Precambrian Research, 64 (1993) 295-309 295 Elsevier Science Publishers B.V., Amsterdam The late Svecofennian granite-migmatite zone of southern Finland—a belt of transpressive deformation and granite emplacement Carl Ehlers*, Alf Lindroos and Olavi Selonen Department of Geology and Mineralogy, Abo Akademi University, SF-20500 Abo, Finland Received March 21, 1991; revised version accepted November 3,1992 ABSTRACT The late Svecofennian granite-migmatite (LSGM) zone in southwestern Finland is a ~ 100 km wide and 500 km long belt transecting the southern Svecofennides from WSW to ENE. It was formed in an area of thin pillow lavas, volcaniclastic sediments and limestones. The area is interpreted as having been an early basin of crustal extension which was the locus of an inherited zone of weakness in the Proterozoic crust. Early recumbent folding was followed by crustal thickening and intrusions of ~ 1.89-1.8 8 Ga old plutonics. The LSGM-zone is characterized by 1.84-1.83 Ga old rhomboidal sheets of late Svecofennian microcline granite and is bounded by ductile shears. Amongst the two major phases of deformation defined in the LSGM-zone, the earlier one (Dl) affected only the supracmstals and the 1.89-1.88 Ga old early plutonics. In contrast, the later phase (D2) also deformed the late Svecofennian migmatites and granites. Dl represents a complex and long-lasting deformation event which in- cluded overturning and thrusting of the Svecofennian strata. D2 comprised ENE-WSW directed drag accompanied by NNW-SSE compression. The Svecofennian crust was thick- ened further and anatectic microcline granites intruded along thrusts. The rhomboidal outline of the late Svecofennian granite sheets indicates a sense of movement in agreement with measured dextral strike-slip in the shears delimiting the LSGM-zone. Imbricated feldspar megacrysts in the granites indicate thrusting towards the west during the stage of granitic magmatism. The gently dipping early Svecofennian gneisses and the late granite sheets were folded into upright F2 folds with gently plunging axes. Locally, the F2 axial surfaces were intruded by late Svecofennian granite mobilisates. 1. Introduction 1986; Patchett and Kouvo, 1986). They rep- resent an episode of massive formation of con- The Svecofennian domain of Finland and tinental crust and comprise I-type rocks rang- Sweden (Fig. 1, inset) is a large crustal seg- ing from diorites through dominant tonalites ment of ~2.0-1.8 Ga age, consisting of calk- and granodiorites to red microcline-rich gran- alkaline plutons and metavolcanic rocks with ites (Front and Nurmi, 1987; Gaal and Gor- interspersed metagreywackes and metapelites. batschev, 1987). Plutonism commenced during the calk-alka- In southern Finland, the 2.0-1.8 Ga old crust line volcanism and granitoids occupy most of was intruded by a second generation of gran- the crustal volume. The oldest known zircons ites. These late Svecofennian, S-type granites found in metagreywackes are 2.1 Ga old have ages of ~ 1.84-1.83 Ga (Huhma, 1986; (Huhma, 1990), but no source rocks of that Vaasjoki and Sakko, 1988; Suominen, 1991). age have been found. Most of the early grani- They form a 500 km long and 100 km wide belt toids have ages of 1.89-1.88 Ga (Huhma, of intrusions (Fig. 1) and are associated with high-grade metamorphism and extensive mig- Corresponding author. matites which have been described by Seder- 0301-9268/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved. SSDI0301 -9268 (93 )EO061-G 296 CEHLERSETAL. lARCHAEAN '••'•••••.| Rapakivi granites шщ Late Svecofennian granite sheets 1.84-1.83 Ga щш Early Svecofennian ^™ granitoids 1.89-1.88 Ga | I Supracrustal rocks Fig. 1. The Svecofennian rocks of southern Finland (after Simonen, 1980). The zone of late Svecofennian 1.84-1.83 Ga old granites and migmatites (LSGM) transects the earlier crust. S=Sulkava, T— Tampere, (7= West Uusimaa, Mustio and Orijarvi, P=Pernio, AT=Kemio, #=Hanko, Pa = Pargas, 7V=Nagu, To=Torsholma. holm (1926), Hopgood et al. (1976), Edel- datings and numerous new geophysical and man and Jaanus-Jarkkala (1983), Hopgood et geological data facilitate a different approach al. (1983),Korsmanetal. (1984), and others. where distinct structural and stratigraphic units There has been a tendency to describe the within the large Svecofennian domain can be Svecofennian geology of Finland on the basis distinguished. of detailed studies in only a few key areas (e.g. The aim of this paper is to give an overview Simonen, 1960). This is a legacy of J.J. Seder- of the late Svecofennian granite-migmatite holms excellent pioneer papers from the begin- zone of southern Finland, the LSGM-zone, as ning of the century. His strong influence on a distinct structural unit. We have tentatively geological thinking has subsequently led to defined two main phases of deformation (Dl overly generalized inferences about stratigra- and D2) separated by a time lapse of some 50 phy and deformation. At present, new U-Pb Ma as shown by their relationships to radi- THE LATE SVECOFENNIAN GRANITE-MIGMATITE ZONE OF SOUTHERN FINLAND 297 ometrically dated granitoid intrusions. Phase number of new U-Pb age determinations from Dl comprises deformational episodes mostly the Torsholma area are referred to as personal preceding the intrusion of the 1.89-1.88 Ga old communications by M. Vaasjoki, 1991. granitoids, but also includes episodes during The red microcline granites along the south- and soon after these intrusions. In contrast, ern margin of the large plutonic complex in phase D2 also affects the 1.84-1.83 Ga old central Finland are chemically similar to those granites. We made no attempts to distinguish of the LSGM-zone, but are older and belong to between different episodes of deformational the same age group as the predominant 1.89- phase Dl. Thus, SI designates all the domi- 1.88 Ga old granodiorites and tonalites nant subhorizontal schistosities formed during (Huhma, 1986; Patchett and Kouvo, 1986). In the Dl events. D2 is confined to the LSGM- fact, no 1.84-1.83 Ga granite ages have been zone and must not be confused with deforma- recorded in the Finnish part of the Svecofen- tion phases in other parts of the Svecofennian nian domain outside the LSGM-zone (M. domain. Later deformations (D3 etc.) as de- Vaasjoki, pers. commun., 1991). scribed in the literature (e.g. Schreurs and The LSGM-zone of late Svecofennian gran- Westra, 1986) are not discussed in this paper. ite massifs extends to the shore of the Gulf of Our conclusions are based on observations Finland in the south. Its southwestern part is made in the western part of the LSGM-zone. separated from the realm of massive early Sve- cofennian granites by a steeply dipping re- 2. The late Svecofennian granite-migmatite gional shear zone with a considerable strike-slip (LSGM) zone of southern Finland component (Fig. 2 in this paper, and Ehlers and Lindroos, 1990a). South of this marked 2.1. Age determinations and geographical shear zone, all granitic rocks belong to the 1.89- extent 1.88 Ga age group (Suominen, 1987, 1991). Maps based on numerous drillings through Although rather few U-Pb zircon age deter- the Palaeozoic cover of northern Estonia into minations are available, the results are consis- its Precambrian basement show no extensive tent: the migmatites and granites forming the granite and migmatite occurrences (Puura et LSGM-zone have ages of ~ 1.84-1.83 Ga. Age al., 1983). This is in accordance with earlier determinations on the migmatites of the Sul- descriptions of the islands in the southern part kava area (1833 ±16 Ma; Korsman et al., of the Gulf of Finland (Ramsay, 1891). Fur- 1984), the Hanko granite (1830± 10 Ma; thermore, published geological maps and de- Huhma, 1986), the Nagu granite (1842 ±31 scriptions indicate that the early Svecofennian Ma; Suominen, 1991) and the Kumlinge gran- granitoids within the LSGM-zone are more ite in the southwestern archipelago (1840 ±4 deformed than those in the 1.89-1.88 Ga old Ma; Suominen, 1991) all plot within this pe- granitoid massifs of the central Finnish early riod (Figs. 1 and 2A). Suominen (1991) also Svecofennian granitoid complex. reports three new U-Pb ages from the LSGM From the data now available we can deline- granites in the Pernio area (1829 ±14; ate approximately the boundaries of a roughly 1840 ± 8; 1829 ± 14 Ma) and one from Kemio 500 km long and 100 km wide zone of re- (1840±8 Ma). Undeformed post-orogenic peated strong deformation and high-grade dykes and intrusions with ages of ~ 1.80 Ga metamorphism accompanied by extensive have been reported from different parts of the migmatitization and intrusions of late Sveco- LSGM-zone (Nykanen, 1983, 1988; Korsman fennian granites along the southern coast of et al., 1984; Vaasjoki and Sakko, 1988). A Finland (Fig. 1). 298 С. EHLERSETAL. 40 km THE LATE SVECOFENNIAN GRANITE-MIGMATITE ZONE OF SOUTHERN HNLAND 299 Fig. 2. (A) Schistosity trends as traced from geological map sheets covering 7000 km2 of the southwestern archipelago of Finland. The area is well exposed and the trends in this map (from Ehlers and Lindroos, 1990a) are based on measured field observations. A steeply dipping shear zone transects the area in a NW direction. The LSGM-zone with characteristic dome- and basin-structures in gently dipping migmatitic supracrustal rocks is northeast of the shear zone. On the south- western side, the steep schistosity in the 1.89-1.88 Ga old granitoids is deflected by dextral movement. The localization of Fig. 5 (Torsholma area) is indicated. S=Sottunga area. (B) Gently dipping stretching lineations and mineral linea- tions in the LSGM-zone.