)

E)

A159

6 A1

369

A013

5 A1

333

A019

9 A0

769

A075

9 A0

977

A136

8 A1

223

A048

1 A0

671

A067

2 A0

242

A138

1 A1

529

A187

5 A1

402

A072

9 A0

196

A183

8 A1

688

A091

8 A1

457

JA-0

5 A0

760

JD-0

5

1600

1800

2000

2200

2400

2600

Age

(Ma)


Iisalmi complex mafic rock ages (n=27)

F)

12.2 Range of εNd – evidence for heterogeneous mantle and crustal contamination

In order to constrain the origin and processes involved in the formation of crust, the Sm–Nd method has been used at GTK since 1981. The avail-able Sm–Nd data comprise ca. 2600 analyses, with ca. 1100 of them being from mafic rocks in Finland (ca. 800 from the Karelia province). The main Sm–Nd results of the mafic-ultramafic rocks and some related felsic rocks within the Karelia province are summarised in Table 1 and shown in the εNd vs. age diagrams (Fig. 125a–e). Most of the initial εNd val-ues for mafic rocks are based on Sm–Nd mineral isochrons and should thus give reliable estimates for the initial isotope composition of the rocks in question. It should be noted, however, that some results are based on only a few analyses of whole-rock samples. Some points in Figure 125 have a relatively large error in age, which is shown by the evolution line following typical compositions of the rock units in question. Nevertheless, a wide range in the initial Nd isotope compositions is evident. Some rocks were clearly derived from depleted mantle sources, whereas others show a major contribution from old enriched lithosphere. It can be questioned whether this is due to crustal contamination in the final magma chamber, at deep crustal levels, or an inherited feature from the heterogeneous subcon-tinental lithospheric mantle.

High-REE mantle-derived rocks should provide useful information on the isotope composition of the source, since they are less sensitive to crus-tal contamination. Such rocks in Finland include

the 2.6 Ga Siilinjärvi carbonatite, 2.0 Ga Kortejärvi and Laivajoki carbonatites (Nykänen et al. 1997), 1.97 Ga Jormua OIB dykes and 1.8 Ga Kaavi lampro-phyres (Huhma 1981, Woodard & Huhma 2015), for which Sm–Nd analyses are presented in Appendix 1. They suggest that mantle reservoirs with a nearly chondritic Nd isotope composition were the dom-inant sources for the 2.6 Ga carbonatite, Jormua OIB, and 1.8 Ga lamprophyres. Similar results have been obtained for 1.8 Ga shoshonitic magmas in the Svecofennian domain, as well as Russian Karelia (Patchett & Kouvo 1986, Eklund et al. 2000). In con-trast, the 2.0 Ga Kortejärvi and Laivajoki carbon-atites provide clearly positive initial εNd(T) values (+2.5) and suggest an origin from a mantle source with time-integrated depletion in LREE. These and other available data thus indicate that the mantle had long-term heterogeneity.

Some features of the chemical composition of the mafic rocks are given in the 147Sm/144Nd vs. Nd diagrams (Figs. 126, 127), which mirror the level of LREE enrichment and the quantity of incompat-ible elements. In this diagram, one “typical” sam-ple from the data set was selected to represent the whole mafic unit. This may obviously result in bias, as a range of compositions is present due to crystal fractionation in magma chambers. Nevertheless, most of the age groups and magma types appear to form distinct systematic trends or groupings in the diagram.

127127



128128

Fig. 125. Epsilon-Nd vs. age diagram for 1.8–2.6 Ga mafic rocks (green – volcanic rocks) and some related felsic lithologies (data sources shown in Table 1 and Appendix 6). Evolution lines are presented if the error in age is in excess of 20 Ma. The trend of the line follows the typical composition of the rocks in question and the length of the line approximates the error in age. Depleted mantle evolution is according to DePaolo (1981). CHUR is the Bulk Earth evolution (De Paolo & Wasserburg 1976). The evolution of typical Neoarchaean granitoid is also shown (A1611, Mutanen & Huhma 2003).

CHUR

Siilinjärvi

Junttilanniemi

Humppi Siunaussalmi/Tulisaari

Raatelampi Kettukallio

Nieminen

Otanmäki

Jormua main suite

Jormua OIB suite

Lapinlahti

Syväri Vuotjärvi

Siilinjärvi Parkkila

Siilinjärvi Vuorimäki

-7

-5

-3

-1

1

3

5

1780 1980 2180 2380 2580

Nd-

epsi

lon

Age (Ma)

Mafic rocks/ Iisalmi complex

E)

CHUR

Tshokkoaivi Koitelainen Akanvaara

Lehtomaa Salla

Onkamonlehto

Ahvenvaara Haaskalehto Silmäsvaara

Rantavaara

Tanhua Kannusvaara

Keivitsa

Keivitsa NI-PGE

Moskuvaara

Rovasvaara

Selkäsenvuoma

Pittarova

Keivitsa dyke

Lotto Tainio

Vesmajärvi

Puijärvi

Satovaara Nyssäkoski

Kapsajoki Veikasenmaa

Kiimaselkä Yräjärvi

Latvajärvi

Koutoiva

Mäntyvaara

Sakiamaa

Möykkelmä

Akanvaara

Kannusvaara Tanhua

Matarakoski

-7

-5

-3

-1

1

3

5

1800 2000 2200 2400 2600

Nd-

epsi

lon

Age (Ma)

Mafic rocks & Felsic porphyries/ Lapland

A)

CHUR

Suoperä (BD) Porttivaara

Syöte

Karkuvaara Laivajoki Kortejärvi

Törninkuru

-7

-5

-3

-1

1

3

5

1800 2000 2200 2400 2600

Nd-

epsi

lon

Age (Ma)

Mafic rocks/ Taivalkoski block

B)

CHUR

Vengasvaara

Penikat Kemi

Runkausvaara

Kuusivaara Koppakumpu

Rytijänkä Hirsimaa

Tikanmaa

-7

-5

-3

-1

1

3

5

1800 2000 2200 2400 2600

Nd-

epsi

lon

Age (Ma)

Mafic rocks/ Pudasjärvi & Peräpohja

C)

CHUR

Lohisärkkä Mustikkarinne Arola

Tohmäjärvi

Hattuselkonen

Kivikevätti Koli Paukkajavaara

Peräaho

Niinivaara

Horsmanaho

Huutokoski

-7

-5

-3

-1

1

3

5

1780 1980 2180 2380 2580

Nd-

epsi

lon

Age (Ma)

Mafic rocks/ Kuhmo block

D)

129129


Tab

le 1

. Su

mm

ary

of t

he

Sm-

Nd

resu

lts

on m

afic-

ult

ram

afic

rock

s in

th

e K

arel

ia p

rovi

nce

in

Fin

lan

d.

Nam

e/ L

ocat

ion

Rock

type

(dyk

e tr

end)

D(1

Age

(Ma)

±ep

sAg

e (M

a)(4

±Re

f*Ag

e (M

a)±

n*Re

f*sa

mpl

esco

mm

ents

Map

YKJ-N

orth

YKJ-E

ast

U-P

bU

-Pb

Sm-N

dSm

-Nd

Lapl

and:

Tsho

hkko

aivi

mafi

c in

trus

ion

L24

9911

-1.8

2499

11x

2458

816

xA1

317,

A13

18, A

360

U-P

b ag

e on

A36

018

3401

7624

845

3296

659

Koite

lain

enm

afic

intr

usio

nL

2439

3-2

.024

393

124

3849

2215

A120

4, e

tc37

4101

7524

300

3507

700

Akan

vaar

am

afic

intr

usio

nL

2436

6-2

.224

366

124

3226

615

A131

237

3304

7462

900

3553

500

Peur

atun

turi

mafi

c in

trus

ion

L24

4830

-1.5

2448

305

xA1

474

with

in T

unts

a su

ite47

1411

7507

142

3617

874

Koul

umao

iva

mafi

c in

trus

ion

L24

6434

-1.0

2464

344

xA1

475

with

in T

unts

a su

ite47

1306

7490

727

3603

128

Leht

omaa

Sal

lam

afic

intr

usio

nL

2424

5-3

.424

245

x1

xA1

525

cros

scut

s Sa

lla G

roup

4614

0273

8292

636

0056

9

Onk

amon

leht

obo

nini

te-n

orite

dy

ke (N

E)L

2403

3-0

.724

033

x1

4, x

A140

5cr

ossc

uts

Salla

&

Kuus

amo

Gro

ups

4612

1273

9982

335

8819

1

Haa

skal

ehto

“Kar

jalit

e/ G

WA”

L22

116

0.7

2211

67

2187

448

7A1

408

etc.

plag

exc

lude

d37

1303

7487

530

3466

640

Silm

äsva

ara

“Kar

jalit

e/ G

WA”

L21

8535

0.0

c. 2

220

821

8535

47

A143

0U

-Pb

age,

cf.

A281

(Ref

8)

3712

0774

9900

034

4030

0

Ahve

nvaa

ra“K

arja

lite/

GW

A”L

2231

270.

8c.

222

08

2231

274

7A1

431

U-P

b ag

e, c

f. A2

81 (R

ef 8

)36

4103

7429

990

3508

700

Rant

avaa

ram

afic

intr

usio

nL

2148

113.

121

4811

822

3325

9x

A133

7, A

900,

A15

86pl

ag d

iseq

uilib

rium

3732

0174

9890

035

0270

0

Tanh

ua

Kann

usva

ara

mafi

c in

trus

ion

L21

487

2.0

2148

7x

2089

335

xA1

446,

A16

74U

-Pb

age

on

gran

ophy

re A

1674

3732

0774

9490

035

2490

0

Väkk

äräv

aara

mafi

c in

trus

ion

L21

00?

-3.0

2100

?x

1x

A171

5ag

e?37

1404

7499

262

3476

945

Keiv

itsa

mafi

c in

trus

ion

L20

585

-3.5

2058

51,

x20

4926

23x

A122

6, A

1316

, A1

319,

A13

90, e

tc.

3714

1275

1146

034

9917

0

Keiv

itsa

NI-P

GE

min

eral

izat

ion

L20

585

-6.4

2069

313

xR6

95/6

7.65

-67.

7037

1412

7512

500

3498

820

Mos

kuva

ara

mafi

c in

trus

ion

L20

3914

-5.2

2039

14x

1x

A143

6cr

ossc

uts

Savu

kosk

i Gro

up37

1411

7503

250

3496

250

Rova

svaa

ram

afic

intr

usio

nL

2055

5-4

.220

555

81

xA8

2037

3102

7471

250

3501

900

Puijä

rvi

mafi

c in

trus

ion

L20

358

0.5

2035

8x

2x

A228

8-9

age

min

imum

7522

500

3491

350

Sato

vaar

am

afic

intr

usio

nL

2025

8-0

.520

258

x1

xA2

290

age

min

imum

7509

260

3500

925

Selk

äsen

vuom

am

afic

“she

eted

” dyk

eL

2008

32.

820

083

x2

17, x

A156

3w

ithin

Kitt

ilä G

roup

2741

0475

2876

133

8730

0

Nut

tiom

afic

dyke

(c

alc-

alka

line)

L20

00?

2.3

217

R4/9

.40

with

in K

ittilä

Gro

up37

2108

7534

560

3447

180

Pitt

arov

am

afic

intr

usio

nL

1996

8-0

.419

968

12, x

1x

A127

2cr

ossc

uts

Savu

kosk

i Gro

up75

2555

933

8806

6

Keiv

itsa

dyke

oliv

ine

gabb

ro d

yke

L19

1667

5.1

1916

675

xA1

366

3714

1275

1230

034

9875

0

Lott

o, In

ari

mafi

c dy

keL

1804

44-5

.218

0444

4x

A191

638

3302

7597

510

3540

160

Tain

io

mafi

c in

trus

ion

(app

inite

)L

1796

4-5

.017

964

2417

7454

3x

A166

526

4212

7452

883

3408

843

Kuot

kola

mpr

ophy

re d

yke

L18

00?

-5.8

1x

A116

8 (R

308)

2744

0475

5057

234

3381

6

Palo

vaar

a Ki

ttilä

lam

prop

hyre

dyk

eL

1800

?-0

.61

xA1

439

2734

0675

1198

634

2595

6

Värr

iöm

afic-

ultr

amafi

c in

trus

ion

L?

-3…

-5

?12

x47

11/0

6/R2

5 et

c.ne

gativ

e ep

s at

an

y pr

ot a

ge47

1174

8916

335

8753

7


130130

Tab

le 1

. Con

t.

Nam

e/ L

ocat

ion

Rock

type

(dyk

e tr

end)

D(1

Age

(Ma)

±ep

sAg

e (M

a)(4

±Re

f*Ag

e (M

a)±

n*Re

f*sa

mpl

esco

mm

ents

Map

YKJ-N

orth

YKJ-E

ast

U-P

bU

-Pb

Sm-N

dSm

-Nd

Taiv

alko

ski b

lock

(nor

ther

n pa

rt o

f the

Len

tua

com

plex

):Po

rttiv

aara

mafi

c in

trus

ion

T24

365

-2.1

2436

525

2388

7523

2659

-W-7

3 et

c.72

9924

035

4878

0

Syöt

em

afic

intr

usio

nT

2436

5-1

.724

365

255

xB7

-11.

56 e

tc.

3532

0972

7890

835

2751

0

Pääj

ärvi

(XD

3)Fe

-tho

leiit

ic

dyke

(NW

)T

2476

351.

724

7635

3x

A141

4 (3

8-VE

N-9

4)46

3373

4612

436

7997

0

Pääj

ärvi

(XD

1)ga

bbro

nori

te

dyke

(NN

W)

T24

2132

-1.4

2421

324

xA1

412

(35-

VEN

-94)

4633

7346

042

3679

957

Pääj

ärvi

opx-

phyr

ic d

yke

T24

40??

-2.2

2x

A146

5 (4

2-VE

N-9

4)op

x ph

enoc

r.

dise

quili

briu

m46

3373

4603

936

7985

2

Pääj

ärvi

(XD

4)Fe

-tho

leiit

ic d

yke

(W)

T23

4924

1.0

2349

243

xA1

492

(39-

VEN

-94)

4633

7346

133

3679

953

Suop

erä

(BD

)ga

bbro

nori

te

dyke

(NN

W)

T24

466

-2.4

2447

10x

2420

293

xA1

415

(122

-VEN

-94)

4541

7304

580

3650

896

Taiv

alko

ski

Fe-t

hole

iitic

dyk

e (W

)T

2407

351.

624

0735

3x

A146

6 (W

D 1

4)35

3472

7675

035

5860

0

Törn

inku

ruFe

-tho

leiit

ic d

yke

(W)

T23

3918

1.2

2339

1828

2404

534

xA1

797

(AD

13-8

)an

alys

is o

f lig

ht

plag

exc

lude

d35

3403

7270

390

3548

690

Kalli

onie

mi

Fe-t

hole

iitic

dyk

e (N

E)T

2352

620.

523

5262

3x

A179

6 (2

-JIV-

03)

4512

0572

6826

335

7307

9

Kark

uvaa

ra/

Nyr

hino

jam

afic

intr

usio

nT

2306

81.

823

068

x23

1927

6x

A145

6, A

988

3531

1272

4036

035

3404

0

Tils

anva

ara

Fe-t

hole

iitic

dy

ke (N

W)

T22

1937

0.2

2219

374

xA1

794

(AD

89-1

0)

3534

0572

6324

735

5010

7

Tils

anva

ara

2 th

olei

itic

dyke

(WN

W)

T22

3342

1.0

2233

424

xA1

795

(AD

89-8

) 35

3405

7263

323

3550

074

Kont

iolu

oma

Kuus

amo

Fe-t

hole

iitic

dyk

e (N

E)T

?3

xA1

798

(AD

85-3

)he

tero

gene

ous?

4521

0772

8588

335

8508

4

Mur

hini

emi

Fe-t

hole

iitic

dyk

e (N

E)T

?3

xA1

800

(AD

91-3

)he

tero

gene

ous?

45

1205

7268

724

3574

957

Hir

sika

ngas

Fe-t

hole

iitic

dy

ke (N

W)

T?

3x

A180

1 (M

LJ40

-1)

hete

roge

neou

s?35

3405

7263

963

3559

318

Koiv

uvaa

raFe

-tho

leiit

ic

dyke

(NW

)T

2058

35-0

.120

5835

3x

A180

2 (M

LJ29

-1)

3534

0272

6723

135

4488

6

Laiv

ajok

ica

rbon

atite

2001

72.

420

017

x4

xA4

97, A

1443

,…35

4211

7320

400

3535

150

Kort

ejär

vica

rbon

atite

1999

92.

619

999

x4

xA1

444,

…35

4207

7310

700

3529

500

131131


Tab

le 1

. Con

t.

Nam

e/ L

ocat

ion

Rock

type

(dyk

e tr

end)

D(1

Age

(Ma)

±ep

sAg

e (M

a)(4

±Re

f*Ag

e (M

a)±

n*Re

f*sa

mpl

esco

mm

ents

Map

YKJ-N

orth

YKJ-E

ast

U-P

bU

-Pb

Sm-N

dSm

-Nd

Puda

sjär

vi c

ompl

ex:

Peni

kat

mafi

c in

trus

ion

P24

448

-1.6

2444

8x

2422

525

16A1

012,

A60

3, A

703

2544

0473

1862

234

1770

3

Kem

im

afic

intr

usio

nP

2433

4-1

.624

334

21

xA6

6225

4111

7301

076

3395

397

Lolju

nmaa

(LD

)bo

nini

te-n

orite

dy

ke (W

NW

)P

2433

?-1

.21

xLD

-4-9

3fe

eder

dyk

e

for

Peni

kat?

2544

7314

041

3418

244

Uol

evin

leht

o (U

D)

thol

eiitc

dyk

e (N

W)

P24

33?

0.4

2378

(m

in)

x24

4716

05

xA1

410

Nd

dise

qulib

rium

3541

1072

8838

035

0290

0

Veng

asva

ara

mafi

c in

trus

ion

P24

444

-5.4

2444

4x

2507

723

xA1

744

SHRI

MP

35

1404

7259

000

3472

600

Runk

ausv

aara

“Kar

jalit

e/ G

WA”

P22

1020

0.2

2210

202

2196

190

516

plag

dis

equi

libri

um25

4473

2996

534

2857

8

Kuus

ivaa

ra/

Perä

pohj

a be

lt m

afic

dyke

P21

4011

3.5

2140

113

1x

A208

7w

ithin

Pe

täjä

skos

ki F

m26

3303

7366

297

3412

956

Kopp

akum

pu/

Perä

pohj

a be

ltm

afic

dyke

P21

305

3.2

2130

5x

1x

A121

4 w

ithin

Tik

anm

aa

Fm26

3302

7353

659

3406

326

Rytij

änkä

mafi

c in

trus

ion

P20

8411

1.0

2084

11x

1x

A854

2613

1173

5471

533

6262

0

Palo

maa

gabb

ro d

yke

(NW

)P

2077

341.

120

7734

3x

A174

335

1404

7263

183

3468

985

Terv

onka

ngas

Ra

nua

mafi

c dy

ke (N

E)P

?4

xA1

808

“mix

ing

line

age”

ca

. 2.6

Ga?

3523

0373

0300

034

6614

7

Iisal

mi c

ompl

ex:

Siili

njär

vica

rbon

atite

I26

104

0.0

2610

4x

3x

A300

, A71

, A1

87, A

376

3331

1270

0014

035

3725

0

Junt

tilan

niem

im

afic

intr

usio

nI

2444

4-1

.824

444

x2

xA1

595-

6da

ta fr

om

fels

ic r

ocks

7140

340

3527

140

Paha

Ka

pust

asuo

mafi

c in

trus

ion

I24

40??

-0.6

x1

xA1

373

7179

750

3530

010

Hum

ppi

mafi

c dy

ke (N

W)

I23

2313

1.3

2323

135,

x22

7038

3x

A135

3332

0870

2569

035

2156

0

Siun

auss

alm

i/Tu

lisaa

rim

afic

dyke

(W)

I22

955

1.8

2295

56

2350

403

xA1

369

3343

0170

4536

035

4430

0

Petä

iska

ngas

mafi

c dy

keI

2300

??1.

023

2228

?x

1930

423

xA1

362

3432

0671

5804

035

1410

0

Raat

elam

pim

afic

intr

usio

nI

2222

7-1

.022

227

x1

xA1

9971

0300

035

5450

0

Hon

kani

emi

mafi

c in

trus

ion

I22

00??

-2.7

x1

xA6

5171

9405

035

3180

0

Kett

ukal

liom

afic

intr

usio

nI

2202

5-0

.4x

1x

A759

7184

580

3525

100

Nie

min

enth

olei

itic

dyke

(W)

I21

217

2.5

2121

7x

2127

4212

14, x

A122

3, A

1368

3334

0370

3082

035

4038

0

Ota

nmäk

im

afic

intr

usio

nI

2058

13-0

.820

5815

920

4319

9x

A138

134

3102

7111

630

3508

490

Koir

akos

kim

afic

dyke

(NW

)I

1968

380.

519

6838

3x

A187

533

4111

7056

220

3537

990

Jorm

ua m

ain

suite

basa

lts (m

ain

suite

)19

535

2.0

1953

513

1936

4317

13, x

at le

ast 1

7 an

alys

es34

3401

7137

220

3545

060

Jorm

ua O

IB

suite

OIB

-sui

te d

ykes

1953

5-0

.119

535

1319

6858

713

, xat

leas

t 7 a

naly

ses

3434

7139

070

3544

630

Lapi

nlah

tim

afic

intr

usio

nI

1888

5-6

.518

885

5,x

2x

A168

8, A

708

3332

0970

3085

035

2187

5

Syvä

ri N

ilsiä

lam

prop

hyre

I17

845

-0.6

1784

523

621

JD-0

570

2244

035

5402

0

Vuot

järv

i Nils

iäla

mpr

ophy

reI

1790

4-0

.317

904

231

21JA

-05

7008

800

3566

160


132132

Nam

e/ L

ocat

ion

Rock

type

(dyk

e tr

end)

D(1

Age

(Ma)

±ep

sAg

e (M

a)(4

±Re

f*Ag

e (M

a)±

n*Re

f*sa

mpl

esco

mm

ents

Map

YKJ-N

orth

YKJ-E

ast

U-P

bU

-Pb

Sm-N

dSm

-Nd

Kuhm

o bl

ock

(in t

he L

entu

a co

mpl

ex):

Viia

nki (

VD)

boni

nite

-nor

ite

dyke

(NE)

K24

40?

-1.2

2395

(m

in)

x24

0946

5x

A135

644

4171

8975

236

4632

9

Lohi

särk

käm

afic

dyke

(WN

W)

K23

2121

1.5

2321

21x

1x

A191

472

1852

736

1003

1

Mus

tikka

rinn

em

afic

dyke

K22

1010

0.7

2210

10x

1x

A198

7104

520

3564

650

Arol

a“K

arja

lite/

GW

A”K

2210

200.

622

1020

721

2575

57

4412

7150

240

3598

495

Koli

“Kar

jalit

e/ G

WA”

K22

1020

1.0

2210

207

2201

5810

7A1

220,

A12

21pl

ag e

xclu

ded

4313

1169

9543

636

5017

7

Kape

a-ah

oFe

-tho

l dyk

e (E

)K

2133

290.

721

3329

5x

A121

2E-

tren

d. c

omb.

, Sm

-Nd:

4423

0771

6459

836

3216

0

Ora

vaar

a To

hmäj

ärvi

mafi

c in

trus

ion

2103

81.

621

038

11, x

311

, xA3

9869

0481

836

7337

1

Veits

ivaa

raFe

-tho

l dyk

e (E

tr)

K20

5440

0.3

2054

403

xA1

489C

2106

±40

(MSW

D=1

.9)

4423

7168

402

3611

623

Veits

ivaa

raFe

-tho

l dyk

e (N

W)

K20

0540

0.3

2005

404

xA1

489B

NW

-tre

nd.

com

b. S

m-N

d:44

2371

6721

336

1188

0

Hat

tuse

lkon

enm

afic

dyke

K19

899

-0.7

1989

922

122

A207

143

4102

7058

711

3655

111

Kivi

kevä

tti (

KD)

Fe-t

hol d

yke

(NW

)K

1980

40.

319

804

x20

1433

3x

A140

920

10±2

6 (M

SWD

=0.6

)44

2371

8753

136

3242

5

Rom

uvaa

rabo

nini

tic &

Fe

-tho

leiit

ic d

ykes

K?

12x

ROM

op

en s

yste

ms?

4413

1271

2684

036

4365

7

Pauk

kaja

vaar

ath

olei

itic

dyke

(NW

)K

1963

81.

419

638

1019

6741

410

A114

443

3101

6984

529

3655

282

Perä

aho

Ilom

ants

im

afic

dyke

K19

7313

-0.6

1973

13x

1x

A151

9

5311

0169

8991

237

3017

7

Niin

ivaa

ra K

aavi

lam

prop

hyre

K17

844

0.4

1784

423

121

A159

4311

0769

8966

435

9496

6

Panj

avaa

raca

rbon

atite

K18

00?

-0.3

730

OKU

-1-J2

-2, …

4312

7026

000

3593

000

Out

okum

pu/

Hor

sman

aho

mafi

c in

trus

ion

K19

7118

2.5

1971

1811

411

, xO

KU-6

02,…

6970

714

3614

763

Out

okum

pu/

Huu

toko

ski

mafi

c in

trus

ion

K19

595

0.5

1959

519

2x

A102

969

8125

136

2279

2

Tab

le 1

. Con

t.

133133


Nam

e/ L

ocat

ion

Rock

type

(dyk

e tr

end)

D(1

Age

(Ma)

±ep

sAg

e (M

a)(4

±Re

f*Ag

e (M

a)±

n*Re

f*sa

mpl

esco

mm

ents

Map

YKJ-N

orth

YKJ-E

ast

U-P

bU

-Pb

Sm-N

dSm

-Nd

Volc

anic

roc

ksM

öykk

elm

ä/

Kuus

amo

Gr

ande

site

sL

2400

?-3

.71

17A1

435

3714

0675

1050

034

7315

0

Män

tyva

ara

Fm/

Kuus

amo

Gr

kom

atiit

esL

2400

?-2

.48

1746

2112

7427

294

3586

268

Kunt

ijärv

i Fm

/ Ku

usam

o G

ran

desi

tes

T24

00?

-2.7

5x

R1/7

9/ 4

.4m

youn

ger

than

A1

868

(242

8±4

Ma)

4524

0673

4063

436

0769

5

Mat

inva

ara

Fm/

Kurk

ikyl

ä G

r an

desi

tes

T24

00?

-2.8

3x

A654

3531

1072

2914

035

3854

0

Vari

snie

mi F

mba

salts

I24

00?

-2.1

4x

7140

447

3526

517

Hon

kava

ara

Fm/

Soda

nkyl

ä G

rba

salts

L22

50?

-1.9

6x

A969

3712

1074

9190

034

5660

0

Petä

jäva

ara

Fm/

Soda

nkyl

ä G

rba

salts

T

2250

?2.

82

x10

J-HAH

-80

(“Gre

enst

one

Fm

II”, K

uusa

mo)

4613

0473

4180

536

0539

6

Ruuk

inva

ara

Fm/ S

odan

kylä

G

r

basa

ltsT

2250

?0.

64

x11

B-H

AH-8

0(“G

reen

ston

e Fm

III

”, Ku

usam

o)46

1306

7364

349

3605

716

Runk

aus

Fm/

Kiva

lo G

rba

salts

(1. fl

ow)

P23

00?

0.0

1216

A985

etc

.25

4408

7329

965

3428

578

Runk

aus

Fm/

Kiva

lo G

rba

salts

(oth

er fl

ows)

P23

00?

-1.0

1416

2544

0873

2996

534

2857

8

Jout

tiaap

a Fm

/ Ki

valo

Gr

basa

ltsP

2140

?3.

921

0550

1616

, xA1

009

etc.

age

infe

rred

fr

om A

2087

2631

0873

5470

233

9502

1

Tika

nmaa

Fm

/ Ki

valo

Gr

tuffi

tes

P21

30?

4.1

2x

olde

r th

an A

1214

(2

130+

-5 M

a)73

4314

534

1783

7

Hir

sim

aa F

m/

Kiva

lo G

rtu

ffite

sP

2106

70.

921

067

291

xA1

788

2633

0473

4634

234

1869

5

Jees

iöro

va

Satt

asva

ara

Fmko

mat

iites

L20

5625

3.9

2056

2512

18ra

nge

of e

psilo

n27

3409

7512

831

3434

872

Jees

iöro

va

Satt

asva

ara

Fmko

mat

iites

L20

5625

2.1

1118

rang

e of

eps

ilon

2734

0975

1101

734

3460

0

Satt

asva

ara

Fm/

Savu

kosk

i Gr

kom

atiit

esL

2060

?3±

120

x66

21 (8

5/95

2-4)

open

sys

tem

s37

1405

7502

820

3473

360

Peur

amaa

/ Sa

vuko

ski G

rpi

crite

sL

2060

?3.

72

1880

0.56

-LVP

86ag

e, c

f. Je

esiö

rova

2741

1075

2819

834

0549

6

Link

upal

o Fm

/ Sa

vuko

ski G

rba

salts

, var

iolit

ic la

vaL

2060

?1.

112

xA9

7227

3206

7519

277

3386

699

Vesm

ajär

vi/

Kitt

ilä S

uite

basa

ltsL

2017

53.

820

175

1219

8241

1517

cf. f

elsi

c po

rphy

ry37

2107

7520

904

3439

534

Kaut

osel

kä/

Kitt

ilä S

uite

basa

ltsL

2017

?-1

.26

17ag

e in

ferr

ed

from

Ves

maj

.37

1208

7507

727

3443

041

Siili

njär

vi

Park

kila

ba

salts

I20

625

-2.6

x2

20ag

e in

ferr

ed

from

A48

133

3108

6997

430

3526

520

Siili

njär

vi

Vuor

imäk

iba

salts

I20

625

2.2

x20

8961

620

age

infe

rred

fr

om A

481

3331

0869

9628

035

2852

0

Kiih

tely

svaa

ra

Hyy

piä

basa

ltsK

2100

?-1

.71

272B

4241

6930

826

3670

161

Tab

le 1

. Con

t.


134134

D(1

: Dom

ain,

L=

Lapl

and,

P=P

udas

järv

i co

mpl

ex, T

=Tai

valk

oski

blo

ck in

Len

tua

com

plex

(& P

ääjä

rvi),

K=

Kuhm

o bl

ock

in L

entu

a co

mpl

ex, I

=Iis

alm

i com

plex

Fm=F

orm

atio

n, G

r= G

roup

Age

(Ma)

eps

Age

(Ma)

(4: U

-Pb

age

in M

a, (m

in)=

min

imum

ag

e de

duce

d fr

om d

isco

rdan

t ana

lyse

sn*

: num

ber

of S

m-N

d an

alys

es

Ref*

: Ref

eren

ce fo

r is

otop

e da

ta:

x) T

his

volu

me

1) M

utan

en &

Huh

ma

(200

1)2)

Per

ttun

en &

Vaa

sjok

i (20

01)

3) K

yläk

oski

et a

l. 20

124)

Man

nine

n &

Huh

ma

(200

1)5)

Paa

vola

(198

8)6)

Höl

ttä

et a

l. (2

000)

7) H

ansk

i et a

l. (2

010)

8) R

äsän

en &

Huh

ma

(200

1)9)

Tal

vitie

& P

aarm

a (1

980)

10) V

uollo

et a

l. (1

992)

11) H

uhm

a (1

986)

12) R

asta

s et

al.

(200

1)13

) Pel

tone

n et

al.

(199

6, 1

998)

14) T

oivo

la e

t al.

(199

1)15

) Han

ski e

t al.

(200

1c)

16) H

uhm

a et

al.

(199

0)

17) H

ansk

i & H

uhm

a (2

005)

18) H

ansk

i et a

l. (2

001b

)19

) Pel

tone

n et

al.

2008

20) L

ahtin

en e

t al.

2015

b21

) Woo

dard

& H

uhm

a 20

1522

) Mik

kola

et a

l. 20

1323

) Woo

dard

et a

l. (2

014)

24) V

äänä

nen

(200

4)25

) Ala

piet

i (19

82)

26) K

arin

en (2

010)

27) H

uhm

a (1

987)

28) S

alm

inen

et a

l. 20

1429

) Kar

hu e

t al.

2007

30) T

orpp

a &

Kar

hu 2

007

Tab

le 1

. Con

t.

Tsohkoaivi

Vengasvaara Koitelainen

Akanvaara Peuratunturi

Koulumaoiva Penikat

Kemi

Lehtomaa Salla

Loljunmaa Pääjärvi 35-VEN

Suoperä Pääjärvi 42-VEN

Pääjärvi 38-VEN

Porttivaara

Syöte Viianki

Uolevinlehto Värriö

Paha Kapustasuo

Onkamonlehto

Taivalkoski Kallioniemi Pääjärvi 39-VEN

Törninkuru

Lohisärkkä

Humppi db Siunaussalmi

Karkuvaara

Petäiskangas

Tilsanvaara Raatelampi

Haaskalehto

Tilsanvaara

Mustikkarinne Runkausvaara

Murhiniemi

Kettukallio

Silmäsvaara

Ahvenvaara

Koli

Arola

Hirsikangas

Rantavaara

Tanhua Kannusvaara Koppakumpu Tervola

Oravaara

Kuusivaara Nieminen Kapea-aho

Veitsivaara Kontioluoma

Rytijänkä

Palomaa

Otanmäki

Koivuvaara

Keivitsa

Keivitsa Ni PGE Rovasvaara

Moskuvaara

Puijärvi

Satovaara

Selkäsenvuoma

Veitsivaara

Nuttio

Pittarova Kittilä

Hattuselkonen

Kivikevätti

Peräaho

Koirakoski Paukkajavaara Jormua Eastern block

Mantle primitive

NMORB

EMORB

OIB

Crust, bulk

Crust, Upper Crust, Middle

Crust, Lower

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0 10 20 30 40

147 S

m/14

4 Nd

Nd (ppm)

Mafic intrusions

2.4-2.5 Ga

2.3-2.4 Ga

2.22 Ga

2.1-2.15 Ga

1.96-2.06 Ga

A)

-1.8 -5.3

-1.8 -2.4 -2.1

-1.2

-1.6

-1.6

-3.4

-1.2 -1.3

-2.4 -2.2

1.4

-1.3 -1.0 -1.2

-0.3 -2.5

-0.6

-0.7

1.6 0.4

0.8 1.0

1.5

1.2 1.5

1.7

1.0

0.9 -1.0

0.4 0.4

0.7 0.5

-0.1

-0.4

0.0

0.8

0.9 0.9

0.3

3.2

1.7 3.2

1.6

3.5 2.5

0.4 0.6 -0.7

1.0

1.0

-0.9

-0.1

-3.5

-6.3 -4.3

-5.1

0.3

-0.5

2.8

0.2

2.3

-0.4

-0.7

0.2

-0.6

0.5

1.2 2.1

Mantle primitive

NMORB

EMORB

OIB

Crust, bulk


Crust, Lower

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0 10 20 30 40

Mafic intrusions

2.4-2.5 Ga 2.3-2.4 Ga ca. 2.22 Ga 2.1-2.15 Ga 1.96-2.06 Ga

Nd (ppm)

147 S

m/14

4 Nd

B)

135135


126. 147Sm/144Nd vs. Nd (ppm) diagram for mafic intrusive rocks divided into different age groups, with Figure B giving the initial εNd values for intrusions named in Figure A. One representative whole-rock analysis was selected from each intrusive unit. The age grouping for few samples is not based on absolute dating but is inferred (Värriö, Pääjärvi 42-VEN, Kontioluoma, Murhiniemi, Hirsikangas). Global reference compositions are from Rudnick & Gao (2003) and Klein (2003).


12.3 The 2.44–2.50 Ga intrusions and dykes

Since the pioneering U–Pb dating studies by Kouvo (1977), the large 2.44 Ga mafic layered intrusions in northern Fennoscandia have been targets for sev-eral isotopic studies (e.g., Alapieti 1982, Mutanen & Huhma 2001, Hanski et al. 2001c). These intrusions manifest the first major Palaeoproterozoic mafic magmatism and have generally been considered a sign of break-up of an Archaean (super)conti-nent (Buchan et al. 2000, Mertanen et al. 1999). Our studies have brought new occurrences to this family, i.e., the mafic intrusions of Tshokkoaivi in NW Lapland (ca. 2.5 Ga), Lehtomaa, Peuratunturi and Koulumaoiva in eastern Lapland, Vengasvaara in the Pudasjärvi block and Junttilanniemi in the Kainuu schist belt, and several dykes cutting the Archaean crust.

In their review of the published geochrono-logical data on the 2.44–2.50 Ga magmatism in the Fennoscandian Shield, Hanski and Melezhik (2012) critically evaluated the notion of two major apparently distinct magmatic phases, with the older one having taken place at ca. 2.50 Ga and

the younger one at ca. 2.44–2.45 Ga. The younger one is widespread, being found as intrusions and lavas in different parts of the Archaean basement of the shield. In contrast, the older event is only represented by three mafic–ultramafic layered intrusions (Monchegorsk, Fedorovo-Pansky, Mt. Generalskaya) dated by Amelin et al. (1995), occur-ring in the Kola Peninsula, i.e. in the Kola craton, with no examples found in the Karelian craton. The isotopic data published by Amelin et al. (1995) were generated in a highly respected geochronological laboratory (the Royal Ontario Museum, University of Toronto), and there appear to be no reasons to doubt their validity. However, there is a problem with the geochronological data obtained from the three mentioned intrusions, because other studies have produced variable dates from a single intru-sion, ranging down to ca. 2.44 Ga (see Fig. 3.10 in Hanski & Melezhik 2012). Regarding the dating results of Amelin et al. (1995) as trustworthy ages for the three Kola craton intrusions, the establish-ment of the presence of two age populations has

136136

Mäntyvaara Fm

Kuntijärvi Fm

Matinvaara Fm Möykkelmä

Honkavaara Fm

Petäjävaara Fm

Ruukinvaara Fm

Runkaus Fm

Runkaus Fm, 1. flow

Jouttiaapa Fm, upper part

Jouttiaapa Fm, lower part

Hirsimaa Fm Hyypiä

Jeesiörova komatiite cumulates

Jeesiörova komatiites

Peuramaa Linkupalo Fm

Vesmajärvi

Köngäs, Vesmajärvi

Kautoselkä Fm.

Pechenga Fe-picrite

Pechenga tholeiite

Siilinjärvi, lower Parkkila

Siilinjärvi, Vehkasuo

Siilinjärvi, Vuorimäki

Mantle primitive

NMORB

EMORB

OIB Crust, bulk


Crust, Lower

Island arc tholeiite

Island arc calc-alk basalt BCR-1

Tikanmaa Fm

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0 10 20 30 40 50 60 70 80

147 S

m/14

4 Nd

Nd (ppm)

Mafic volcanic rocks

ca. 2.4 Ga

2.2-2.3 Ga?

2.1-2.15 Ga

1.98-2.06 Ga

Text color - initial epsilon: red >+1.5, blue <-1.5

Fig. 127. 147Sm/144Nd vs. Nd (ppm) diagram for mafic-ultramafic volcanic rocks. One representative whole-rock analysis is selected from each rock formation. Global reference compositions are from Rudnick & Gao (2003) and Klein (2003).

implications for the potential correlation of early Palaeoproterozoic magmatic and related ore-for-mation events between different cratons (Karelia, Kola, Superior, Hearne, Wyoming). Within the Fennoscandian Shield, the Kola and Karelian cratons are separated by the Belomorian mobile belt and the Lapland granulite belt, and they are thought to have collided together during the Kola-Lapland orog-eny at ca. 1.91–1.93 Ga (Daly et al. 2001, Lahtinen et al. 2005). Thus, the cratons potentially had dif-ferent histories before that tectonic event, and the magmatism in the early Palaeoproterozoic might have been different, consistent with the appar-ent lack of ca. 2.50 Ga intrusions in the Karelian craton. However, the situation is not as simple, as this work has shown that the Karelian craton was magmatically not entirely dormant at ca. 2.50 Ga. The new isotopic data for the Tshokkoaivi layered intrusion in NW Lapland indicate that its emplace-ment took place at 2499 ± 11 Ma. Also, some felsic rocks (Rookkiaapa Formation) in Central Lapland have an age close to 2.50 Ga.

Rocks of the 2.44–2.50 Ga group are gener-ally characterised by negative initial εNd values of –1 to –2 (Figs. 125, 126b). Similar negative initial εNd values have been obtained for 2.4–2.5 Ga mafic

intrusions in Russia (e.g., Amelin & Semenov 1996, Balashov et al. 1993), and many felsic lithologies related to the layered intrusions also share this feature (e.g., Lauri et al. 2006). Moreover, roughly coeval mafic and ultramafic metavolcanic rocks, for example, the Mäntyvaara Formation (Kuusamo Group) in the Salla area, eastern Lapland, provide initial εNd values at 2.44 Ga close to –2 (Hanski & Huhma 2005; Figs. 125, 127). The age of the Mäntyvaara Formation is constrained by the cutting Onkamonlehto dyke (A1405), providing a minimum age of 2.4 Ga for the volcanic rocks. These LREE-enriched komatiites resemble the komatiites from the Vetreny belt (Puchtel et al. 1997). All these mafic rocks of the “2.44 Ga” group with negative initial εNd values are characterised by significantly LREE-enriched chondrite-normalised REE patterns (Figs. 126, 127).

Based on their field relationships and geochemi-cal and isotopic characteristics, the 2.4–2.5 Ga dyke swarms have been divided into five groups by Vuollo & Huhma (2005): (1) NE–SW-trending boninite-noritic dykes, (2) NW–SE-trending gabbro-norite dykes, (3) NW–SE-trending tholeiitic dykes, (4) NW–SE- and E–W-trending Fe-tholeiitic dykes, and (5) E–W-trending orthopyroxene-plagioclase-phyric

137137


0.510

0.511

0.512

0.513

0.514

0.04 0.08 0.12 0.16 0.20 0.24 0.28

143 N

d/14

4 Nd

147Sm/144Nd

"2.22 Ga intrusions" Age = 2223 ± 28 Ma

eps = +0.6 MSWD = 3.0 n=21

"2.44 Ga intrusions" Age = 2446 ± 31 Ma

eps = -1.9 MSWD = 7.7 n=50

Keivitsa intrusion Age = 2053 ± 26 Ma

eps = -3.3 MSWD = 7.2 n=27

Fig. 128. Sm–Nd isochron diagram illustrating the limited variation in the initial ratio within each rock associa-tion. Data sources are presented in Table 1.


dykes. Groups 1 and 2 represent a (boninitic) magma type with negative initial εNd(T) values, consistent with the results obtained for most layered intru-sions. These dykes may be coeval with the 2.44 Ga intrusions and relate to boninite-like Cr-rich (group 1) and Cr-poor (group 2) parental magmas. The data on groups 3 (A1410 Uolevinlehto) and 4 (A1414 Pääjärvi, A1466 Taivalkoski) show slightly posi-tive initial εNd(T) values and are related to tholei-itic parental magmas. The chemical distinction is also evident in the 147Sm/144Nd vs. Nd diagram (Fig. 126), as the Fe-tholeiites tend to have higher Nd and Sm/Nd ratios. These Fe-tholeiitic dykes appear to share isotopic signatures with slightly younger 2.3 Ga mafic rocks.

The low εNd values suggest that the REE were largely derived from lithospheric sources that were enriched in LREE in late Archaean times. Debate has surrounded whether this is a feature inherited from the subcontinental lithosphere or is due to crustal contamination deep in the crust or in the final magma chamber. In spite of a few exceptions, the available Sm–Nd data on the layered intrusions suggest that the variation in the initial Nd isotope composition is very limited over the shield and within intrusions (Huhma et al. 1990, Tolstikhin et al. 1992, Balashov et al. 1993, Amelin and Semenov 1996, Hanski et al. 2001c, Hanski 2012, this study). This is shown in the isochron diagram (Fig. 128), in which 50 Sm–Nd analyses conducted on all intru-sions in Finland give an “age” of 2446 ± 31 Ma and an εNd value of –1.9. The MSWD of 7.7 sug-gests scatter in excess of analytical error, which is mainly due to metamorphic effects experienced by the Koitelainen and Akanvaara intrusions (Hanski et al. 2001c), not due to variation between intru-sions. A possible exception is provided by the large Burakovka Complex in Russia, where εNd(T) in the lower zone appears to be marginally higher (from –1 to 0) than in the upper zone (from -2 to -1, Amelin & Semenov 1996). A similar trend has also

been observed in the world’s largest layered intru-sion, the Bushveld Complex, where Sm–Nd data suggest that crustal contamination in the upper part of the intrusion was slightly higher than in the lower part (Maier et al. 2000).

According to Mutanen (1997), several lines of pet-rographic, geochemical and mineralogical evidence suggest that crustal contamination at the final site of emplacement has been common in large layered intrusions. Provided that during emplacement the magma was already contaminated by crustal mate-rial at depth, it is conceivable that the further in situ contamination processes may be minor in terms of isotope diagnostics.

Based on the combined Nd and Os isotopic sys-tematics of the Koitelainen and Akanvaara intru-sions, Hanski et al. (2001c) suggested that extensive crustal contamination took place deep in the crust, while in situ contamination after the final emplace-ment of the magma was relatively insignificant. The fact that the Os isotope compositions remained almost immune to crustal contamination, whereas the Nd isotope compositions were strongly affected, led Hanski et al. (2001c) to conclude that the pri-mary magma producing the layered intrusions was a high-Os, low-Nd magma, potentially a komati-ite or komatiitic basalt. They suggested that this magma evolved through ACF processes in a deep-crustal magma chamber into a contaminated, low-Ti basaltic composition, and it was later emplaced into upper crustal Archaean gneisses and overly-ing, broadly coeval volcanic rocks. This hypothesis is supported by the Nd–Os isotopic systematics of the contemporaneous, crustally contaminated komatiitic basalts in the Onkamo Group (Hanski & Huhma 2005) and in the Vetreny belt, Russian Karelia (Puchtel et al. 2001). It is generally thought that the formation of 2.45 Ga rocks was related to upwelling of a mantle plume, which was related to the fracturing and rifting of a supercontinent.

12.4 The 2.3 Ga mafic rocks

The reduction in magmatic activity on Earth begin-ning about 2.45 Ga and lasting for 200–250 Ma has been emphasised by Condie et al. (2009) and is also evident in the database obtained from Finland (e.g., Huhma et al. 2011). However, several new occur-rences of ca. 2.3 Ga mafic intrusions and dykes have been recognised during recent studies. Currently, U–Pb results are available from six localities yield-

ing ages between 2295 ± 5 Ma and 2339 ± 18 Ma. In addition to these, Sm–Nd mineral data from two other sites suggest emplacement ages within this range. All samples in this age group are located in eastern Finland, in the Lentua and Iisalmi com-plexes. NW–SE-trending ca. 2.31 Ga Fe tholeiitic dykes have also been identified in Russian Karelia (Stepanova et al. 2014a). The Sm–Nd results for

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all these rocks provide clearly positive initial Nd epsilon values, suggesting that the mafic magma had an origin from a depleted mantle source with-out major communication with Archaean LREE-enriched material (Figs. 125, 126). These rocks are thus very distinct from the majority of the 2.44 Ga magmatism. The dykes and intrusions of the 2.3–2.4 Ga group have relatively high levels of REE with a moderate enrichment in LREE (Fig. 126), and resemble continental tholeiitic basalts in chemi-

cal composition (Vuollo & Fedotov 2005). The dated 2.3 Ga mafic rocks are mainly dykes and do not form sills in the Jatulian metasediments, suggesting that the Jatulian was deposited after a strong post- 2.3 Ga erosional stage of the Karelian craton. The bulk of the Sm–Nd data available on the 2.22–2.44 Ga mafic volcanic rocks do not yield clearly positive initial Nd epsilon values (Table 1). This suggests that these volcanic rocks are not related to the 2.3 Ga high-epsilon dykes.


The differentiated mafic intrusions of the ca. 2.22 Ga gabbro-wehrlite association (GWA) are widespread in eastern and northern Finland, and often located near the Archaean-Palaeoproterozoic unconform-ity, intruding conformably into Jatulian quartzites. In Central Lapland, they occur as sills in quartz-ites of the Sodankylä Group (Fig. 3). Because of the extensive occurrence of these sills in the Karelian formations and the ease of discovering zircon in their gabbroic differentiates, this igneous phase has become the most frequently dated episode of mafic magmatism in Finland, as summarised by Hanski et al. (2001a). Zircon in these rocks often displays a turbid colour and porous texture due to a high degree of metamictisation, resulting in a strong U–Pb discordance, but spot analyses on pristine zir-con domains have still yielded concordant ages of 2210–2220 Ma (Hanski et al. 2010). We have used laser ablation MC-ICP-MS to analyse some previ-ously studied samples and obtained similar results. Despite common albitisation and alteration, the Sm–Nd data from rocks of the GWA association form a coherent group yielding an initial εNd(2220 Ma) value of ca. +0.6 (Fig. 128, Fig. 126; Hanski et al. 2010). This composition was also obtained for a granophyre in the Koli intrusion. As the roof rocks at Koli consist of Archaean gneisses, in situ con-tamination was not important in the genesis of the granophyre or the whole-rock association.

Given the widespread regional distribution of the 2.22 Ga sills in Finland, Hanski et al. (2001a) regarded it as surprising that the existence of simi-lar intrusive bodies has not been reported from the Russian part of the Fennoscandian Shield. Recently, Bingen et al. (2015) published geochronological evi-dence for the presence of 2.22 Ga mafic rocks in the Palaeoproterozoic Kautokeino greenstone belt, northern Norway, which is plausible given the fact that the Karelian formations continue from

Central Finnish Lapland to northern Norway as the Kautokeino and Karasjok belts.

A special feature of the 2.22 Ga magmatism is that it occurs overwhelmingly as sill-like bodies near the Archaean-Palaeoproterozoic unconformity; there are no known volcanic equivalents, and steeply dipping dykes cutting the Archaean basement are very rare. One of the confirmed dykes belonging to this magmatic event is found within the Archaean Kuhmo greenstone belt. It occurs as a mostly ultra-mafic, strongly flow-differentiated dyke, penetrated by drilling at Petäjäniemi, with its strike being approximately north–south, i.e. the same as that of the greenstone belt (Hanski 1984). Differentiated sills, similar to those of the 2.22 Ga magmatism elsewhere, have also been recognised within the Kuhmo belt (Hanski et al. 2010). For example, in the Ensilä area, they trend approximately perpendicu-larly to the narrow greenstone, but are not found in the granitoids outside the belt (Hanski 1984, Tulenheimo 1999). This suggests that the green-stone belt has faulted contacts to the surrounding granitoids in this area and the Palaeoproterozoic intrusions were preserved in a graben-like structure within the belt due to its post-2.22 Ga downward displacement. The same structure may have also preserved some potentially Jatulian-age quartzites within the belt (at Hietaperä), also known to be spa-tially associated with 2.22 Ga sills. In summary, as opposed to the several mafic dyke swarms of the basement, the emplacement of the 2.22 Ga intru-sions appears to be unrelated to fracturing of the Archaean crust due to shield-wide tensional forces. Instead, they were emplaced as horizontal sills with an extensive lateral flow of magma and strong in situ gravitative differentiation. The sills were fed from sparse, rarely identified magma conduits, which were channelled through earlier zones of weakness, such as the narrow Kuhmo greenstone belt.

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12.6 The 2.1–2.15 Ga mafic rocks

Ages of 2.1–2.15 Ga have been obtained for mafic rocks from ca. 20 localities, mostly from north-ern Finland (Räsänen & Huhma 2001, Perttunen & Vaasjoki 2001, Manninen et al. 2001, Pekkarinen & Lukkarinen 1991, Huhma 1986, this paper). The intrusions analysed for Sm–Nd isotopes are characterised by positive εNd(T) values (Fig. 126), some approaching the composition of the (model) depleted mantle (Fig. 125). The most positive εNd(T) value for the 2.1 Ga mafic rocks (+4.2) has been obtained for the Jouttiaapa Formation metabasalts in the Peräpohja belt (Huhma et al. 1990). In this for-mation, the first lava flows are extremely depleted in LREE, and the Sm–Nd data on 16 whole-rock samples yield an age of 2103 ± 50 Ma for the major Sm–Nd fractionation, which can be also considered as an estimate for the age of eruption (recalculated using three additional analyses). Recent analyses of the roughly coeval Tikanmaa Formation as well as two dykes (Koppakumpu, Kuusivaara) yielded similar positive εNd values (Fig. 125c). These dykes

also provide a minimum age of 2130 ± 5 Ma for the Jouttiaapa Formation.

The E–W-trending Fe-tholeiitic dykes in the Kuhmo block appear to belong in this age group (e.g. A1212 Kapea-aho), whereas the NW–SE-trending dykes (e.g. A1409 Kivikevätti) are younger, being ca. 2 Ga. Dykes of this family have also been rec-ognised from Russian Karelia by Stepanova et al. (2014b), who report ages of 2140 ± 3 Ma and an initial εNd of +3.

The positive εNd(T) may indicate that major attenuation of the lithosphere took place at ca. 2.1 Ga and eventually allowed material from the con-vective mantle to escape to the surface. This is also well exemplified by the Jeesiörova (and Sattasvaara) komatiites (Savukoski Group) in Central Lapland, for which a slightly younger age of 2056 ± 25 Ma was obtained using several Sm–Nd analyses on pyroxene separates and whole-rock samples (Hanski et al. 2001b).

12.7 The 1.95–2.06 Ga mafic rocks

Mafic intrusions with an age of ca. 2050 Ma exist in many places in northern Finland, particularly within the Savukoski Group (Rastas et al. 2001, Räsänen & Huhma 2001). These include the Ni ore-bearing Kevitsa mafic-ultramafic body (Mutanen 1997, Mutanen & Huhma 2001) and recently dated Moskuvaara, Puijärvi and Satovaara intrusions. In central Finland, this age group is represented by the Otanmäki intrusion (Talvitie & Paarma 1980). The available Sm–Nd results provide initial Nd isotope compositions very distinct from the coeval depleted mantle and suggest the involvement of old enriched lithosphere in their genesis (Fig. 125). In particu-lar, the Ni–PGE-bearing ore type from Kevitsa with εNd(T) of –6.4 has Nd isotope characteristics typi-cal for late Archaean crust, suggesting significant crustal contamination (Huhma et al. 1995, Hanski et al. 1997, Hanski & Huhma 2005).

Rifting of the Archaean continent finally led to its breakup and the formation of seafloor at ca. 2.0 Ga. The ~2015 Ma felsic porphyries and the associated (Vesmajärvi Formation) mafic metavolcanic rocks have yielded initial εNd compositions close to the depleted mantle (Fig. 125). This, together with the overall characteristics, has led to the conclusion

that the Kittilä Group contains products of bimodal magmatism generated within an oceanic domain (Hanski & Huhma 2005).

The other two sites of voluminous ca. 2.0 Ga mafic magmatism in the Fennoscandian Shield are the Onega region and Pechenga belt in Russia. In the Onega region, basalts with an initial εNd(T) of +3 suggest an origin from depleted mantle, but the lower initial εNd values in some volcanic rocks have been interpreted to result from significant crus-tal contamination, and in this sense, these lavas resemble lavas from other continental flood basalt provinces (Puchtel et al. 1998, Huhma et al. unpub-lished). The Pechenga ferropicrites have provided a Sm–Nd age of 1977 ± 52 Ma, which was supported by U–Pb zircon analysis on a felsic rock (Hanski et al. 1990, Hanski 1992, Hanski et al. 2014). The initial εNd of +1.4 suggests that the source had slight time-integrated LREE depletion. A similar εNd(T) value was also obtained from the nearby Nyasyukka dyke, which has yielded well-constrained ages of 1941 ± 3 Ma (U–Pb on baddeleyite) and 1956 ± 19 Ma (Sm–Nd ) (Smolkin et al. 2015). The analyses con-ducted on associated tholeiitic basalts, instead, have an initial εNd value of +3.5 and suggest an

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origin from a depleted mantle, which also appears to be the case with slightly older Kolosjoki Formation tholeiites (Hanski et al. 2014).

Several Sm–Nd analyses are also available from the Jormua ophiolite complex, which has been dated at 1953 ± 5 Ma by U–Pb on zircon (Peltonen et al. 1996) and refined here to 1952 ± 2 Ma using concordant CA-TIMS analyses. The Jormua ophi-olite represents a 1.95 Ga seafloor from an ocean to continent transition zone that mainly consisted of Archaean subcontinental lithospheric mantle (Peltonen et al. 2003). The E-MORB-type basalt suite yielded a Sm–Nd age of 1936 ± 43 Ma and an initial εNd value of +2, whereas the OIB-type dykes in the mantle peridotites gave a Sm–Nd age of 1968 ± 58 Ma with a nearly chondritic initial εNd(T) (Peltonen et al. 1996, 1998). The source for the main E-MORB-type suite is thus not directly

connected to the depleted mantle, which is rep-resented, for example, by the high εNd(T) basalts from the Vesmajärvi Formation or the Kevitsa dyke (Fig. 125a).

Other alkaline rocks associated with the 2.0 Ga rifting occur ca. 180 km north of Jormua, where the Kortejärvi and Laivajoki carbonatites are found in a N–S-trending shear zone between two Archaean blocks (Nykänen et al. 1997). In contrast to the Jormua OIB, these carbonatites provide a clearly positive initial εNd(T) of ca. +2.5 (Fig. 125b). As these rocks have high concentrations of REE, the obtained initial Nd isotope composition should well repre-sent the average mantle source that contributed to the formation of carbonatitic magma at 2.0 Ga. Evidently, the mantle below the Archaean craton was heterogeneous.

13 CLOSING REMARKS

Nearly 400 Sm–Nd analyses of whole-rock samples and mineral separates from ca. 70 mafic intrusions and ten mafic volcanic formations in the Finnish Karelia province are reported in this volume. These data, together with abundant U–Pb age results and the previously published Sm–Nd analyses on ca. 30 mafic rock units, provide tools for constrain-ing igneous petrogenesis and the Palaeoproterozoic evolution in the Fennoscandian Shield. Over a time period of 700 Ma, mafic magmas were emplaced in several episodes occurring at ca. 2.50 Ga, 2.44 Ga, 2.3 Ga, 2.22 Ga, 2.15 Ga, 2.12 Ga, 2.05 Ga, 2.0 Ga, 1.95 Ga, 1.88 Ga and 1.78 Ga. Many of the rock associations formed during these events may be regarded as examples of ancient large igneous provinces, in this case related to long-lasting episodic rifting of the Archaean lithosphere.

The emphasis of the Sm–Nd studies was on most pristine mafic rocks available, which generally pro-duced Sm–Nd mineral ages consistent with the available U–Pb zircon ages. As many of the initial εNd values were based on Sm–Nd mineral isochrons, they should give reliable estimates of the initial iso-tope composition of the studied rocks. The initial εNd values range from very positive to strongly nega-tive and suggest that some rocks were derived from a depleted mantle source, whereas others have a large contribution from old enriched lithosphere. Contamination of ultramafic magma in well-mixed

reservoirs deep in the crust may explain many fea-tures observed in mafic-ultramafic rocks, such as the near constant εNd of -2 in the 2.44 Ga intrusions, but the isotopic results also show that various man-tle sources with distinct isotope compositions have existed during the Palaeoproterozoic. Examples are provided by high-REE mantle-derived rocks showing a range of initial εNd values from nearly chondritic (e.g., 2.6 Ga Siilinjärvi carbonatite, 2.0 Ga Jormua OIB, 1.8 Ga lamprophyres) to highly positive (e.g., the 2.0 Ga Laivajoki and Kortejärvi carbonatites).

Contamination with country rock material in the final site of intrusion has also been considered important in modifying the chemical composition of some rocks, for example in the 2.06 Ga Kevitsa mafic intrusion, with an εNd value of –3.4, and par-ticularly the Kevitsa Ni-PGE mineralisation with an εNd value of –6.4.

The age and initial Nd isotope composition, together with other relevant information, should be used to correlate dykes, intrusions and mafic extrusive units in the Fennoscandian Shield. Although in situ contamination may occasionally cause problems, the Nd isotope studies suggest that the chemical characterisation of various for-mations should provide useful tools for correlation. The results could also be used in correlating events in different cratons, particularly across the Atlantic

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to the Canadian Shield, from where compilations of magmatic events (LIPs) have been published by Ernst & Buchan (2004) and Ernst & Bleeker (2010). Some of these events are analogous on both sides of the Atlantic, but differences also seem to exist. The global occurrence of 2.44 Ga mafic magmatism is well known (e.g., Alapieti 1982, Heaman 1997). It has been termed the Matachewan event in the Superior Province. According to Ernst & Buchan

(2004), 2.3 Ga mafic rocks appear to be absent in Canada, but the 2.2 Ga event is well represented by the Ungava dykes and Nipissing sills. In Finland, 2.2 Ga mafic rocks form layered intrusions and sills close to the Archaean craton margins, but in Canada, they also occur as dyke swarms. The 2.1 Ga event is found in the Superior, for example, as the Marathon linear dyke swarm.

ACKNOWLEDGEMENTS

We want to express our gratitude to Olavi Kouvo for the farsighted vision he showed when initiating the isotope research in Finland. His contribution to the data presented in this paper is also enormous. The personnel of the isotope laboratory are also greatly acknowledged, particularly Tuula Hokkanen, Arto Pulkkinen, Leena Järvinen, Matti Karhunen, Marita Niemelä, Mirja Saarinen and Hugh O’Brien.

We acknowledge the laboratories in Stockholm (NORDSIM/ Martin Whitehouse) and St. Petersburg (VSEGEI, SHRIMP/ Sergei Sergeev) for SIMS analy-ses and Sandra Kamo in Toronto for U–Pb bad-deleyite data.

The NORDSIM facility has been supported by the Research Councils of Denmark, Norway and Sweden, the Geological Survey of Finland and the Swedish

Museum of Natural History. This is NORDSIM con-tribution number #547.

Tom Andersen is acknowledged for providing the program used for MC-ICPMS data reduction.

Excellent sample material was provided by seve-ral geologists: Tapani Mutanen, Vesa Perttunen, Jorma Räsänen, Petri Peltonen, Perttu Mikkola, Vesa Nykänen, Jorma Palmen, Tuomo Karinen, Juha Karhu, Jorma Paavola, Erkki Luukkonen and Heikki Juopperi. We appreciate discussion with these geologists, as well as discussions with Pentti Hölttä, Jarmo Kohonen and Raimo Lahtinen, and comments on the manuscript by Jouni Luukas. The review comments by Stefan Claesson are greatly acknowledged. E.H. acknowledges support from Academy of Finland grant 281859.

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Mafic igneous rocks have been a major focus of isotope research since the establishment of the isotope laboratory at the Geological Survey of Finland in the 1960’s. This volume presents abundant isotope data on Palaeoproterozoic mafic rocks of the Karelia Province in eastern and northern Finland. The previously unpublished data consist of ca. 400 Sm-Nd analyses on 80 mafic rock formations and ca. 1700 U-Pb anal-yses by TIMS, SIMS and LA-MC-ICP-MS on 80 samples. These results together with published material provide tools for constraining the age and petrogenesis of mafic magmas and the overall geological evolution in the Fennoscandian Shield.

ISBN 978-952-217-394-2 (PDF)ISSN 0367-522X (print)ISSN 2489-639X (online)

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Sm–Nd and U–Pb isotope geochemistry of the ...tupa.gtk.fi/julkaisu/bulletin/bt_405.pdf · Sm–Nd and U–Pb isotope geochemistry of the Palaeoproterozoic mafic magmatism in eastern