ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2017

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1595

Deep subduction of theSeve Nappe Complex in theScandinavian Caledonides

IWONA KLONOWSKA

ISSN 1651-6214ISBN 978-91-513-0139-6urn:nbn:se:uu:diva-332525

Dissertation presented at Uppsala University to be publicly examined in Hambergsalen,Villavägen 16, Uppsala, Friday, 8 December 2017 at 10:00 for the degree of Doctor ofPhilosophy. The examination will be conducted in English. Faculty examiner: Professor JaneGilotti (University of Iowa).

AbstractKlonowska, I. 2017. Deep subduction of the Seve Nappe Complex in the ScandinavianCaledonides. Digital Comprehensive Summaries of Uppsala Dissertations from theFaculty of Science and Technology 1595. 63 pp. Uppsala: Acta Universitatis Upsaliensis.ISBN 978-91-513-0139-6.

This thesis seeks to improve our understanding of the processes involved in continental collisionzones, with a particular focus on subduction-exhumation. The main objective of this work hasbeen to define the tectonometamorphic evolution of the deeply subducted Seve Nappe Complex(SNC) in the Scandinavian Caledonides. I utilize mineralogy, petrology and geochronology toconstrain the P-T-t paths of the SNC rocks in Sweden.

The research has focused on the high grade rocks of the SNC and resulted in the discovery ofmetamorphic diamonds within the gneisses in west-central Jämtland and southern Västerbotten.Microdiamonds provided evidence for the ultra-high pressure metamorphism (UHPM) andsubduction of continental rocks to mantle depths. The UHPM in these rocks was confirmedby calculations of the P-T conditions. The UHPM is further recorded by eclogites and garnetpyroxenites from northern Jämtland and eclogites from Norrbotten. All these findings providecompelling evidence for regional UHPM of vast parts of the SNC (at least 400 km along thestrike of this allochthonous unit). The SNC rocks followed nearly isothermal decompressionpaths and paragneisses have locally experienced partial melting during exhumation. Formationof the peculiar Ba- and Ti-enriched dark mica in the Tväråklumparna metasediments is relatedto the latter stage.

In-situ monazite dating of the diamond-bearing gneisses from west-central Jämtland supportsprevious geochronological data inferring that the peak of metamorphism is probably MiddleOrdovician and was followed by Early Silurian partial melting. The exact timing of the UHPMhere still remains to be resolved. The Lu-Hf garnet and U-Pb zircon dating of eclogite and gneissfrom northern Jämtland confirms the Middle Ordovician age of the UHP-HP metamorphism ofthe SNC rocks. The chemical dating of monazite from the Marsfjället gneiss suggests an earlierUHP history of the Seve rocks in southern Västerbotten as a post-UHP uplift is dated to ca.470 Ma.

Based on the P-T-t data obtained in this thesis, particularly on the evidence for MiddleOrdovician UHPM and subsequent Silurian exhumation, a new tectonic model for theScandinavian Caledonides has been proposed. The outcomes of this thesis therefore improveour understanding of the tectonometamorphic history of the Caledonides.

Keywords: geothermobarometry, thermodynamic modelling, Raman spectroscopy, diamond,ultrahigh-pressure metamorphism, eclogite, peridotite, gneiss

Iwona Klonowska, Department of Earth Sciences, Villav. 16, Uppsala University, SE-75236Uppsala, Sweden.

© Iwona Klonowska 2017

ISSN 1651-6214ISBN 978-91-513-0139-6urn:nbn:se:uu:diva-332525 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-332525)

For my family and friends

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Majka, J., Rosén, Å., Janák, M., Froitzheim, N., Klonowska, I.,

Manecki, M., Sasinková, V. and Yoshida, K. (2014) Microdia-mond discovered in the Seve Nappe (Scandinavian Caledonides) and its exhumation by the “vacuum-cleaner” mechanism. Geol-ogy, 42: 1107–1110.

II Klonowska, I., Janák, M., Majka, J., Froitzheim, N. and Kośmińska, K. (2016) Eclogite and garnet pyroxenite from Stor Jougdan, Seve Nappe Complex, Sweden: implications for UHP metamorphism of allochthons in the Scandinavian Caledonides. Journal of Metamorphic Geology, 34: 103-119.

III Klonowska, I., Janák, M., Majka, J., Petrík, I., Froitzheim, N., Gee, D.G. and Sasinková V. (2017) Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Com-plex of the Scandinavian Caledonides. Journal of Metamorphic Geology, 35: 541–564.

IV Fassmer, K., Klonowska, I., Walczak, K., Andersson, B., Froitz-heim, N., Majka, J., Fonseca, R.O.C., Münker, C., Janák, M. and Whitehouse, M. Middle Ordovician subduction of continental crust in the Scandinavian Caledonides - an example from Tje-liken, Seve Nappe Complex, Sweden (in review)

V Bukała, M., Klonowska, I., Barnes, C., Majka, J., Kośmińska, K., Janák, M., Fassmer, K., Broman, C., Luptáková, J. UHP met-amorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P-T path and tectonic implications (submit-ted)

VI Majka, J., Kruszewski, L., Rosén, Å. and Klonowska, I. (2015) Ba- and Ti-enriched dark mica from the UHP metasediments of the Seve Nappe Complex, Swedish Caledonides. Mineralogia, 46: 41-50.

VII Klonowska, I., Janák, M., Majka, J., Petrík I., Holmberg, J., Sas-inková, V. and Yoshida, K. Lower Ordovician UHP metamor-phism of the Baltoscandian margin recorded by the Seve Nappe Complex in southern Västerbotten, Scandinavian Caledonides (manuscript)

Reprints were made with permission from the respective publishers. The au-thor also contributed to the following journal publications that are not included in the thesis:

• Klonowska, I., Majka, J., Janák, M., Gee, D.G. and Ladenberger, A.

(2014). Pressure-temperature evolution of a kyanite-garnet pelitic gneiss from Åreskutan: implications for (U)HP metamorphism of the Seve Nappe Com-plex, west-central Jämtland, Swedish Caledonides, in Corfu, F., Gasser, D., and Chew, D.M., eds. New Perspectives on the Caledonides of Scandinavia and Related Areas: Geological Society of London, Special Publications, 390: 321-336.

• Majka, J., Janák, M., Andersson, B., Klonowska, I., Gee, D.G., Rosén,

Å. and Kośmińska, K. (2014) Pressure-temperature estimates on the Tjeliken eclogite: new insights into (ultra)-high pressure evolution of the Seve Nappe Complex in the Scandinavian Caledonides, in Corfu, F., Gasser, D., and Chew, D.M., eds. New Perspectives on the Caledonides of Scandinavia and Related Areas: Geological Society of London, Special Publications, 390: 369–384.

• Kośmińska, K., Majka, J., Mazur, S., Krumbholz, M., Klonowska, I.,

Manecki, M., Czerny, J. and Dwornik, M. (2014) Blueschist facies metamor-phism in Nordenskiold Land of west-central Svalbard. Terra Nova, 26: 377-386.

• Meade F.C., Troll V.R., Ellam R.M., Freda C., Font L., Donaldson C.H.

and Klonowska I. (2014) Bimodal magmatism produced by progressively in-hibited crustal assimilation. Nature Communications, 5: 1-11.

• Lorenz, H., Rosberg, J.E., Juhlin, C., Bjelm, L., Almqvist, B.S.G., Berthet, T., Conze, R., Gee, D.G., Klonowska, I., Pascal, C., Pedersen, K., Roberts, N.M.W. and Tsang, C.F. (2015) COSC-1–drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia. Sci-entific Drilling, 19: 1–11.

• Almqvist, B.S.G., Biedermann, A.R., Klonowska, I. and Misra, S. (2015)

Petrofabric development during experimental partial melting and recrystalli-zation of a mica‐schist analogue. Geochemistry, Geophysics, Geosystems, 16(10): 3472–3483.

• Almqvist, B.S.G., Misra, S., Klonowska, I., Mainprice, D. and Majka, J.

(2015) Ultrasonic velocity drops and anisotropy reduction in mica-schist ana-logues due to melting with implications for seismic imaging of continental crust. Earth and Planetary Science Letters, 425: 24-33.

• Majka, J., Mazur, S., Kośmińska, K., Dudek, K. and Klonowska, I.

(2016) Pressure-temperature estimates of the blueschists from the Kopina Mt., northern Bohemian Massif, Poland - constraints on subduction of the Saxo-thuringian continental margin. European Journal of Mineralogy, 28: 1-11.

Personal Contributions

The papers included in this thesis are the result of collaboration with several co-workers. My individual contributions to each paper are listed below:

Paper I: I took part in the field work and rock sampling at Tväråklumparna Mt. I worked together with Å. Rosén in the Raman Laboratory in Kraków, prepared some of the figures and shared the writing. Paper II: I collected samples by the lake Stor Jougdan, carried out the EMP analyses together with J. Majka and M. Janák, performed the geothermo-barometic calculations and the thermodynamic modelling, interpreted the data, made the illustrations and shared the writing.

Paper III: I collected samples, carried out the EMP and Raman spectroscopy analyses, performed the thermodynamic modelling, interpreted the data, made the illustrations and shared the writing. Paper IV: I collected gneiss samples, prepared the zircons and ran the SIMS analyses, carried out the EMP analyses, performed the thermodynamic mod-elling, interpreted the data, made part of the illustrations and shared the writ-ing.

Paper V: I carried out Raman spectroscopy analyses, helped with the EMP and the thermodynamic modelling, and shared the writing. Paper VI: I took part in the field work and rock sampling, carried out the EMP analyses together with J. Majka and Å. Rosén, helped with the text and the illustrations.

Paper VII: I took part in the field work and rock sampling, carried out the EMP and Raman spectroscopy analyses, performed the thermodynamic mod-elling, interpreted the data, made the illustrations and shared the writing.

Contents

1. Introduction ............................................................................................... 13 1.1. Scope of the thesis ............................................................................. 13 1.2. Ultra-high pressure metamorphism (UHPM) .................................... 15 1.3. Subduction-exhumation processes .................................................... 16 

2. Geological background – Seve Nappe Complex in the Scandinavian Caledonides ................................................................................................... 18 

3. Methods .................................................................................................... 24 3.1. Elemental microanalysis.................................................................... 24 3.2. Whole rock analysis .......................................................................... 25 3.3. Raman spectroscopy .......................................................................... 25 3.4. Conventional geothermobarometry ................................................... 26 3.5. Thermodynamic modelling ............................................................... 26 3.6. Ti-in-quartz geothermometry ............................................................ 27 3.7. Zr-in-rutile geothermometry .............................................................. 27 3.8. Quartz-in-garnet geobarometry ......................................................... 28 3.9. Geochronology .................................................................................. 29 

3.9.1. Th-U-Pb monazite dating .......................................................... 29 3.9.2. U-Pb zircon dating ..................................................................... 29 3.9.3. Lu-Hf garnet dating ................................................................... 30 

4. Summary of the papers ............................................................................. 31 4.1. Paper I ............................................................................................... 31 4.2. Paper II .............................................................................................. 34 4.3. Paper III ............................................................................................. 37 4.4. Paper IV............................................................................................. 38 4.5. Paper V .............................................................................................. 41 4.6. Paper VI............................................................................................. 43 4.7. Paper VII ........................................................................................... 45 

5. Conclusions and future directions ............................................................. 47 

Summary in Swedish .................................................................................... 51 

Acknowledgement ........................................................................................ 53 

References ..................................................................................................... 56 

Abbreviations

BSE EDS FE-EPMA FWHM HP LOI P-T-t SEI SIMS SNC QuiG TitaniQ UHPM WDS WGR XRF

Backscattered electron Energy Dispersive Spectroscopy Field Emission Electron Probe Micro Analyzer Full width at half maximum High Pressure Loss on ignition Pressure-Temperature-time Secondary Electron Image Secondary Ion Mass Spectroscopy Seve Nappe Complex Quartz-in-garnet geobarometry Titanium-in-quartz geothermometry Ultrahigh-pressure metamorphism Wavelength Dispersive Spectroscopy Western Gneiss Region X-ray Fluorescence

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1. Introduction

1.1. Scope of the thesis High-pressure metamorphic mineralogy and petrology is a rapidly developing branch of geology. Studies of high-pressure rocks provide the basis for an im-proved understanding of major continental collision zones and can provide models for other collisional belts, including the Himalaya that resulted from a long period of intra-continental thrusting.

The recent discovery of evidence for ultrahigh-pressure metamorphism (UHPM) in the Seve Nappe Complex (SNC) in the Scandinavian Caledonides in Sweden (Janák et al. 2013a; Klonowska et al. 2014) is providing the basis for new investigations of the Caledonian subduction history of the Baltoscandian margin of continent Baltica. The Caledonian bedrock in the mountains of western Scandinavia provides a comprehensive geological cross-section of a Himalayan-type orogen. It formed as a result of the collision of the continents Laurentia and Baltica, but many of the mountain building processes involved still remain enigmatic, including Ordovician subduction-collision events which can be traced in the allochthonous units thrust onto the Baltoscandian platform, as well as mechanisms governing exhumation of these allochthons. Of particular interest for this thesis is the Middle Alloch-thon, which represents the Baltoscandian rifted margin and continent-ocean transition zone. The Middle Allochthon overlies the Lower Allochthon, com-prising successions deposited on the Baltoscandian platform and the foreland basins, and it is overlain by the Upper Allochthon (Iapetus derived units) and the Uppermost Allochthon (terranes with Laurentian margin affinity; Gee 1975a; Stephens & Gee 1985; Gee et al. 2013). The Seve Nappe Complex is the highest grade unit of the Middle Allochthon and bears evidence of deep subduction of the continental crust already during the Ordovician. Thus, it serves as a key for understanding the fifty million year period of the Iapetus ocean closure and formation of the Scandinavian Caledonides. Notably, these traces of the high- to ultrahigh-pressure (HP-UHP) metamorphism within the SNC are chronologically much earlier (ca. 30-60 Myr) than the long-studied Western Gneiss Region (WGR), an archetypical UHP area within this moun-tain belt. In the WGR, mainly represented by parautochthonous Baltoscandian basement, the UHPM is restricted to the western allochthonous parts and was formed during the final stages of the Scandian collision of Baltica and Lau-

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rentia in the Early Devonian; it provides little evidence of the earlier subduc-tion history. Therefore, there is a need for detailed study of the Middle Allochthon (especially the SNC) to explore the Ordovician-Silurian history and derive a comprehensive model of Caledonian orogeny in Scandinavia.

The project is based on mineralogical, petrological and geochronological studies of the SNC lithologies. Petrographic observations and quantitative P-T-t data generated in this thesis contribute to a better understanding of deep subduction-exhumation processes in continental collision zones and improve our knowledge about the tectonometamorphic evolution of the Scandinavian Caledonides. Moreover, this project is connected to the ICDP "Collisional Orogeny in the Scandinavian Caledonides (COSC)" drilling programme and provides important insights into the project’s main objective to develop a coherent tectonic model for Scandinavian Caledonides and its application to other orogens.

Interestingly, the Åreskutan Mt. (part of the SNC) was already a key local-ity for understanding the long distance thrusting in the Caledonides in the 19th century. A theory was developed in 1888 by the Swedish geologist Alfred Elis Törnebohm (Fig. 1), which has become generally accepted after many years of mapping and laboratory studies; our on-going work is discovering new evidence of Åreskutan’s even deeper and longer journey, down into the mantle and back to the surface.

Figure 1. A drawing showing Törnebohm’s theory of a thrusting in the Caledonian orogen (Törnebohm 1888, 1896). Cartoon by E. Erdmann (1896) in Gee and Kum-pulainen (1980).

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1.2. Ultra-high pressure metamorphism (UHPM) Discovery of coesite, an index mineral of ultrahigh-pressure metamorphism (Fig. 2), in the early 1980’s in crustal rocks from two different orogens revo-lutionalized our view on mountain building processes. The first coesite was discovered in pyrope-bearing quartzites from Dora Maira in Italy by Chopin (1984) and this discovery was followed shortly after by Smith (1984) who identified coesite in an eclogite from the WGR in Norway. The second UHPM index mineral, diamond was discovered at Kumdy Kol, Kokchetav massif in Kazakhstan by Sobolev and Shatsky (1990) in three different lithologies including gneisses, schists and eclogites. All of the metamorphic diamonds (generally µm in size) from these rocks were found as inclusions in garnet. Since these discoveries, the role of continental crust in geodynamic processes has had to be reconsidered. The clear evidence for subduction of the buoyant continental crust to mantle depths started a new chapter in petrology called ultrahigh-pressure metamorphism.

UHPM can be diagnosed not only by the presence of the index minerals but also by calculation of P-T conditions that plot at pressures in the stability field of coesite (Gilotti 2013; Fig. 2). Also, additional information about the former UHPM can be read from pseudomorphs after UHP minerals formed during transformation to their lower pressure equivalents and characteristic textures devoloped during such transition. A good example is the coesite-to-quartz transformation, where the coesite pseudomorph - polycrystalline quartz forms radial cracks within the rigid host mineral, e.g. garnet. In this study I have focused on a careful search for UHPM indicators using microscopic observa-tions (UHP minerals and their pseudomorphs), Raman spectroscopy and phase equilibrium modelling.

Gilotti (2013), following Liou et al. (2009), presented a map of the UHP terranes recognized worldwide (Fig. 3). There are more than 20 diamond and/or coasite-bearing terranes known around the world, the majority situated in Phanerozoic continent–continent collision belts. Thanks to the research in this thesis, we can add three new localities to the world’s map from the Seve Nappe Complex in the Scandinavian Caledonides (Fig. 3; see also Fig. 4 for more details).

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Figure 2. Metamorphic facies diagram highlighting a field of ultrahigh-pressure (UHP) metamorphism (yellow). A lower boundary of the UHPM is defined by the quartz-coesite transition line.

Figure 3. Map of worldwide ultrahigh-pressure (UHP) terranes, from Gilotti (2013), with updated information from this thesis for the Seve Nappe Complex (SNC) in the Scandinavian Caledonides in Sweden.

1.3. Subduction-exhumation processes Recognition of the UHPM within increasing number of collisional orogens has led to a focus on seeking explanations for deep subduction and subsequent exhumation of continental crust from mantle depths. The early models were mainly based on the assumption that the UHP units can be exhumed by the

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remaining positive buoyancy of the SiO2-rich continental rocks (e.g. Che-menda et al. 1995). However, if the terrane is subducted to UHP conditions, the transformation of quartz (2.65 g/cm3) to coesite (2.92 g/cm3) results in an increase of the rocks’ density and therefore the decrease of buoyancy. Hence, a purely buoyancy driven mechanism being responsible for exhumation of the UHP terranes is highly unlikely. As summarized in work by Hacker et al. (2013), many of the UHP terranes vary in size, rate of exhumation and/or de-gree of melting during exhumation. Some on them have never made it back to crustal levels. Numerical models supported by field observations and pres-sure-temperature-time-deformation (P-T-t-D) data have resulted in develop-ment of a wide range of mechanisms for subduction-exhumation of UHP terranes. Numerical and conceptual models include e.g. slab break-off and eduction (e.g. Duretz et al. 2012), extensional collapse (e.g. Platt 1993), the Plunger effect (Warren et al. 2008), traction-driven corner flow and serpentin-isation of the mantle wedge (e.g. Gerya & Stöckhert 2006), slab rollback (e.g. Brun & Faccenna 2008), sideway tectonic removal of overburden (Boutelier & Chemenda 2008; Webb et al. 2008) and extraction (e.g. Froitzheim et al. 2003; Shyu et al. 2011).

Many of the thrust sheets in the Seve Nappe Complex are dominated by SiO2-rich metasedimentary rocks, others comprise metamorphosed basalts, gabbros and doleritic dyke swarms. Eclogitization is widespread but not in all units. The mechanism driving the subduction-exhumation of this UHPM com-plex remains to be resolved.

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2. Geological background – Seve Nappe Complex in the Scandinavian Caledonides

The SNC (Fig. 4) has been recognized over a distance of 1000 km along the length of the Caledonides in Sweden (e.g. Zachrisson 1973); it has been in-ferred to reach both northwards into northern Norway and southwards into southwesternmost parts of Norway (e.g. Gee et al. 2013). These allochthons have been inferred to be derived from the Baltoscandian outer margin and continent-ocean transition zone (Gee 1975b; van Roermund 1982; Andréas-son 1994; Andréasson et al., 1998). Cryogenian rifting led to early Ediacaran opening of the Iapetus Ocean with the intrusion of dolerite dyke swarms (Solyom et al. 1979; Hollocher et al. 2007) at ca. 600 Ma (Svenningsen 2001). The innermost parts of the margin are lacking dykes and the number of these mafic intrusions increases towards the outermost margin, where in addition to dyke swarms gabbros and ultramafites occur. Substantial parts of this margin were subducted during the closure of the Iapetus Ocean in the Ordovician and metamorphosed to different degrees (Gee et al. 2013). By the Early Silurian the two continents Baltica and Laurentia were colliding (e.g. Gee 1975b; Dallmeyer & Gee 1986; Gee et al. 2013)

In this study we have focused on the SNC areas where the former evidence for at least HP metamorphism has been previously reported either by the pres-ence of eclogites (northern Jämtland and southern Norrbotten) or the estima-tion of eclogite-facies P-T conditions in areas where eclogites were suspected, but had not been confirmed (west-central Jämtland and southern Västerbot-ten).

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Figure 4. Tectonostratigraphic map of the Scandinavian Caledonides, modified after Gee et al. (2013). Localities where HP and UHP metamorphism has been recognized within the Seve Nappe Complex are indicated: Tväråklumparna (Snasahögarna Mountains; Papers I & VI) and Åreskutan (Paper III) – central Jämtland; Saxnäs (Paper VII) – southern Västerbotten; Friningen, Stor Jougdan (Paper II), Tjeliken (Paper IV) and Sippmikk – northern Jämtland; Tsäkkok and Vaimok (Paper V) – southern Norrbotten; Torneträsk and Treriksröset – northern Norrbotten; SNC-like Lindås and Jæren nappes – southern Norway. In bold are marked the localities stud-ied in this thesis.

In west-central Jämtland (Papers I, III & VI) the SNC is subdivided into Lower, Middle and Upper Seve nappes (Sjöstrom 1983; Zachrisson & Sjöstrand 1990, Fig. 5). The Lower Seve is dominated by quartzofeldspathic metasediments (mainly psammitic) and marbles, with abundant amphibo-litized dolerite dykes (Arnbom 1980). Subordinate are gabbros and perido-tites. It is metamorphosed under amphibolite, perhaps higher facies conditions

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and is extensively overprinted by retrogression in greenschist facies. Within the Midde Seve Nappe, the metasedimentary rocks are more pelitic and the unit is of higher metamorphic grade. It is dominated by paragneisses and mig-matites with subordinate metabasites and cut by pegmatitic dykes. No eclo-gites have been found so far in this area. The Upper Seve mostly consist of amphibolites with minor metasediments.

Figure 5. Simplified geological map of west-central Jämtland area, Sweden. Locali-ties of diamond-bearing gneisses from the Middle Seve Nappe are marked. Modified from maps in Papers I & III.

In northern Jämtland the SNC is also divided into subunits (Fig. 6; Papers II & IV). Here within both Lower and Middle Seve nappes eclogites and garnet peridotites occur (van Roermund & Bakker, 1984; Strömberg et al., 1984; van Roermund, 1985; Zachrisson & Sjöstrand, 1990). The Lower Seve is domi-nated by psammites with mica schists and amphibolites, and subordinate gneisses. The Middle Seve rocks are more pelitic and gneisses here show par-tial melting, in contrast to the Lower Seve gneisses where no migmatization is observed.

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Figure 6. Simplified geological map of northern Jämtland and southern Västerbotten area, Sweden. Modified from Gee et al. (2013). Localities of UHP rocks in Jämtland are marked by black stars (bold text shows localities from this study; Papers II & V). Marsfjället UHP gneiss is marked by diamond (Paper VII).

In southern Västerbotten (Fig. 6; Paper VII) the same subdivision as for the northern Jämtland applies. However, eclogites sensu-stricto have not been re-ported from the southernmost parts of this area. The Middle Seve here consists of gneisses and migmatites, with minor pyroxene-garnet–bearing metabasites (Trouw 1973). The Upper Seve is dominated by amphibolites and mica schists with minor gneisses and ultramafics.

In Norrbotten the SNC is subdivided into Vaimok, Sarek and Tsäkkok lenses (from bottom to top, respectively; Fig. 7). Eclogites are found within Vaimok and Tsäkkok lenses (e.g. Nicholson 1984; Stephens & van Roermund 1984; Kullerud et al. 1990; Albrecht 2000). The area discussed in Paper V, the Vaimok lens is further divided into the Lower Seve Nappe (eclogite-ab-sent) and the Grapesvare and Maddåive nappes (eclogite-bearing).

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Figure 7. Simplified bedrock map of the Vuoggatjålme area (southern Norrbotten, Sweden) compiled after Albrecht (2000) and Bedrock Map K222:2 of Swedish Geo-logical Survey. From Paper V.

Even farther north, at Torneträsk, perhaps as far north as Treriksröset (Fig. 4) the occurrence of eclogites in the SNC has been described by Kathol (1989). No further research of the rocks from this area has been done.

In southwestern Norway, within the high grade units interpreted as being equivalents of the SNC (Fig. 4), the Jæaren and Lindås nappes, eclogites and eclogitized lower crustal granulites occur (e.g. Austrheim 1987; Jamtveit et al. 1990; Smit et al. 2008; Roffeis et al. 2012).

In addition to the far-travelled SNC that is part of the Middle Allochton, HP-UHPM is known from two other structural units. One occurs withtin the structurally highest allochthonous unit, the Tromsø Nappe in the Uppermost Allochthon in northern Norway (Fig. 4) where UHP eclogites and diamond-bearing gneisses occur (Ravna & Roux 2006; Janák et al. 2012; Janák et al. 2013b). Although the Tromsø Nappe has been interpreted to be a part of the Uppermost Allochthon (Gee et al. 1985), it has been proposed by Janák et al. (2012) that it may well be an out-of-sequence thrust sheet and a part of the Baltoscandian margin like the SNC. The other and the lowest structural HP-UHPM complex is the famous WGR in southwestern Norway (e.g. Smith 1984; Dobrzhinetskaya et al. 1995; Cuthbert et al. 2000; Carswell et al. 2003, 2006; Root et al. 2005; Butler et al. 2013) and its possible northern equivalent – Lofoten (Froitzheim et al. 2016).

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Regarding the timing of the metamorphism in the SNC, in west-central Jämtland the migmatization age of the Middle Seve Nappe is constrained by various methods to ca. 440 Ma (e.g. Claesson 1982; Williams & Claesson 1987; Gromet et al. 1996; Ladenberger et al. 2014; Majka et al. 2012). A mon-azite age of ca. 455 Ma was interpreted by Majka et al. (2012) to record the near peak pressure conditions. The thrust emplacement was dated to ca. 424 Ma (Majka et al. 2012). In northern Jämtland, the peak stage of metamorphism is dated to ca. 460 Ma (Sm-Nd; Brueckner & van Roermund 2007), but also younger age of ca. 446 Ma was recorded (U-Pb TIMS zircon age; Root & Corfu 2012). In southern Västerbotten the Sm-Nd age of ca. 462; Ma inter-preted as reflecting prograde or peak stage of metamorphism was derived by Grimmer et al. (2015). The same authors have dated the exhumation of the Upper and Middle Seve in this area to ca. 434 Ma. The high grade metamor-phism in Norrbotten is generally older than in other parts of the SNC and ranges from ca. 500 Ma to 475 Ma (Ar/Ar, Dallmeyer & Gee 1986; Sm/Nd, Mørk et al. 1988; U/Pb titanite, Essex et al. 1997; U/Pb zircon, Root & Corfu 2012) related to early Caledonian collision with an island arc.

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3. Methods

3.1. Elemental microanalysis An electron-probe microanalyser (EPMA) is an instrument that was used in all Papers to provide raw data in the form of major element oxides (Wave-length Dispersive Spectroscopy (WDS) spot analyses and Energy Dispersive Spectroscopy (EDS) spot and areal analyses), X-ray elemental maps and images obtained by backscattered electron (BSE) and cathodoluminescence imaging (CL).

EPMA analyses were carried out using different instruments in following

laboratories:

Jeol JXA8530F Hyperprobe Field Emission Electron Probe Microa-nalyser (FE-EPMA) at the Department of Earth Sciences, Uppsala Uni-versity, Sweden. This instrument was used for WDS and EDS analyses (Papers I-VII), EDS areal measurements (Papers II & III), X-ray map-ping (Papers III & V) and BSE (Papers I-V & VII) and CL (Paper IV) imaging.

Jeol SuperProbe electron microprobe at the Faculty of Geology, Geophys-ics and Environment Protection, AGH University of Science and Tech-nology (AGH-UST) in Kraków, Poland. We used this instrument for standard major element oxides WDS and EDS analyses, Zr-in-Rutile measurements, BSE imaging and X-ray mapping (Paper V) and CL im-aging (Paper IV).

CAMECA SX-100 electron microprobe at the Dionýz Štúr Institute of Geology in Bratislava, Slovakia. This instrument was used for standard mineral chemistry WDS analyses and BSE imaging (Papers II, III & VII) and monazite dating (Papers III & VII).

JEOL Superprobe JXA 8200 at the Steinmann Institute in Bonn, Ger-many. BSE images, mineral chemistry and element maps were meas-ured/obtained with this instrument (Paper V).

Information regarding the specific run conditions and mineral standards used can be found in each Paper.

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In addition to EPMA, a secondary electron microscope (SEM) was utilized for observations of microdiamond morphology. SE imaging (SEI) was per-formed at the Institute of Materials and Machine Mechanics, Slovak Academy of Sciences in Bratislava, Slovakia using field-emission scanning microscope (FE-SEM) JEOL JSM 7600F equipped with the EDAX microanalytical sys-tem. The SEI images were acquired at voltage of 10kV (Papers I, III & VII).

3.2. Whole rock analysis The bulk composition of the eclogite from Paper IV was analyzed using a PANanalytical Axios X-Ray fluorescence (XRF) spectrometer at the Stein-mann Institute, Bonn and of the gneiss with a LiBO2/Li2B4O7 fusion ICP-ES located at the ACME labs in Canada. The bulk-rock composition of the eclo-gites from Papers II & V was analysed with a LiBO2/Li2B4O7 fusion followed by XRF at the Bureau Veritas Mineral Laboratories in Canada. In addition, the areal EDS measurements of a thin section were used to obtain the bulk composition of the rocks in Papers II & III.

3.3. Raman spectroscopy Identification of diamonds and associated micro inclusions was performed in three different laboratories at (a) Institute of Chemistry, Slovak Academy of Sciences in Bratislava, Slovakia (Papers I, III & VII) (b) AGH University of Science and Technology in Kraków, Poland (Paper I) and (c) Department of Geology and Mineralogy, Kyoto University, Japan using spectrometers Thermo Scientific DXR (in a&b) and NRS-3100 (JASCO; in c; Papers I & VII). The Ar-ion laser with a green 514.5 nm line and the Nd:YAG laser with 532 nm line were used. The instrument in Bratislava was also used for the Raman mapping (Paper III).

Micro-Raman spectroscopy was also used to measure quartz inclusions in garnet (Paper V). For this purpose we used a Horiba spectrometer LabRAM HR 800 equipped with an Olympus BX41 light microscope in two laborato-ries: (a) at the Department of Geological Sciences, Stockholm University, Sweden and (b) at the Earth Science Institute, Slovak Academy of Sciences, Banská Bystrica, Slovakia. Raman spectra were excited by 514 nm and 532 nm lasers.

The measurements were performed at room temperature. The collected Ra-man spectra were further analyzed using Fityk software version 0.9.3 (Wojdyr 2010).

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3.4. Conventional geothermobarometry Conventional, or “classical”, geothermobarometry is used to calculate P-T metamorphic conditions based on specific chemical reactions (assuming equi-librium) between different mineral pairs. A range of geothermobarometric methods was applied in this study.

For eclogites (Papers II, V), we have used calculations for garnet + clino-pyroxene + phengite mineral assemblage using coupled method of Fe2+-Mg exchange thermometer for garnet + clinopyroxene (Ravna 2000) and net-transfer geobarometer for reaction 6diopside + 3muscovite = 3celadonite + 2grossular + pyrope (Ravna & Terry 2004). This approach provides reliable P estimates in phengite eclogites (Ravna & Terry 2004), however, obtained T might be underestimated due to uncertainties in Fe2+/Fe3+ ratio in pyroxene, providing an error of ± 60 °C (Ravna & Paquin 2003).

For garnet pyroxenites (Paper II), a combination of Fe-Mg exchange and ‘solvus’ thermometers, together with a single pyroxene barometer, was used for a mineral assemblage garnet + orthopyroxene + clinopyroxene. The calcu-lations were done for the following mineral pairs: garnet-orthopyroxene (Har-ley 1984), garnet-clinopyroxene (Ravna 2000) and orthopyroxene-clinopy-roxene (‘solvus’, Brey & Köhler 1990). For pressure calculations the Al-in-orthopyroxene was taken (Mac Gregor 1974; Nickel & Green 1985; Brey & Köhler 1990). PTEXL computational spread sheet created by T. Koehler and A. Girnis was used.

3.5. Thermodynamic modelling In addition to conventional geothermobarometric methods, thermodynamic (phase equilibrium) modelling is a standard method used in a metamorphic petrology to calculate P-T conditions and reconstruct the metamorphic evolu-tion of a rock. A pseudosection is a particular type of phase diagram that is calculated for a specific bulk-rock composition showing different stability fields of mineral assemblages in equilibrium. Pseudosections are a function of two variables, the most common variety is a P-T pseudosection, but also e.g. P-H2O (at constant T) or T-CO2 (at constant P) pseudosections can be calcu-lated. Gibbs free energy of a mineral is a basis of the thermodynamic model-ling used by petrologists. There are several programs that have been developed to calculate thermodynamic equilibria and create phase diagrams. Each of them requires an internally consistent database of reliable thermodynamic data.

In my research I have used a Perple_X software (Connolly, 1990, 2005) with an internally consistent dataset of Holland and Powell (1998; hp02ver.dat). P-H2O (Papers II, III & VII) and P-T pseudosections (Papers II, III, IV, V & VII) were calculated for different lithologies including gneiss,

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eclogite and garnet pyroxenite. The bulk rock composition was either obtained by XRF analyses or by areal EDS measurements on a thin section. For samples in which chemical zoning in minerals was preserved and for which different stages of the P-T path were calculated, the effective bulk rock composition was acquired by extraction of the average composition of the mineral’s zone of interest and its modal proportion from the initial bulk composition.

3.6. Ti-in-quartz geothermometry Ti-in-quartz geothermometry (Wark & Watson 2006; Thomas et al. 2010, 2015) is an experimentally calibrated and reliable method especially for rocks formed at temperatures above ∼600 °C. This simple method of T estimation using trace amounts of Ti in quartz is based on substitution of Ti4+ for Si4+ in quartz in the presence of rutile (aTi = 1; Thomas et al. 2010). This method was applied to estimate T of a granulite facies metamorphism in gneiss (Paper III). Titanium in quartz was measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the Department of Chemistry, Masaryk University in Brno, Czech Republic.

3.7. Zr-in-rutile geothermometry Rutile has emerged recently as another mineral that potentially can provide the information about the temperatures recorded in high pressure rocks. Ru-tile, despite the fact that it typically occurs in a low abundance in rocks, con-tains a significant portion of the high field strength element (HFSE) budget. For natural samples in which zirconium (Zr) content in rutile is buffered by quartz and zircon, it has been proven that Zr content in rutile has a first-order temperature dependence (e.g. Zack et al. 2004, Ferry & Watson 2007). Further experimental studies by Tomkins et al. (2007) have shown that zirconium sol-ubility in rutile is also pressure dependent. The authors demonstrated a de-crease in solubility of Zr with increasing pressure, but this relationship seems to diverge from the expected trend in higher pressure conditions (>3.0 GPa). It has been speculated that the change in trend can be related with a change of the substitution mechanisms responsible for Zr incorporation in rutile, but more experimental data is needed to settle this issue (Tomkins et al. 2007). Moreover, rutile inclusions in other minerals (e.g. garnet) provide excellent candidates for Zr-in-rutile geothermometry since they are protected from reequilibration in lower-T conditions and thus record the temperature of their formation. However this approach is limited as it requires rutile to be present in the mineral assemblage for entrapment and preservation of the previous conditions to occur. Matrix rutile, especially in HT and UHT rocks may pro-vide underestimated results due to diffusive reequilibration (Zack et al. 2004).

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Importantly, Kohn et al. (2016) proved rutile grains do not maintain equilib-rium with matrix grains along grains surface. Thus, Zr-in-rutile is a useful, easily applicable geothermometric tool for rutile-bearing rocks. However, this method still has to be used with caution due to remaining questions and un-certainties. Zr-in-rutile was applied in Paper V to calculate temperature of early stages of garnet growth in eclogite. Measurements were performed by a Jeol SuperProbe electron microprobe at the Faculty of Geology, Geophysics and Environment Protection, AGH-UST in Kraków, Poland.

3.8. Quartz-in-garnet geobarometry Quartz-in-garnet geobarometry is a method that is independent of mineral phase equilibria (in contrast to conventional geothermobarometry and phase equilibrium modelling) and is based on systematic peak shifts in Raman spec-tra of quartz as a function of pressure. This newly developed method can be used as a barometer in rocks in which quartz inclusions in garnet are pre-served. This geobarometer exploits contrasting physical properties of both minerals with quartz being a soft and compressible mineral, whereas garnet is nearly incompressible and thus serves as an isotropic host for the quartz (e.g. Enami et al. 2012; Ashley et al. 2014; Kohn et al. 2014). During the growth of garnet, both the host garnet and quartz (inclusion) experience the same P-T conditions. However, during the exhumation the external P-T conditions de-crease. As a result of the differences in elastic properties of these two minerals (as stated above), garnet may preserve the internal pressure of entrapped quartz (Ashley et al. 2014) as the incompressible garnet host will not allow relaxation of quartz grains with decreasing pressure. This physical effect is manifest in the pressure-temperature dependent Raman peaks of α-quartz (Schmidt & Ziemann 2000; Ashley et al. 2014) as the two main bands (iden-tified at atmospheric pressure) at 464 and 206 cm-1 will shift towards higher wavelengths with increasing pressure recorded by the quartz inclusions. There are three factors influencing the degree of residual pressure (internal pressure of inclusion): entrapment P-T, P-T dependence of both minerals’ molar vol-ume and elastic properties of the host mineral (e.g. Guiraud & Powell 2006; Kohn 2014).

This method was used in Paper V to obtain information about pressure conditions during the early stage of garnet growth (entrapment of quartz in-clusions).

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3.9. Geochronology 3.9.1. Th-U-Pb monazite dating In-situ monazite dating was used in Papers III & VII and the Th-U-Pb age was determined by EPMA. HREE, Y, and Eu contents in monazite provide an excellent record of the mineral phases that coexist(ed) with monazite (e.g. pla-gioclase, garnet and melt). Monazite in metamorphic rocks commonly pre-serves chemical zoning which may reflect stages of monazite growth, thereby preserving multiple stages of metamorphic evolution (e.g. Terry et al. 2000b; Hermann & Rubatto 2003; Kohn et al. 2005; Majka et al. 2012; Petrík et al. 2016; Regis et al. 2016)

The EPMA in Bratislava was used for chemical dating and compositional maps of monazites were obtained by the instrument in Uppsala. For monazite analyses the applied acceleration voltage was 15 kV, beam current was 100-130 nA, and counting times (peak + background) are as follows: 150 s for Pb, 45 s for Th, 75 s for U, 45 s for Y and for all other elements it was 25–35 s. Depending on the thickness of monazite zone, the beam diameter that we used ranged between 3 and 10 μm. Dating was performed by the ‘age calibration’ method. Using this method, an apparent age acquired from a spot-analysis is corrected against five age standards. The standards are accurately dated by isotopes using SHRIMP and TIMS methods. Following standards were used: REE and Y phosphates (XPO4) for REE and Y, ThO2 for Th, UO2 for U, galena (PbS) for Pb, Al2O3 for Al and wollastonite (CaSiO3) for Si and Ca. Further details on this dating methodology are given in Petrík & Konečný (2009). Isoplot software (Ludwig 2001) was used to calculate weighted aver-ages of monazite ages.

3.9.2. U-Pb zircon dating In-situ dating of zircon provides important constraints on the metamorphic evolution of high grade rocks. Metamorphic zircon rims are commonly formed not only during HT conditions, but also at UHP conditions, and in both cases their formation is related to a growth in a presence of a fluid or a melt (e.g. McClelland & Lapen 2013; Rubatto 2017). Paper V includes U-Pb iso-tope data obtained from ion probe analysis of zircon from the high grade gneiss.

Prior to the analysis, zircon separates were obtained from the rock using standard techniques of crushing, sieving, Wilfley water table density separa-tion, magnetic separation, and hand picking under a binocular microscope. The separated zircons were mounted in epoxy together with standards. After polishing, the zircon mount was coated with gold and examined by BSE and CL imaging to select suitable analytical points. Zircon morphology and its textures were carefully examined.

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Secondary ionization mass spectrometry (SIMS) analysis was carried out at the NordSIM laboratory at the Department of Geosciences, Swedish Mu-seum of Natural History, Stockholm, using a CAMECA IMS 1280 ion micro-probe. Analytical procedures of Whitehouse and Kamber (2005) were fol-lowed during the analysis. Depending on thickness of zircon metamorphic zones, a spot size of 10 μm, or smaller, was used. The primary beam current applied was 5 - 10 nA at an acceleration voltage of 13 kV. Analyses used 12 cycles through the mass stations with a mass resolution of 4500. Standard zir-con 91500 (Wiedenbeck et al. 1995) was measured every sixth analysis for instrumental drift correction. In-house Excel macros were used for data reduc-tion. Isoplot 3.75 add-in program for Excel (Ludwig 2012) was used for plot-ting concordia diagrams and age calculations. The amount of common lead (206Pb) in measured 206Pb was estimated from measured 204Pb and corrected using a present day terrestrial Pb isotopic composition (Stacey & Kramers 1975).

3.9.3. Lu-Hf garnet dating Lu-Hf dating of garnet (Paper V) was performed on a Thermo-Neptune MC-ICPMS instrument at Steinmann Institute, Bonn by Kathrin Fassmer, PhD student at University of Bonn in Germany. The Lu-Hf dating of the HP-UHP rocks from the Scandinavian Caledonides is part of her PhD project: Subduc-tion, exhumation and resubduction in the Scandinavian Caledonides: studying the deep levels of a Himalaya-type orogen. Further details about the method can be found in our publication (Paper V).

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4. Summary of the papers

4.1. Paper I Microdiamond discovered in the Seve Nappe (Scandinavian Caledonides) and its exhumation by the “vacuum-cleaner” mechanism In this paper we present the first discovery of metamorphic diamonds in the pelitic gneisses of the Middle Seve Nappe in the Scandinavian Caledonides and at the same time the evidence for the UHPM and the deep subduction of the continental crustal rocks to depths of more than 100 km. The in situ mi-crodiamonds from the SNC at Tväråklumparna Mt. in central Jämtland, Swe-den (Figs 4 & 5) were identified using the microRaman spectroscopy. The new tectonic model for subduction-exhumation processes in the central Scan-dinavian Caledonides was proposed.

The diamond-bearing gneiss is composed primarily of garnet, aluminosili-cates (both sillimanite and kyanite), quartz, biotite, K-feldspar and plagio-clase, with minor phengite, graphite, Al-spinel, rutile and ilmenite. Garnets are chemically homogeneous, almandine dominated (almandine62–64pyrope31–

33grossular3–4spessartine1–2) and contain abundant mineral inclusions. Micro-diamonds of 4 to 7 μm in size were identified within the characteristic micro inclusion clusters within garnet porphyroblasts (Fig. 8a-b); some of the clus-ters resemble the euhedral shape of the garnet (Fig. 8a). The Raman peak of Tväråklumparna diamonds occurs between 1327-1330 cm-1 (Fig. 8c) and is downshifted in comparison to the Raman peak of ideal diamond (1332 ± 0.5 cm-1; Solin and Ramdas, 1970). The Raman band of the investigated crystals is also relatively wide (FWHM = 6-22 cm-1 vs 1.65 ± 0.02 cm-1 of ideal dia-mond). The SEI of the diamond inclusion (Fig. 8d) shows its spheroidal, bleb like shape, a direct and cohesive contact with the host garnet and associated cavities. The cavities may indicate former presence of fluids in the inclusions that are now exposed on the surface.

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Figure 8. (a-b) Microphotograph of the typical micro inclusion clusters in garnet (Grt) within which diamonds are identified. In (a) some of the clusters outline the former euhedral shape of garnet. (c) Selected Raman spectra of diamond (Dia; up-per) and diamond together with graphite (Dia + Gr; lower). Diamond band occurs at 1328 cm-1. (d) SEI of microdiamond inclusion in the host garnet. Figs (c & d) are modified from Majka et al. (2014b; Paper I).

Diamond gives information on the metamorphic history of the Tväråklumparna gneiss, indicating that the rocks reached the UHPM. Unfor-tunately, more precise estimation of the peak metamorphic conditions was im-possible due to the complete homogenization of the garnet porphyroblasts dur-ing the high temperature overprint.

The new recognition of the UHPM within the crustal rocks of the far-trav-elled allochthonous unit, the SNC, in addition to the well-known UHPM of the basement rocks in the WGR required reexamination of the existing tec-tonic models for the formation of the Scandinavian Caledonides and exhuma-tion of the deeply subducted continental rocks in general. The two independ-ent UHP events (ca. 460-450 Ma in the SNC and ca. 420-400 Ma in the WGR) require subduction, exhumation and partial resubduction of the continental crust (Brueckner and van Roermund 2004). The processes involved in such an evolution are still poorly understood, but it is clear that the first occurred prior to Baltica-Laurentia collision and the latter during the continent-continent col-lision.

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We proposed in this paper that the SNC in the central Scandinavian Cale-donides, where in addition to the UHP gneisses, the HP/UHP eclogites and peridotites occur and are hosted by metasedimentary rocks, was subducted to mantle depths during an island arc – continent collision. The exhumation of the diamond-bearing Seve rocks and other deeply subducted UHP continental crustal rocks known worldwide could have been driven by the local pressure reduction caused by the forearc lithospheric block extraction (Fig. 9). This exhumation mechanism, introduced here (Fig. 9), was called the "vacuum cleaner" being based on the model of slab extraction by Froitzheim et al. (2006). The "vacuum cleaner" model was further applied to interpret the tec-tonic evolution of the SNC UHP garnet peridotites/pyroxenites and eclogites from northern Jämtland (Paper II) and the UHP eclogites from Norrbotten (Paper V). The applied model to the Caledonide orogen in Scandinavia is described in more detail in the summary of Paper II.

Figure 9. The "vacuum cleaner" model explaining UHP rocks exhumation for a two subduction zones scenario. The exhumation is driven by subduction/elimination of overlying lithosphere. The middle lithospheric slab (dotted line) sinks away due to its negative buoyancy and causes strong reduction of horizontal compressive stress and opening of two virtual gaps (A, B). The ongoing convergence moves the upper plate and fills the gap “A”. Gap “B” is filled by subducted rocks that rise diapirically due to the remaining buoyancy and tectonic under-pressure. From Majka et al. (2014b; Paper I).

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4.2. Paper II Eclogite and garnet pyroxenite from Stor Jougdan, Seve Nappe Complex, Sweden: implications for UHP metamorphism of allochthons in the Scandina-vian Caledonides

Investigation of the Stor Jougdan eclogites and garnet pyroxenites in northern Jämtland, Sweden (Figs 4 & 6) provided evidence for the UHPM of the Lower Seve Nappe. Conventional geothermobarometry and thermodynamic model-ling applied to both rock types constrained the peak pressure conditions of metamorphism within the coesite stability field. In this paper we have sum-marized all the available data for the peak metamorphic conditions of the SNC HP-UHP rocks as well as of the metamorphic ages from the SNC rocks of northern Jämtland and southern Västerbotten. We highlight the importance of the lateral extent and timing of the HP/UHPM within the different parts of the SNC as well as its kinematics that are crucial for reconstructing the mountain building processes in the Scandinavian Caledonides. While the origin of the HP/UHP eclogites within the SNC is well defined (they represent eclogitized mafic dykes of the Baltica continental margin), the origin and tectonic position of the HP/UHP garnet peridotites and associated pyroxenites is unclear. The tectonic model proposed for the central Scandinavian Caledonides in Paper I provides an alternative scenario for the origin of the SNC garnet peridotites. The new model is discussed with two other previously existing tectonic mod-els proposed by Brueckner and van Roermund (2004) and Gee et al. (2013).

The Stor Jougdan eclogite is a phengite-bearing eclogite that has preserved information about the prograde, peak pressure and retrograde stages of the metamorphic evolution. Particularly well-defined is the peak pressure stage, characterized by the stable mineral assemblage garnet + omphacite + phengite + rutile + coesite(?) which is suitable for the geothermobarometric calcula-tions calibrated by Ravna & Terry (2004) and Ravna (2000) as well as the phase equilibrium modelling. The rock reached P-T conditions of 3.5 GPa and 800 °C (2.8–4.0 GPa and 750–900 °C outlined by the compositional iso-pleths). A small and fresh garnet pyroxenite vein found within the strongly retrogressed garnet peridotite body located around 2 km from the phengite-eclogite consists primarily of garnet, orthopyroxene and clinopyroxene with minor olivine, defining the peak pressure assemblage. Conventional geothermobar-ometric calculations combining several calibrations available for garnet – orthopyroxene, garnet – clinopyroxene and orthopyroxene – clinopyroxene minerals pairs and Al – in – orthopyroxene (MacGregor 1974; Harley 1984; Nickel & Green 1985; Brey & Köhler, 1990; Ravna 2000) were used together with pseudosection modelling. The calculations yield conditions of 3.4 GPa and 865 °C (2.3–3.8 GPa and 810–960 °C outlined by the compositional iso-pleths). Retrograde metamorphism is inferred from an amphibole replacing

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the peak mineral assemblage. Both the eclogite and garnet peridotite-pyroxe-nite are hosted by a quartzofeldspathic gneiss.

The documentation of the UHPM in the eclogites and garnet pyroxenites of the Lower Seve Nappe in northern Jämtland (this study and Majka et al. 2014a) provides new insights into the tectonometamorphic evolution of this unit. The most recently proposed extrusion wedge model by Grimmer et al. (2015) for exhumation of the SNC HP-UHP rocks from Jämtland and Väs-terbotten had to be reconsidered as it assumed that the base of the extrusion wedge is located along the Lower/Middle Seve Nappe boundary. Our new finding of the UHPM in the Lower Seve suggests that the major shear zone forming the lower boundary of the extrusion wedge should be located below the Lower Seve Nappe. The tectonic model proposed by Majka et al. (2014b; Paper I) assumes that the SNC has formed the extrusion wedge, however no geochronological data are available so far to test such a scenario.

Taking into account the origin of the garnet peridotites that is interpreted to be subcontinental (Brueckner et al., 2004), three tectonic scenarios describ-ing their origin are possible. First model, proposed by Gee et al. (2013) sug-gests the formation of the UHPM of the SNC during, or just before the initial stage of Laurentia – Baltica collision. The second model favors microconti-nent – Baltica collision (Brueckner & van Roermund 2004) in the Middle Or-dovician. The third scenario implies the collision of the outer margin of Bal-tica with an island arc (Majka et al. 2014b; Paper I). The latter model allows for deep subduction and exhumation of the UHP SNC crustal rocks (Papers I & III) and capturing of the peridotites from the lithosphere of the subducting plate, Baltica. Importantly, this model also explains the resubduction of the SNC together with the WGR and exhumation of both units from extreme depths. Figure 10 shows the hypothetical model for the tectonometamorphic evolution of the Scandinavian Caledonides assuming the third scenario (arc-continent collision) and exhumation that is driven by extraction of the forearc lithosphere block. The pre-collisional setting (Fig. 10a) shows intra-oceanic subduction and formation of an island arc at ca. 470 Ma. At ca. 450 Ma the Baltica have collided with an arc and Baltica outer margin (the SNC) was sub-ducted (Fig. 10b), partly to mantle depths as indicated by the UHPM in the Stor Jougdan eclogite (Paper II) and findings of the microdiamonds in gneisses (Papers I & III). The ongoing convergence caused the formation of a rupture of the thermally weakened lithosphere under the arc. The negative buoyancy of the lithospheric arc block led to its subduction and subsequent extraction. The elimination of this block decreased the horizontal compressive stress allowing the exhumation of the SNC rocks at ca. 430 Ma, as they rose into the void left behind by the sinking slab (Fig. 10c). This mechanism can drive the exhumation of the deeply subducted continental margin material; it also may drag peridotites from the Baltica lithosphere into the exhuming slab. The second subduction event, with Laurentia – Baltica collision and Scandian

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underthrusting of Baltica beneath Laurentia (ca. 435-390 Ma) led to deep sub-duction (ca. 420-400 Ma; Fig. 10d) of the basement of Baltica (its inner parts; WGR) together with the attenuated parts of the SNC (called Blåhø Nappe in Norway). The WGR exhumation at ca. 400 Ma resulted from extraction of the Iapetus plate (Fig. 10e).

Figure 10. Model for the tectonic evolution of the Scandinavian Caledonides with a focus on the Seve Nappe Complex, including the Stor Jougdan rocks, showing (a) intra-oceanic subduction in the Iapetus Ocean (b) island arc-Baltica margin collision and deep subduction of the SNC as a part of the outer margin of Baltica (c) extrac-tion of a forearc lithosphere block leading to drop of horizontal compressive stress and an exhumation of the SNC (d) Continued plate convergence leading to the Lau-rentia-Baltica collision and subduction of Baltica basement and cover of the Western Gneiss Region (WGR) to extreme depths involving resubduction of the already ac-creted SNC); Iapetus plate elimination by subduction under Laurentian margin (e) exhumation of the WGR together with the resubducted SNC as a result of extraction of trailing edge of Iapetus plate. Modified after Majka et al. (2014b; Paper I) and Klonowska et al. (2016; Paper II).

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4.3. Paper III Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides The Åreskutan Mt., that has been famous since 1888 as a result of Törnebohm’s revolutionary theory about the thrusting in the Caledonides, has become an important place on geological maps once again thanks to the dis-covery of the microdiamonds. In this paper we report the second finding of the metamorphic diamonds in pelitic gneisses of the Middle Seve Nappe in Jämtland (Figs 4 & 5).

The microdiamonds are found in situ as inclusions in garnet. Thermody-namic modelling constrains the peak pressure conditions to 4.1 – 4.2 GPa at 830 – 840 °C, within diamond stability field, and the peak temperature condi-tions to 850 – 870 °C at 1.0 – 1.1 GPa. The temperature of the granulite facies overprint was also obtained by the Ti-in-quartz thermometry (Wark & Watson 2006; Thomas et al. 2010, 2015). TitaniQ gives corresponding results (T = 875 °C for P = 1 GPa) to the thermodynamic modelling. The P-T path of the Åreskutan gneiss is compared to the P-T paths derived for the other UHP rocks (eclogites and pyroxenites) from the SNC.

We present here the results of the in-situ Th-U-Pb monazite dating applied directly on the diamond-bearing gneisses from Åreskutan and Tväråklumparna (discovery of the diamonds presented in Paper I). The chem-ical dating yields an average age of 441.2 ± 3.0 Ma for Åreskutan and 445.1 ± 3.5 Ma for Tväråklumparna. In both localities the monazite dates the post-UHP exhumation event. For the Åreskutan gneiss, the evolution of monazite along an inferred P-T path was proposed. Three chemically (U, Th, Y, Eu) distinctive zones show younging ages from cores through interiors to rims and the formation of each zone is correlated with different processes during the rock evolution, including decreasing modal garnet abundance, partial melting and plagioclase crystallization.

The confirmation of the Ordovician UHPM within paragneisses of the Mid-dle Seve Nappe on Åreskutan and Tväråklumparna (Paper I) in west-central Jämtland, in addition to the evidence for the UHPM of the same age in eclo-gites, peridotites and pyroxenites in northern Jämtland (Janák et al. 2013a; Gilio et al. 2015; Klonowska et al. 2016 – Paper II) provide evidence for the large-scale, regional UHPM in the SNC. However, the finding of the UHPM within rheologically distinct lithologies, strong eclogites and garnet perido-tites – pyroxenites and weak gneisses, provides the basis for the ongoing dis-cussion about the role of tectonic overpressures and lithostatic pressure in met-amorphic rock complexes in the collisional orogens (Mancktelow 2008; Li et al. 2010; Pleuger & Podladchikov 2014; Gerya 2015; Schenker et al. 2015). We provide arguments that disagree with the concept of local tectonic over-pressure being responsible for formation of the UHPM within the SNC rocks.

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For reconstruction of the tectonometamorphic history of this allochthons, particularly the subduction – eduction processes, it is important to take also into account the long-distance (at least 400 km) transport of this unit from the subduction zone onto the platform (Gee et al. 2013). The emplacement oc-curred under amphibolite and subsequently even lower facies conditions and strongly overprinted the former mineralogical evidence of the deep subduction within the rocks. The unambiguous evidence for the UHPM is found only in parts of the SNC; some of it could have been partly or completely erased, but a careful search for the UHP remnants along the entire SNC is needed. How-ever, different P-T-t paths of the SNC rocks along the entire unit indicate that only parts of the Baltica outer margin were subducted to mantle depths. The high grade metamorphism along the SNC in Sweden varies from ca. 500 Ma in Norrbotten, where only recently the first UHP eclogites were identified (Bukała et al. submitted; Paper V), to ca. 450 Ma in Jämtland. The monazite dating of the diamond-bearing gneisses shows that the exhumation of the Seve rocks to lower crustal depths occurred ca. 445-440 Ma and is in agreement with previously obtained ages for the same metamorphic stage. The emplace-ment of the external SNC rocks on the Baltoscandian platform occurred at ca. 420-400 Ma, during Scandian collision, while the SNC correlatives in Norway (Blåhø Nappe) were, at the same time, deeply resubducted together with the WGR rocks (e.g. Terry et al. 2000a; Brueckner 2006).

4.4. Paper IV Middle Ordovician subduction of continental crust in the Scandinavian Cale-donides - an example from Tjeliken, Seve Nappe Complex, Sweden

The results presented in the Papers I-III as well as in the previous work by Brueckner et al. (2004), Janák et al. (2013a), Majka et al. (2014a) and Gilio et al. (2015) provided compelling evidence for the UHPM within different parts of the Seve Nappe Complex in Jämtland. These studies show that UHPM in this far-travelled allochthonous unit is regional. The exact timing of the deep subduction is still not well-defined but crucial for understanding the tectono-metamorphic evolution of the orogen.

In this paper we have studied phengite – garnet gneiss and eclogite from the Tjeliken Mt. in northern Jämtland (Figs 4 & 6). The P-T conditions of the peak metamorphism for the eclogite were estimated previously by Majka et al. (2014a) by conventional geothermobarometry, the averaged P-T THER-MOCALC method and phase equilibrium modelling to 2.5-2.6 GPa at 650–700 °C (Fig. 11). Additionally, the textural observations suggest that the rock could have experienced the UHPM as the radial cracks around quartz are com-mon in the host omphacite. Here, we have dated the same eclogite using the

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Lu-Hf garnet-whole rock method. The garnet has preserved the prograde zon-ing indicated by the bell-shaped distribution of Mn and Lu (Fig. 12). Such a zoning gives information that the Lu-Hf will date early growth of garnet. The isochron based on three garnets and two whole rock aliquots yields an age of 458 ± 1.0 Ma (Fig. 13a). To examine if the gneiss that is underlying the eclo-gite had experienced similar metamorphic conditions we calculated the ther-modynamic model. Garnet and phengite isopleths constrained the peak P-T to 2.5-2.7 GPa at 680-760 °C within the garnet – phengite – jadeite – quartz – rutile – H2O stability field (Fig. 11). SIMS U-Pb Concordia age of 458.9 ± 2.5 Ma was obtained from the zircon metamorphic rims (Fig. 13b). This age is interpreted to date the crystallization of zircon rims during the high grade metamorphism.

Figure 11. P-T pseudosection calculated for the Tjeliken gneiss using Perple_X soft-ware. Compositional isopleths of garnet (Grt) and phengite (Ph) define the peak P conditions (yellow rectangle). The P-T conditions of Tjeliken eclogite from Majka et al. (2014a) are marked by a green countour. Modified from Fassmer et al. (in revi-sion; Paper IV).

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Figure 12. (left) X-ray concentration map of Mn in garnet from the Tjeliken gneiss. The dots across the porphyroblast show the laser spots after measuring the Lu abun-dance. The Lu profile is shown in the right diagram. Modified from Fassmer et al. (in revision; Paper IV).

Figure 13. (a) Lu-Hf isochron for the Tjeliken eclogite, uncertainties are 2σ. Calcu-lated initial values and ages are based on λ176Lu = 1.865 x 10-11 yr-1 (Scherer et al. 2001), Grt 1-3: garnet separates, WR(tt): tabletop-digested whole rock split, WR(b): Parr-bomb digested whole rock split. (b) U-Pb concordia diagram for analyses on metamorphic zircon rims from the Tjeliken gneiss. Modified from Fassmer et al. (in revision; Paper IV).

New P-T estimations and geochronological data show that the Tjeliken eclogite and underlying phengite-garnet gneiss have experienced similar peak metamorphic conditions (Fig. 11) and yielded the same metamorphic age of ca. 458 Ma (Fig. 13). The age obtained in this study by Lu-Hf garnet and U-Pb zircon dating overlap with the Sm-Nd age of 463.7 ± 8.9 Ma determined by Brueckner and van Roermund (2007) for the Tjeliken eclogite. In addition to the well-constrained timing of the subduction, the same age obtained for the Lu-Hf and Sm-Nd systems provides important information about the duration of garnet growth and thus about the dynamics of the subduction exhumation-processes. It is inferred that the garnet growth/subduction was fast, i.e. lasted only a few million years, as there is no difference in the Lu-Hf and Sm-Nd ages.

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The metamorphic age of the Tjeliken rocks is also the same as the Sm-Nd ages obtained by Brueckner and van Roermund (2007) for the Lower Seve Nappe Stor Jougdan garnet peridotite (459.6 ± 4.2 Ma) and it overlaps with the Sm-Nd age of the Middle Seve Nappe Friningen garnet pyroxenite and Sippmikk eclogite (452.9 ± 5.3 Ma; composite age calculated for four sam-ples). These results indicate that the Lower and Middle Seve HP/UHP rocks in northern Jämtland could have been subducted together at ca. 458 Ma.

In this paper we also discuss two different tectonic scenarios explaining the difference in the metamorphic age of the SNC HP/UHP rocks in northern Jämtland (ca. 458 Ma) and Norrbotten (490-470 Ma; less precisely con-strained), the two eclogite-bearing areas in the SNC located ca. 300 km from each other. The first scenario implies two independent tectonic processes (col-lisions) and the second scenario speaks for one continuous process involving the arc-continent collision migrating southwards.

4.5. Paper V UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P-T path and tectonic implications

Detailed field, petrographic, geothermobarometric and geochronological in-vestigations of the Seve Nappe Complex rocks from Jämtland during recent years have resulted in substantial improvement of our understanding of the metamorphic evolution of this allochthon. The UHPM in this region is well documented in eclogites, garnet peridotites and pyroxenites, and gneisses as presented in the previous four papers, summarized above. In this paper we have studied eclogites farther north, in Norrbotten, from the Grapesvare Nappe within the Vaimok lens (belonging to the lower part of the SNC; Figs 4 & 7), where HP metamorphism was documented previously by Stephens and van Roermund (1984), Andréasson et al. (1985), Santallier (1988) and Albrecht (2000). Here we reported the first evidence of UHPM within the SNC in Norrbotten.

The detailed P-T path of the Grapesvare eclogite was reconstructed thanks to a good preservation of mineral inclusions within garnet porphyroblasts and a chemical zoning within garnet, omphacite and phengite (Fig. 14a). The P-T conditions were constrained by application of the multimethod approach. The garnet nucleation (stage E0) records P = 1.5-1.6 GPa and T = 620-660 °C and was calculated using combined QuiG geobarometry and Zr-in-rutile geother-mobarometry (Fig. 14b). Conventional geothermobarometry (Ravna, 2000; Ravna & Terry, 2004) was applied to estimate P-T conditions of the following three metamorphic stages (Fig. 14), E1 stage recording peak pressure meta-morphism (P = 2.9 GPa, T = 702 °C), E2 and E3 stages reflecting retrogres-sion/exhumation (P = 2.5 GPa at T = 730 °C and P = 2.1 GPa at T = 735 °C,

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respectively). The calculation of each stage is based on three distinctive chem-ical zones in garnet, omphacite and phengite (Fig. 14a). It was also possible to constrain the P-T conditions of E1 and E2 by thermodynamic modelling. For calculation of the pseudosection for stage E1, the bulk rock composition was used, whereas for stage E2 the effective/modified bulk composition was obtained by subtraction of an averaged composition of the garnet, omphacite and phengite cores (regarding their modal proportion) from the ‘E1 bulk’. Compositional isopleths of garnet, phengite and omphacite for the ‘E1 pseu-dosection’ plotted within the garnet + omphacite + phengite + rutile + SiO2 (coesite) stability field and yielded P-T of 2.8-3.1 GPa and 660-780 °C. For the ‘E2 pseudosection’ the isopleths overlap in the quartz stability field. P-T conditions for this stage are estimated to 2.2-2.8 GPa and 680-780 °C. The E4 stage (Fig. 14b; late exhumation) is inferred from the amphibole ± plagioclase ± quartz symplectites replacing omphacite.

Figure 14. (a) Model showing mineral zones representative for metamorphic stages E1, E2 and E3. (b) The P-T path was inferred based on geothermobarometry. Purple lines show results of garnet-clinopyroxene thermometer (Ravna 2000) and blue lines of garnet – clinopyroxene – phengite geobarometer (Ravna & Terry 2004). Lines ‘QuiG1’ and ‘QuiG2’ show nucleation conditions of atoll and regular garnet, respec-tively. Green and orange fields represent the temperature range for rutile inclusions and matrix grains, respectively. Modified from Bukała et al. (submitted; Paper V).

The Grapesvare eclogite has followed a clockwise P-T path and experienced UHP conditions during its metamorphic evolution related to deep subduction. It has also preserved three exhumation stages providing new insights into the tectonic evolution of the northern part of the Scandinavian Caledonides. Im-portantly, the HP/UHP rocks of the SNC in Norrbotten are substantially older

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(ca. 490-480 Ma; Root & Corfu 2012; Fassmer et al. 2017) than those known from the SNC in Jämtland (ca. 460-430 Ma; e.g. Claesson 1987; Gromet et al. 1996; Brueckner and van Roermund 2007; Grimmer et al. 2015; Majka et al. 2012; Root & Corfu 2012; Fassmer et al. in revision (Paper IV)). Taking into account the newly derived P-T path and coupling it with the geochronological data we have proposed a new subduction-exhumation tectonic model for the Vaimok lens of the SNC (Fig. 15).

Figure 15. The subduction-exhumation tectonic model for the Vaimok lens of the SNC. The model is partly based on the “vacuum cleaner” model proposed by Majka et al. (2014b; Paper I) and a simplified cartoon illustrating the subducted SNC in Gilio et al. (2015). (a) Arc-continent collision and SNC subduction. Metamorphic stage E0 shows garnet nucleation that started at depths of ca. 50 km. Stage E1 repre-sents the maximum burial of the SNC to ca. 100 km depth. The age of the peak met-amorphism is from (1)Fassmer et al. (2017) and (2)Root & Corfu (2012). (b) Extrac-tion of the forearc lithospheric block and exhumation of parts of the SNC. E2 and E3 represent two stages of stagnation during the SNC exhumation. E4 shows the late exhumation stage to higher crustal levels. Small insets represent the inferred P-T path. Figure from Bukała et al. (in review; Paper V).

4.6. Paper VI Ba- and Ti-enriched dark mica from the UHP metasediments of the Seve Nappe Complex, Swedish Caledonides

Investigations of the Tväråklumparna paragneisses (Figs 4 & 5) resulted not only in discovery of the first metamorphic diamonds within the SNC rocks as presented in Paper I but also in finding yet another interesting, though un-common mineral, the Ba- and Ti-enriched dark mica. The main end-members of this dark mica are oxyannite (KFe2Ti(AlSi3O10)O2), oxy-ferrokinoshitalite BaFe2Ti(Al2Si2O10)O2 and siderophyllite (KFe2Al(AlSi3O10)(OH)2) and the mean crystallochemical formula can be written as (K0.54Ba0.39Na0.02Ca0.01)Σ0.96

(Fe1.37Mg0.85Ti0.50Al0.29Mn0.01Cr0.01)Σ3.03(Si2.59Al1.41)Σ4.00O10(OH1.30O0.66F0.02

S0.01)Σ1.99. Such a mineral is described for the first time from the UHP metased-imentary rocks; however, its formation is most likely associated with the

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granulite facies overprint that the Tväråklumparna gneisses have experienced during exhumation.

The Ba- and Ti-enriched dark mica was found within calcic gneiss col-lected in the same outcrop as the diamond-bearing gneiss (Paper I). The main mineral assemblage is garnet, clinopyroxene, white mica (phengite and mus-covite), quartz, kyanite, sillimanite, dark mica, K-feldspar and plagioclase, minor minerals are calcite and graphite and accessories are zircon, rutile, titanite, ilmenite and pyrite. Well-developed laths of dark mica are found in the matrix, commonly occurring along grain boundaries of feldspar associated with breakdown textures of phengitic muscovite, and with ilmenite. Textural observations suggest the relatively late stage (post-UHP) growth of dark mica together with feldspar and ilmenite at the expense of white mica.

The dark mica is highly enriched in Ba and Ti with mean concentrations of BaO = 11.54wt% and TiO2 = 7.80wt%. The crystallochemical formula was calculated based on several correlations, or a lack of/weak correlation, between different elements including Ba vs K, (Mg+Fe+VIAl) vs Ti, Ti vs Mg, Ti vs Fe, Ti4++2Al3+ vs R2++Si, Ti4++2O2- vs R2++2OH-, Fe vs Mg, Ti vs IVAl and Ti vs Si. The mean end-member formula is written as Oan19Ofk14Sdp11Oph9Aph8Bfa8Bfm6Oks6Phl4Bma4Eas3Ksh3R5 and all the involved end-members are listed in the Table 1.

Table 1. End members of the main empirical formula of the Tväråklumparna Ba- and Ti-enriched dark mica.

Mineral end-member Abbreviation Formula

oxyannite oxy-ferrokinoshitalite siderophyllite oxyphlogopite annite-phlogopite Hypothetical component Hypothetical component oxykinoshitalite phlogopite Hypothetical component eastonite kinoshitalite *remaining end-members

Oan Ofk Sdp Oph Aph Bfa Bfm Oks Phl Bma Eas Ksh R

KFe2Ti(AlSi3O10)O2 BaFe2Ti(Al2Si2O10)O2 KFe2Al(AlSi3O10)(OH)2 KMg2Ti(AlSi3O10)O2 KFe3

2+(AlSi3O10)(OH)2 to KMg3(AlSi3O10)(OH)2 BaFe2Al(AlSi3O10)O2 BaFe2Mg(Al2Si2O10)(OH)2 BaMg2Ti(Al2Si2O10)O2 KMg3(AlSi3O10)(F,OH)2 BaMg2Al(AlSi3O10)O2 KMg2Al(Al2Si2O10)(OH)2 Ba(Mg)3(Al2Si2)O10(OH,F)2

There are only a few places around the world where Ba- and Ti-enriched

dark micas have been found within metamorphic rocks and characterized in detail and none of the known occurrences is associated with UHP rocks. The Middle Seve metasediments in west-central Jämtland have experienced deep subduction to mantle depths (Papers I & III) and have been subsequently exhumed and undergone anatexis under granulite facies conditions (Paper

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III). The later was driven by the partial melting of phengite during decom-pression. BaO was also measured in muscovite (BaO ≈ 0.5wt%) and along with the textural relations it can be inferred that the white mica was a source of Ba for the studied dark mica.

4.7. Paper VII Lower Ordovician UHP metamorphism of the Baltoscandian margin recorded by the Seve Nappe Complex in southern Västerbotten, Scandinavian Caledo-nides

In this paper we present the third finding of microdiamonds within the Middle Seve Nappe gneisses near Saxnäs in southern Västerbotten (Figs 4 & 6) to-gether with petrographic data, thermodynamic modelling and monazite da-ting. It is this northernmost locality of the diamond-bearing gneisses within the SNC, located 250 km NE (along the strike of the Seve unit) from the Tväråklumparna Mt., where the first metamorphic diamond was discovered (Majka et al. 2014b; Paper I).

Likewise the Tväråklumparna and Åreskutan diamonds (Papers I & III), the Saxnäs diamonds are found in pelitic, kyanite-bearing gneisses. The dia-mond inclusions were identified within the characteristic micro inclusion clus-ters in garnet, dominated by disordered graphite, CO2, carbonates and rutile, and a combination of thereof as polyphase inclusions. In addition to garnet as the diamond host mineral, one diamond inclusion was found in zircon (Fig. 16a). Metamorphic diamond is the most abundant and the best preserved in the Saxnäs samples (in comparison to Tväråklumparna and Åreskutan sam-ples). It occurs here as a single mineral inclusion and in polyphase inclusions. One diamond crystal is detected together with CO2 and carbonate (Fig. 16b). The Raman peaks of diamond inclusions occur between 1335 and 1332 cm-1 (Fig. 16c,d) and are relatively sharp with a FWHM varying between 3.6 and 6.9 cm-1. Pseudosection modelling applied to the diamond-bearing garnet-phengite gneiss yields peak metamorphic conditions of ~3.6 GPa and 750 °C plotting within the diamond-stability field. In-situ monazite Th-U-Pb dating was applied to provide initial constraints on the metamorphic evolution of the diamond-bearing gneisses. U/Pb vs. Th/Pb isochron (Cocherie and Albarède 2001) was calculated and yielded the centroid age of 469.9 ± 3.4 Ma (n = 49, MSWD = 1.3).

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Figure 16. (a-b) Photomicrograph of (a) diamond (Dia) and graphite (Gr) inclusions in zircon (Zrn). Zircon is found as inclusion in K-feldspar (Kfs). Within the same K-feldspar rutile (Rt) needles occur; (b) polyphase inclusion of diamond – carbonate (Cb) – CO2 and rutile. (c-d) Raman spectrum of (c) diamond inclusion in zircon. Di-amond peak occurs at 1333 cm-1. (d) diamond associated with CO2 and carbonate. The diamond peak occurs at 1329 cm-1. Modified from Klonowska et al. (manu-script; Paper VII).

The presence of microdiamonds in the paragneisses in Saxnäs locality in southern Västerbotten provides new information on the regional extent of the UHPM in the SNC. The deep subduction origin (UHP stage) of the Saxnäs gneiss (part of the Marsfjället gneiss; Trouw 1973) is also supported by the results of the phase equilibrium modelling. This third finding of microdia-monds within the SNC encourages careful search for relicts of the UHP min-erals within the gneisses along the strike of the entire unit, despite the fact that these rocks are commonly overprinted under granulite and even lower facies conditions (Majka et al. 2012, Paper I, Klonowska et al. 2017, Paper II). The monazite age of ca. 470 Ma interpreted to record the post-UHP stage of met-amorphism is older than the monazite age also dating the post-UHP conditions of the other SNC diamond-bearing gneisses from Åreskutan and Tväråklumparna (ca. 445-435 Ma; Klonowska et al. 2017, Paper III). The timing of the HP-UHPM along the entire SNC spans from ca. 500 Ma to 450 Ma and still needs to be constrained more precisely as it provides key evidence for the better understanding of the subduction-exhumation processes.

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5. Conclusions and future directions

In this thesis unequivocal evidence for ultra-high pressure metamorphism within vast parts of the Seve Nappe Complex is presented. The UHPM is doc-umented by the presence of microdiamonds in paragneisses in three localities within the Middle Seve Nappe, at Tväråklumparna Mt. (Paper I) and Åresku-tan Mt. (Paper III) in Jämtland and near Saxnäs (Paper VII) in southern Väs-terbotten. The metamorphic diamonds are identified in garnet inclusions and also in zircon (only Saxnäs). The phase equilibrium modelling applied to the Åreskutan and Saxnäs gneisses indicate peak metamorphic conditions within the stability field of diamond (4.1 – 4.2 GPa at 830 – 840 °C and ~3.6 GPa at 750 °C, respectively). In northern Jämtland, within the Lower Seve Nappe, the UHPM was recorded in eclogites and garnet pyroxenites from the Stor Jougdan locality (Paper II) and near-UHPM in eclogites and gneisses at Tje-liken Mt. (Paper IV). P-T conditions estimated by conventional geothermo-barometric methods and pseudosection modelling reached ~3.5 GPa at 870 °C for both rocks from Stor Jougdan. Equilibria among silicate minerals in the Tjeliken gneiss indicate lower P-T conditions of 2.5-2.7 GPa at 680-760 °C. However, the peak metamorphic conditions of ~3.0 GPa and 800 °C for the Middle Seve Nappe in northern Jämtland, the Friningen peridotite body and pyroxenite – eclogite dyke within it, determined by Brueckner et al. (2004), Janák et al. (2013a) and Gilio et al. (2015) lay in-between the results obtained here for the Lower Seve Nappe and show that both units have experienced similar P-T histories. At present, the northernmost extent of the UHPM within the SNC is defined by the eclogites from the Grapesvare Nappe in the Vaimok lens in Norrbotten (Paper V). The eclogites here recorded the peak pressure metamorphism at 2.9 GPa and 702 °C. Despite the fact that all of the investi-gated rocks are stable within the coesite stability field, this UHP index mineral has not been found in the SNC rocks. The possible presence of the coesite is deduced only by the occurrence of its breakdown product, polycrystalline quartz surrounded by radial cracks in garnet or omphacite (e.g. in the Åresku-tan gneiss and the Tjeliken and Vaimok eclogites). The quantitative P-T paths were derived for the diamond-bearing Åreskutan gneiss (Paper III) and the Vaimok eclogites (Paper V) thanks to the preservation of compositional zon-ing in the major minerals. UHPM in the Åreskutan gneiss was followed by decompression melting at granulite facies conditions of 850 – 870 °C at 1.0 – 1.1 GPa. A more detailed P-T path was constrained for the Vaimok eclogite

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by the application of the multimethod approach. The P-T estimates were ob-tained for four metamorphic stages; stage E0 representing garnet nucleation (P = 1.5-1.6 GPa and T = 620-660 °C), peak metamorphic stage E1 (P = 2.9 GPa, T = 702 °C) and two retrogression stages E2 and E3 (P = 2.5 GPa at T = 730 °C and P = 2.1 GPa at T = 735 °C, respectively). The latter study also shows that the application of alternative methods to those based on the equi-librium between different mineral pairs, i.e. QuiG barometry, may well im-prove our understanding of the metamorphic evolution of rocks. All of the presented data provide strong evidence that the UHPM rocks within the SNC have formed as a consequence of deep subduction of the outer continental margin of Baltica.

The timing of the subduction – exhumation processes of the UHP rocks is fundamental in order to understand the tectonometamorphic evolution of the orogen. The recognition of UHPM in an increasing number of localities along the entire SNC makes this far-travelled allochthonous unit crucial for inter-pretation of the tectonic processes in the Caledonides in general; in this thesis the geochronological data provided important constraints on the timing of the UHP-HP subduction and following exhumation in different parts of the SNC (Papers III, IV & VII). Lu-Hf garnet-whole rock and U-Pb zircon dating of the Tjeliken rocks in northern Jämtland yielded an age of ca. 458 Ma for peak metamorphic conditions (Paper IV). On the other hand, the in-situ Th-U-Pb monazite dating of the diamond-bearing gneisses from Åreskutan and Tväråklumparna (Paper III) and Saxnäs (Paper VII) gave information about the timing of the post-UHP exhumation of these rocks. The dating yielded ages of 445-435 Ma for Jämtland and of ca. 470 Ma for Västerbotten. New geochronology together with the already existing metamorphic age data and our compelling evidence for the UHPM in the SNC are opening up new ave-nues for discussion and investigation of subduction models and the tectonic evolution of the Caledonides.

Starting with the southernmost UHP area, the Middle Seve Nappe in west-central Jämtland, the timing of the exhumation and related partial melting is well-constrained in paragneisses to ca. 440 Ma by various methods (e.g. Claesson 1982; Williams & Claesson 1987; Gromet et al. 1996; Ladenberger et al. 2014; Majka et al. 2012, Klonowska et al. 2017, Paper III). The only information about the age of the peak metamorphism was derived by Majka et al. (2012) where authors interpret the monazite age 455 ± 11 Ma of one of the monazite zones to date the subsolidus conditions occurring close to the peak P-T conditions. Taking into account our new monazite data (Paper III) it can be concluded that the deep subduction of the SNC in this area occurred before 440 Ma, probably close to ca. 455 Ma. 150-200 km NE along the strike of the SNC, in northern Jämtland, where garnet peridotites and eclogites of the Lower and Middle Seve nappes have been investigated in detail by e.g. Brueckner and van Roermund (2007) the age of the peak metamorphism was estimated to 460-450 Ma. New precise ages derived from Lu-Hf garnet and

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U-Pb zircon dating (Paper IV) for the Lower Seve Nappe is in a good agree-ment with the data from the previous work. The same age of metamorphism in the Lower and Middle Seve nappes together with the very similar peak P-T conditions of these rocks allows the conclusion that they may have been deeply subducted together to mantle depths during the Caledonian orogeny at a time close to 458 Ma. The least earlier studied is Saxnäs area, located around 50 km NE from the Tjeliken Mt. Here, the post-UHP stage in the diamond-bearing Middle Seve gneiss is dated to ca. 470 Ma (Paper VII). No geochron-ological data have been obtained until now for the peak metamorphic stage. The only credible ages have been presented for the Upper Seve schists in the study by Grimmer et al. (2015), who suggest prograde growth of garnet at 462 ± 3.5 Ma and subsequent exhumation of both Upper and Middle Seve occur-ring at ca. 434 Ma, and reaching mid crustal levels at ca. 430-426 Ma. The monazite age suggests that the UHP event of the Middle Seve here was pre-470 Ma, i.e. older than the UHPM in Jämtland; however it may be coeval with the UHPM 250 km farther north in Norrbotten, where the high pressure met-amorphism in the SNC was dated to ca. 504-475 Ma by Sm-Nd on garnet and U-Pb on titanite (Essex et al. 1997; Mørk et al. 1988), but U-Pb zircon dating gave an age of ca. 482 Ma (Root & Corfu 2012). There is a clear need for more detailed dating of the UHP-HP metamorphism in order to correlate the collisional event(s) occurring in different parts of the SNC.

Thus our data today indicate that the deep subduction of the Baltoscandian outer margin and collision with an island arc during closure of the Iapetus Ocean occurred in the Middle Ordovician in Jämtland rocks. Earlier, Late Cambrian(?)-Early Ordovician subduction-collision in Norrbotten (and possi-bly also in Västerbotten) may be related either to separate arc-continent colli-sion or to the same collision as in Jämtland as a result of slow southward mi-gration of the collision zone. The UHP SNC rocks were exhumed to lower crustal levels in the Silurian. Taking into account possible tectonic scenarios of exhumation of the UHP rocks, the forearc block extraction model proposed by Majka et al. (2014b; Paper I) is favored in this study. The removal of the forearc lithosphere block might have functioned as a driving force for activa-tion of the ‘vacuum-cleaner’ mechanism that in turn led to exhumation of both the UHP continental crustal rocks and mantle rocks from the downgoing Bal-tica plate. These subduction-exhumation processes clearly predate subduction of Baltica’s inner margin and underthrusting of Laurentia. This Scandian col-lision (Silurian-Devonian) involved deep subduction of WGR basement to-gether with the internal parts of the SNC (Blåhø Nappe in Norway) that were then subducted for a second time.

The tectonic model for the Scandinavian Caledonides introduced by Majka et al. (2014b; Paper I) is purely hypothetical and based on the currently avail-able P-T-t paths and Taiwan analogue (e.g. Malavieille et al. 2002; Shyu et al. 2011). It is necessary to test this model by numerical modelling. Nevertheless

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this PhD work provided a broad ‘input’ data for the model. However, compre-hensive P-T-t-D paths still need to be constrained for the entire SNC in Swe-den and also for the SNC in Trøndelag (Blåhø Nappe) and the WGR further south in order to test mechanisms responsible for exhumation of the UHP rocks.

Of particular interest are the following aspects that remain to be resolved in the scope of the work presented here:

Detailed quantitative P-T paths of the HP-UHP rocks along the entire SNC

and careful search for the relicts of the UHPM especially in places where HPM has been previously recognized;

The precise timing of the HP-UHP metamorphism, exhumation and asso-ciated formation of the shear zones (mylonites);

Further investigation of the garnetiferous gneisses and migmatites in the SNC in areas of central and northern Västerbotten and eventually in the potential localities in northern and southern Norway;

Timing of the UHPM in the WGR, with particular focus on the resub-ducted SNC;

Origin of the microdiamonds.

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Summary in Swedish

Inom geologin är högtrycksbergarternas mineralogi och petrologi ett snabbt växande forskningsområde, som ger nya och detaljerade kunskaper om konti-nent-kontinentkollisioner och uppbyggnaden av större bergskedjor. Syftet med denna avhandling är att fördjupa förståelsen av de processer som är verk-samma vid bergskedjebildning och framför allt gällande de skandinaviska Ka-ledoniderna. Denna bergskedja bildades genom en kollision mellan kontinen-talplattorna Baltika och Laurentia då Iapetushavet stängdes för omkring 400 miljoner år sedan. De starkt komprimerande krafterna under kollisionen ledde till att stora sjok, dvs. skollor, av havsbottenberggrund och kontinentalskorpa transporterades hundratals kilometer upp på Baltikakontinenten. Skollorna är överskjutna på varandra och indelas i undre, mellersta, övre och översta skoll-berggrunden och återfinns idag i den svenska och norska fjällkedjan. Ett tvär-snitt genom den kaledoniska berggrunden visar stora likheter med den fortfa-rande aktiva Himalayas uppbyggnad. Innan kollisionen med Laurentia krock-ade Baltika med en vulkanisk öbåge, vilket resulterade i att delar av Baltika pressades ner (genom subduktion) så djupt att bland annat diamanter bildades till följd av det ultrahöga trycket. Omvandling under extremt höga tryck be-nämns ultrahögtrycksmetamorfos och bevis för denna typ av metamorfos åter-finns i den så kallade Seveskollan. Seveskollan utgör den högst belägna skol-lan i den mellersta skollberggrunden och är allmänt känd för sina högmeta-morfa bergarter.

Ultrahögtrycksmetamorfos av kontinental jordskorpa sker på manteldjup och Seveskollan har genom senare tektoniska processer transporterats upp mot ytan igen. Fokus i denna avhandling är att förstå dessa processer och forsk-ningen har därför inriktats på Seveskollan med huvudsyfte att rekonstruera den tektoniska och metamorfa utvecklingen av den tidigare djupt nedpressade skollan. Genom mineralogiska, petrologiska och geokronologiska metoder har tryck, temperatur och ålder för metamorfa och tektoniska händelser bestämts för olika bergarter i Seveskollan i Jämtland, Västerbotten och Norrbotten.

Resultatet visar att Seveskollans högmetamorfa kyanitförande gnejser från Tväråklumparna och Åreskutan i västra och centrala Jämtland samt i Saxnäs i södra Västerbotten innehåller mikrodiamanter. De metamorfa diamanterna vi-sar att delar av Seveskollan har utsatts för extremt tryck, så högt att det endast kan uppnås då kontinental jordskorpa pressas ner djupt i jordens mantel. Tryck- och temperaturberäkningar baserat på gnejsernas mineralkemiska sam-mansättning visar att gnejserna metamorfoserats inom stabilitetsfältet för dia-mant. Ultrahögtrycksmetamorfos har även fastställts i bergarterna eklogit och granatförande pyroxenit från Stor Jougdan, fengit-och granatförande gnejs från Tjeliken i norra Jämtland och längre norrut i eklogit från Vaimok i Norr-botten. Tillsammans utgör dessa tydliga bevis för att förekomsten av ultrahög-trycksmetamorfos är mer utbredd i Seveskollan än vad man tidigare ansett.

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Resultaten visar en regional utbredning på cirka 400 km i nordsydlig riktning längs med den mellersta skollberggrunden. Kemisk U-Th-Pb-datering av mo-nazit från de diamantförande gnejserna från väst-centrala Jämtland är tämligen oprecis men bekräftar att metamorfosen nådde sitt maximum i mitten av den geologiska tidsperioden ordovicium (488-443 miljoner år sedan) och överens-stämmer med tidigare dateringar. Under tidsperioden silur (444-409 miljoner år sedan) utsattes dessa bergarter för partiell uppsmältning, vilket avspeglar en trycksänkning till följd av att bergarterna transporterades upp mot jordytan genom så kallad exhumering. Lu-Hf-datering av granat-bergart och U-Pb da-tering av zirkon på eklogit och gnejs från Tjeliken visar att bergarterna där påverkades av högtrycks- och ultrahögtrycksmetamorfos för cirka 458 miljo-ner år sedan, dvs i mellersta ordovicium, medan U-Pb datering av monazit i de diamantförande Saxnäsgnejserna i södra Västerbotten fastställer att ultra-högtrycks-metamorfosen ägde rum under tidig ordovicium för mer än 470 mil-joner år sedan, dvs. mycket tidigare än längre söderut. En tektonisk modell baserad på tryck-, temperatur- och åldersbestämningar med ultrahögtrycksme-tamorfos i mitten av ordovicium följt av exhumering i silur som visar den me-tamorfa och tektoniska utvecklingen av de skandinaviska Kaledoniderna pre-senteras i avhandlingen. Modellen visar att ultrahögtrycksbergarterna bildades under extrema tryck till följd av Baltikas kollision med en vulkanisk öbåge. De djupt nedpressade bergarterna har återvänt till ytligare nivåer till följd av tryckreduktion, en typ av baksug, som uppstår då en del av Iapetusplattan, mellan den vulkaniska öbågen och subduktionszonen, bryts av och sjunker och på så sätt frigörs ett utrymme med undertryck. Tryckreduktionen tillsam-mans med den positiva bärkraften hos den relativt lätta kontinentalskorpan transporterar dessa djupt begravda bergarter tillsammans med mantelbergarter upp till jordens yta.

Resultaten i denna avhandling bidrar till en ökad förståelse av den tekto-niska och metamorfa utvecklingen av de skandinaviska Kaledoniderna. Den tektoniska modellen för hur dessa ultrahögtrycksbergarter når jordens yta igen kan även tillämpas för andra bergskedjor, inte minst Himalaya.

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Acknowledgement

It might be difficult to believe that it all has started at totally different tectonic boundary. I met my supervisor, Jarek Majka, in Iceland in 2010. Who would have back then thought that exploring Mid-Atlantic Ridge will bring me to the most beautiful journey through my PhD in the Caledonides. Thank you for being such a great supervisor and a true friend. I am grateful for endless num-ber of things I have learnt and experienced during my PhD (and earlier during my MSc project); it would be impossible to count all the ways that you've helped me during these 7 years. Thank you so much for all that you've done and for letting me work so independently. I hope you have enjoyed this scien-tific journey as much as I did.

I count myself lucky for having David G. Gee as my mentor. I met David in 2011 during the Transcand excursion and since then it was clear to me that I will “stay” in Caledonides for longer. Thank you for guiding me through Caledonides and a scientific world. I will always be grateful for all support I have got from you.

My gratitude also extends to my co-supervisors, Marian Janák for your en-couragement, discussions and advice whenever needed and your great hospi-tality at any time I was visiting Bratislava; Bjarne Almqvist and Peter Lazor for inspiring discussions, support and guidance.

I also thank my collaborators for their input and ideas in the field, in the lab, and with the papers, especially Niko Froitzheim, Igor Petrík, Maciek Manecki, Karolina Kośmińska, Michał Bukała, Åke Rosén, Barbro Anders-son, Kathrin Fassmer, Kaśka Walczak, Vlasta Sasinková, and Kenta Yoshida.

I am thankful to all my colleagues from MPT for enjoyable time and wel-coming me so warmly here at the very beginning of my studies in Uppsala: Ester and Johen, Michael, Abi, Steffi, Kirsten, Juliane, Lara, Sylvia, Börje, Frances, David, Franz, and later on Harri, Tobias, Safieh, Erika, Konstantinos and Lei. I am also grateful to Hemin Koyi, Karin Högdahl, Valentin Troll, Hans Annersten, Erik Jonsson, Örjan Amcoff, Hans Harryson, Håkan Sjöström and Jenny Andersson, as each of you have given me at least one unforgettable lesson. Karin and Håkan, thank you also for polishing the Swe-dish summary of my thesis. Barbro, Åke, Sara and Johanna, it has been a unique experience working with you in the field and being (partly) your su-pervisor! Johanna, thank you for all support and help during the last days!

I was really lucky meeting so many friendly people at the Department. I would like to especially thank Małgosia Moczydłowska-Vidal for all discus-sions during fika and a warm smile. Also, a special thank you goes to Fatima, who was very helpful dealing with all administration issues. My time here would have never been so enjoyable without the “other side” of a corridor and all friends from Geophysics: Bojan, Suman, Georgiana, Magnus, Garðar, Shunguo, Laura, Monika with Christos, Maria and Michael with Clara, Livsa,

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Rémi, Théo, Aggela, Peter H., Peter S., Sebastian, Joachim, Daniel, Tegan, Darina, Karin, Sahar, Zeynab, Omid, Mohsen, Ruth, Lars, Silvia and Fred with Tristan, Ruixue, Ping, Michael, Luisa, and Michiel, and from the other Departments: Rūta, Svenja, Maria, Babis, Alizee and Heda. All friends from Geologiska Sektionen – thank you all for fun during field trips, seminars and TGIFs.

COSC team, firstly I would like to thank Chris Juhlin for letting me be a part of the project and financing part of my PhD. I am especially thankful to Henning and Bjarne, and all students working with me during the same shifts. It has really been an enriching experience.

Orogen Dynamics Team – thank you for all the experience in the field, discussions, conferences and team work, it all can be done in such a positive atmosphere and with loads of fun. I wish everyone could have experienced work in such team once in their life! No, I cannot leave it anonymous, my friends, thank you Jarek, Karola, Michał, Kaśka, Marysia, Pauline, Chris, Jo-hanna, Ziemniak (Potato), Karol, Kuba, Paweł, Ula, Maciek, Barbro, Sara, Sofia, Cam and Krzysiek. Once I am already mentioning a group from Kra-ków, it is worth noting, that my scientific journey would have never started without Maciek Manecki (thank you!), who decided to take a group of almost 20 students to Iceland in 2010… Karola Kośmińska, if I start writing about all the things I am grateful for, it would become a second PhD thesis. I think it is better if we meet at least(!) once a year wherever we live to look back together into all the memories from our PhD time.. And remember, one day we will create a new Department together ;-) MiHau (dobry jak chleb), double thanks to you as well, for a great time in the field and now in Uppsala!

Uppsala has relatively quickly become my real home thanks to extended family of close friends. I simply say THANK YOU for every single moment to Anka, Chris, Olaf and Zuza, Alicja, Rūta (My Dearest!), Bojan, Georgiana, João, Little Sara, and Suman. I would have never made it without you!

There is definitely more people who have made my time in Sweden so nice and cheerful: Lorraine, Alex, Przemek (Mr. Bacon), Hagen, Reuben, Rudi, Hesham, Matthijs and Elen, Marta, Dragos, Othmane, Prune, Mats, Leonor, Anna, Łukasz (and rest of Katushka/Sputnik Kollektiv team), “Polish Woman Mafia in Uppsala” especially Anka, Alicja, Agata N., Agata Z., Beata, Ola R., Justyna and Kasia F. Big thank you to all our house mates and guests in the house – it was such a memorable time! My other home is also Kraków, thank you for making me feel still so homey there: Paulina (Koza) and Przemek, Karola Kielczyk and Asia Kuder, Beata, Grześ and Ola, Mati and Ania, and ODT! Niklas thank you for your friendship and.. high gravity sofa. Isiaj, I am grateful for all the moments and for your great support during the last days. Iiro, I really appreciate your support and thank you for showing me life from a different perspective.

Last but definitely not least, my family has always been the biggest source of love, happiness and strength. I thank my parents for always supporting my

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interests and dreams and for all the trust. My Sis, Ala, and my brother Michał (Klonek) and his family Asia and Wojtek for always being close in spite of distance. Dad, I am sure you would have been so proud of me, I love you truly and miss you so much. Kocham Was i dziękuję za wszystko!

Iwa

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