High temperature-Iow pressure metamorphism and deep crustal structures . Meeting of IGCP project 304 ' Deep Crustal Processes ' in Finland, September 16- 20, 1994. Edited by Matti Pajunen Geological Survey of Finland, Guide 37, 27-34, 1994.

TECTONO-METAMORPHIC EVOLUTION IN THE T AMPERE-V AMMALA AREA

by

Timo Kilpeläinen(l, Sergey Korikovsky(2 , Kalevi Korsman(land Mikko Nironen(l

Kilpeläinen, T., Korikovsky, S., Korsman, K. & Nironen, M. 1994. Tectono-metamorphic evolution in the Tampere- Vammala area. Geological Survey of Finland, Guide 37, 27- 34, 6 figures.

Key words (GeoRef Thesaurus, AGI): schi st belts, metamorphic rocks, mi gmatites, structural geology , deformation , metamorphism, tecton ics, Proterozoic, Tampere, Vammala, Finland

/j Geological Survey of Finland, F1N-02150 Espoo, Finland 21 Academy ofSciences ofthe Russia, Staromonetny per. 35, 10901 7 Moscow, Russia

Introduction

The Tampere-Vammala area is part of the Palaeo­proterozoie Svecofennidic belt that runs across south­em Finland from east to west and is composed of metasediments and volcanites varying in lithology and metamorphie grade, and of granitoids mainly intrusive to them.

The Tarnpere schist area and its surroundings were in a key position in the early studies of Sederholm (1897) at the turn of the century. Sederholm (1931) was inspired partly by the well­preserved depositional structures in the Tampere schist area and its continuations and partly by the striking contrast between the schist area and the Vammala tonalite migmatite area to the south of it. According to hirn, the formations were separated by a great time discordance. He interpreted the Bothnian volcanites and sediments of the Tampere schist area as having deposited on the intensely deformed Svionian schists at Vammala. This discordance was, however, eliminated by stratigraphie investigations

and by age determinations. It was eventually con­cluded that the rnigmatite area was merely a marine part of the Tampere island are formation, in which the metamorphie grade increases gradually from the synclinally folded Tampere schist area to the anticlinally folded Vammala rnigmatite area (Sirnonen 1980).

The relation of the migmatite area to the weakly metamorphosed schist area has, however, now been reconsidered in the light of recent tectono-metamor­phic investigations and plate tectonic modelling (e.g. Kähkönen et al. 1994, this volume).

Geological set up

The variation in lithology and metamorphie grade in the Tampere-Vammala area exhibits certain regu­larities (Figs. 1 and 2). The Tampere schist area is composed of island are-type volcanites and their fluvial weathering products and of marine turbidites beneath the volcanites. At the boundary between the

27

IV 00 "0

o ~ N

61·40'

Fig. I. Lithological map ofthe Tampere­Vammala area (map sheets 2 1 12, 21 14, 2121, 2 122 , 2123 and 2124) after Koistinen ( 1994).

Granite

Metatonalite

Granitic and tonalitic neosome

Mafic plutonic rocks

Mica gneiss or mica schist

Felsic metasediments and metavolcanic rocks

- as paleosome or in tercala tions

Mafic and intermediate metavolcanic rocks

- as paleosome or intercalations

Graphitic schist

20 km

[[ll]] ~ ~

Fig. 2. Tectono-metamorphic map ofthe Tampere-Vammala area (map sheets 21 12, 2114, 2 121 , 2 122,2 123 and 2124) (compiled by R. Niemelä and P. Wasenius). V = Värmälä, H = Hämeenkyrö, N = Nokia and T = Tottijärvi granitoid intrusions.

Granitoids and gabbroids

Ms+Chl

And+St

[~~~~] Ms+SiI±Kfs

Kfs+Sil±Ms

m Sil+Crd±Grt

D Sil+Grt±Crd (migm.)

[:::~] Sil+Grt (migm.)

Grt+Crd (migm.)

SO/ SI

./ S2

~S3 0 Subhorizon tal

fold axes

> Dipping fold axes

Z Syncline

20 km

Geological Survey of Finland, Guide 37 Tilllo Kilpeläill ell , Sergey Korikovsky. Kalevi Korsmall and Mikko Niroll ell

migmatite area and the Tampere schist area, the turbidites extend to the migmatite area. The Tampere island arc rocks that overlie the turbidites are, how­ever, lacking in the migmatite area, where there are early-Svecofennian ultramafic lavas substantially older than the Tampere island arc rocks.

The Tampere-Vammala area can be divided metamorphically and stratigraphically into two parts. The volcanites of the Tampere island arc system and their weathering products were metamorphosed un­der greenschist or amphibolite facies conditions, with muscovite in equilibrium and the metamorphic grade increasing towards the migmatite area. The underlying metaturbidites in the Tampere schist area were metamorphosed in a similar manner to the island arc volcanites, i.e. there is no metamorphic discordance between them. In the migmatite area the assem blage gamet -cordieri te-si Bi mani te-bioti te i s in equilibrium. The change in metamorphic grade on the erosion surface between the Tampere schist area and the migmatite area is so sharp that the boundary appears tectonic .

Structural and metamorphic evolution

The earliest structure in the Tampere-Vammala area is the schistosity (S I) parallel to the layering and compositional banding. Observations of the type of o I deformation in the Tampere-Vammala area are scarce. In places, it is accompanied by intense stretch­ing-type lineation, but only a few F I folds have been recorded. D I deformation has, however, overtumed layers here and there, implying that FI folding gave rise to overthrusts and recumbent folds. Important points are, first, that D I deformation gains in inten­sity with the increase in metamorphic grade and, second, that the structures and direction ofthe move­ment associated with it were horizontal even though today the structures are now almost ubiquitously vertical.

The second-generation folds developed through­out the area due to north-south compression. Owing to the low-angle attitude ofthe folding structures and the direction of the prevailing compression, the axes of the F2 folds were originaBy horizontal, trending east-west. The axial plane was vertical and also trended east-west. Younger deformations, however, tumed the D2 structures, and they have preserved their primary attitude only in the Tampere schist belt, which folded into an syncline during D2 deforma­tion.

The decomposition reaction of muscovite in the Vammala migmatite zone, the associated crystalliza­ti on of sillimanite, and the crystallization of gamet and cordierite due to the decomposition of biotite

30

started progressively during 0 I deformation. In the deepest sections ofthe migmatite zone, the crystalli­zation of gamet invariably preceded that of cordierite. The decomposition of biotite probably continued with the crystallization of gamet and cordierite or of cordierite alone (Fig. 3). Cordierite continued to crystallize when D I deformation had come to an end. The above crystallization order shows that the tem­perature rose and the pressure declined in the course of 01-D2 deformation. The highest temperature, 670°C, was reached at 5-6 kb pressure. Considering the intensity of the migmatization this temperature may seem low, butit was precisely the intensity ofthe migmatization that buffered the increase in temper­ature. Migmatization was caused by abundant aque­ous fluid f1uxes rather than by the introduction of great amounts of heat into the system. Infiltration of large volumes of extemal fluid is consistent with the carbon and oxygen isotope ratios measured on the abundant calcite concretions in the sedimentary beds. Their 180 compositions are completely homogenized, but the J3C values vary in a wider range, indicating exchange reactions with a hydrous, CO

2-poor fluid

(pers . comm., J. Karhu 1994). The high abundance of D I biotite in psammites also implies the presence of aqueous fluid , as biotite is in contact with sillimanite throughout the migmatite area.

Characteristic of the migmatite area are the hydra­tion reactions that followed the progressive stage. These include changes in the composition of gamet and cordierite and their biotitization , and the crystal­lization of andalusite.

Schistosity related to D I deformation is only 10-cally visible in the Tampere schist area, whereas in the migmatite area it is predominant. Andalusite, staurolite and gamet crystallized at an early stage of D2 deformation when the schist area was folded into a syncline (Fig. 4). In the course of the progressive metamorphism, gamet and staurolite altered into andalusite, wh ich continued to crystallize after D2 deformation, when sillimanite, too, started to crys­taBize. The crystallization of andalusite and sillimanite was obviously related to the incipient decomposition reaction of muscovite. Hence, the progressive stage continued even after D2 deformation. In the central parts of the Tampere syncline, metamorphism took place at 470°C and 3-4 kb. At the margins of the syncline, close to the migmatite area, the temperature was 570°C and the pressure 3-4 kb.

In the migmatite area, the metamorphism became increasingly progressive mainly during D I deforma­tion and, in the Tampere schist belt, during 02 deformation. Consequently, the temperature and age of metamorphism and the beginning of structural evolution increases as a function of depth.

The isograd surfaces of the Tampere-Vammala

Geological Survey of Finland , Guide 37 Tectono-metamorphic evolution in the Tampere-Vammala area

Fi g. 3. Alteration of gamet into cordierite in the Vammala migmatite area. Gamet and cordierite were not in mutual equilibrium. Photomicrograph, plane-polari zed light. Bt = biotite, e rd = cordierite and Grt = gamet. Photo by M. Paj unen.

Fig. 4. Photomicrograph of staurolite and S2 schistos ity in the Tampere schist area. Plane-polari zed light. Photo by M. Väisänen.

area are further deformed by two conjugate plastic foldings (03 deformation). In the eastern part of the area, dextral, northeast-southwest-trending folds with a vertical axial plane are dominant whereas in the west the corresponding structure is sinistral and the axial plane trends roughly northwest-southeast.

Hence, F3 folding deforms both 0 l and 02 struc­tures. The F2 axis culminations and depressions thus formed have given rise, particularly in the migmatite area, to dome-basin interference structures in which the metamorphic grade varies locally with an appar­ent lack of regularity (Figs. 5 and 6).

31

Geological Survey of Finland, Guide 37 Timo Kilpeläin en, Serge)' Korikol'sky. Kalevi Korsman and Mikko Nironen

Fig . 5. Vammal a mig l11 atite (scale bar is 12 cm). Photo by 1. Ras tas.

The hydration reaetions in the Vammala migmatite area are related to D3 deformation, but migmatization was stilJ going on in the area in the D3 stage. This was, however, mainly granitie, and thus differed from the tonalitie migmatization of the D I (and D2) stage. Some D3 shear zones see m to have experi­eneed elevated temperatures, beeause the mutual equilibration of gamet and eordierite in the shear zones eontinued in the D3 stage. It is possible that D3 migmatization and the loeal thermal pulse reeord the metamorphism of the potassium granite migmatite zone at 1850-1830 Ma (Korja et al. 1994, this volume and Väisänen et al. 1994, this volume).

Temporal evolution of the Tampere-Vammala area

The age of the Tampere island are voleanites falJ in the 1904-1888 Ma range (Kähkönen 1987). The age of the plutonie elasts of the eonglomerates de­posited on them ranges from 1890 to 1888 Ma (Nironen 1989). The tonalites of the Tampere sehist area intruded during D2 deformation 1885 Ma ago. Tonalitie migmatization was already taking plaee in the Vammala migmatite area during Dl deforma­tion. The D2 tonalite intrusions in the Vammala area are 1885 Ma old as they are in the Tampere sehist area.

Age determinations on the eonglomerates of the Tampere sehist belt imply that they deposited 1888 Ma ago at the earliest, i.e. they formed the erosion

32

sUlfaee at that time together with the volcanites of the Tampere sehist area ofthe same age and the sediments deposited on them. As shown by observations of metamorphism and deformation, the turbidites be­neath the Tampere island arc volcanites were elose to the erosion surface 1888 Ma ago. At the present erosion surface, the turbidites that metamorphosed under greenschist facies conditions grade into migmatitic gamet-cordierite gneisses without tectono­metamorphie discordance (at 5 kb and 670°C). The turbidites whieh were at the erosion surface would accordingly have sunk to a depth of at least 15 km and melted to a great extent in the course of 3 Ma. This implies tectonic thickening of the crust and a strong increase in the heat flux. However, direct observations oftectonic crustal thickening are lacking in the Tampere­Vammala area. The area represents a crustal section of such depth that the nature ofthe overthrusts can only be deduced indirectly. Moreover, the age ofthe early stage of D 1 deformation and the associated metamorphism is unknown. Hence, the evolution of the heat flux could have lasted longer than 3 Ma, since in the deepest seetions of the migmatite zone, it would elearly have started before the upper PaJts ofthe Tampere syneline were metamorphosed, i.e. before 1885 Ma. As in the nOlthem part of the Svecofennian island are system, deformation and metamorphism evolved rapidly in the Tampere-Vammala aJ'ea, mainly 1890-1885 Ma aga when the Svecofennian island are system collided with the Archaean crust. The intense thermal pulse at 1830 Ma in southem Finland is only 10caIJy recorded in the Tampere-Vammala area.

Geological Survey of Finland, Guide 37 Tectono-metamorphic evolution in the Tampere-Vammala area

Fig. 6. Three-dimensional representation of folded isograd surfaces in the Tampere-Vammala area. The hi gh stra in zone marked in red between the Tampere schi st belt and the Vammala migmatite zone is located in the Pirkka la area (see Fig. 2). The area is viewed obliquely downwards from the northwes t. For silll plification, the tonalite intru sions are shown as cy lindrica l bodies. Locations of Värlllälä (V), Hällleenkyrö (H), Nokia (N) and Tottijärvi (T) intrusions are shown for ori entati o n (see also Fig. 2).

Geotectonic implications of the Vammala migmatite area

As concluded above, the Tampere island arc sys­tem and the migmatized Vammala zone are not in exotic relation to each other. The same metaturbidites have been found both in the weakly metamorphosed Tampere island arc area and in the migmatite area, and the detrital zircon population of the metaturbidites is the same in both areas (Vaasjoki et al. 1994, this volume). However, the degree of discordance may increase with depth, for so me formations of the island arc system are alm ost totaJly free of D 1 deformation structures. This, however, only implies that the meta­morphic grade increases with depth as does the age of the metamorphism. Hence the variation in metamor­phic grade is mainly due to folding of the isograd surfaces during D2-D3 deformation (Fig. 6). The buffering effect of melting, which consumes great quantities of heat, and of the associated reactions is distinctly visible on the boundary between the Tampere schist area and the Vammala migmatite area. The metamorphic temperature drops sharply over a short distance in the rocks undergoing metamorphism above the migmatite zone, which is, at least partly, the reason for the apparent discordance between the migmatite and schist areas.

Our knowledge of the early tectono-metamorphic evolution of the deepest sections of the Vammala migmatite area is , however, very scanty, as it largely

3 4427048

derives from diamond drill cores of Ni exploration holes. Some observations suggest that the early stage of metamorphism and crystallization of garnet took place at di stinctly higher pres ure (7-8 kb) than that at which the metamorphic peak temperature was reached (5-6 kb). For the time being, it is difficult to establish what impact D 1 deformation had on chang­es in metamorphic grade on the erosion surface.

South of the Vammala migmatite area, in the Turku potassium granite migmatite area, 1885-Ma­old metamorphism is cut by a strong thermal pulse of 1850-1830 Ma (Väisänen et al. 1994, this volume). Thi s younger metamorphi sm is only weakly visible in the Vammala migmatite zone. In the Turku area, the F2 folding with a subhorizontal axial plane is syntectonic with tonalite magmatism, 1885 Ma old. Folding of the same age in the Tampere-Vammala area has a vertical axial plane as has the syncline of the Tampere schist belt. In the Vammala migmatite area, metamorphism mainly evolved during D 1 de­formation, whereas in the Turku area the crystalliza­tion of garnet and cordierite did not start until the D2 deformation. The evolution of deformation and met­amorphism in the Vammala migmatite area preceded that in the Turku migmatite area. Observations sug­gest a suture zone in the Vammala migmatite area between two island arc systems (Kähkönen et al. 1994, this vo1ume). At least part of the Vammala migmatite area is , however, closely related to the Tampere island arc formation .

33

Geolog ical Survey of Finland , Guide 37 Tilllo Kilpeliiin en, Se rge)' Korikovsky, Kale"i Korsman anel Mikko Niroll ell

References

Kähkönen, Y. 1987. Geochemistry and tectonomagmatic af­finities of the metavolcanic rocks of the early Proterozoic Tampere schist belt , southern Fin land . Precambrian Res. 35, 295-3 1 I.

Kähkönen, Y., Lahtinen, R. & Nironen, M. 1994. Palaeo­proterozoic supracrustal belts in southwestern Fin land. Geol. Surv. Fin land, Guide 37, 43-47 .

Koistinen, T. (ed. ) 1994. Precambrian basement ofthe Gulf of Finland and surrounding area, I : I 000 000. Espoo: Geo l. Surv. Finland.

Korja, T., Luosto, U., Korsman, K. & Pajunen, M. 1994. Geophysica l and metamorphic features ofPalaeoproterozoic Svecofennian orogeny and Palaeoproterozoic overprinting on Archaean crust. Geol. Surv. Finland, Guide 37, 11 -20.

Nironen, M. 1989. Emp lacement and structural setti ng of granitoids in the early Proterozoic Tampere and Savo schi st be ils, Finland - implications for contrasting crusta l evolu­tion . Geol. Surv. Finland, Bull. 346, 83 p.

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Sederholm J.J. 1897. Über ei ne archäi sc he Sedi mentformation im Südwestlichen Finnland und ihre Bedeutung für die Erk lärung der Entstehun gsweise des Grundgebirges. Bull . Comm . geo l. Finlande 6, 254 p.

Sederholm, J.J. 1931. On sub-Bothni an unconformity and on the Archean rocks formed by secularweathering. Bull. Comm. geol. Fin lande 95 , 81 p.

Simonen, A. 1980. The Precambrian in Fi nland. Geol. Surv. Finland, Bull. 304, 58 p.

Vaasjoki, M., Huhma, H. & Karhu, J. 1994.Evolut ion of the continental crus t in Finland with special reference to the Svecokarelian orogeny . Geol. Surv. Finland, Guide 37, 2 1-22.

Väisänen, M., Hölttä, P., Rastas, J., Korja, A. & Heikkinen, P. 1994. Deformation , metamorphism and thedeep structure of

the crust in Turku area, southwestern Finland. Geol. Surv. Finl and, Guide 37,35-41.