Bulletin of the Geological Society of Finland (Jun 1999)
Structural and metamorphic evolution of the Turku migmatite complex, southwestern Finland
Abstract
The Turku migmatite complex in southwestern Finland is a representative area for the type of tectonic and metamorphic evolution seen within the Palaeoproterozoic Svecofennian Orogen in southern Finland. The orogeny can be divided into early, late and postorogenic stages. The early orogenic structural evolution of the crust is expressed by a D1/D2 deformation recorded as bedding-parallel S1 mica foliation deformed by tight to isoclinal D2 folds with subhorizontal axial planes and a penetrative S2 axial plane foliation. Syntectonic ca. 1890-1870 Ma tonalites were emplaced during D2 as sheet intrusions. This deformation is attributed to thrust tectonics and thickening of the crust. The late orogenic structural evolution produced the main D3 folding, which transposed previous structures into a NE-SW trend. The doubly plunging fold axis produced dome-and-basin structures. The attitude of the F3 folds varies from upright or slightly overturned to locally recumbent towards the NW. Granite dikes were intruded along S3 axial planes. Large D3 fold limbs are often strongly deformed, intensively migmatized and intruded by garnet- and cordierite-bearing granites. These observations suggest that these potassium-rich granites, dated at 1840-1830 Ma, were emplaced during D3. This late orogenic NW-SE crustal shortening further contributed to crustal thickening. Subvertical D4 shear zones that cut all previous rock types possibly controlled the emplacement of postorogenic granitoids. Steeply plunging lineations on D4 shear planes suggest vertical displacements during a regional uplift stage. Metamorphic grade increases from cordierite-sillimanite-K-feldspar gneisses in the northwest and from muscovite-quartz±andalusite rocks in the southeast to high-temperature granulite facies migmatites in the middle of the study area. Block movements during D4 caused the observed differences in metamorphic grade. Garnet and cordierite are mostly breakdown products of biotite and sillimanite and their growth is mainly syntectonic with D2; they are elongated within the S2 plane and deformed by D3 folds. Leucosome veins were already produced during D2, but in situ melting began during D3 producing granitic leucosomes with euhedral garnet and cordierite along the axial planes of F3. Garnet is typically altered to cordierite and plagioclase in the presence of sillimanite, indicating decompression close to the temperature maximum. Pressure and temperature estimates from garnet and cordierite indicate that the granulites reached temperatures in excess of 800°C at approximately 6 kbar pressure while the adjacent amphibolite facies rocks crystallized at 100-150°C and 1-2 kbar lower temperatures and pressures. Therefore, the granulite areas represent the deepest structural levels in the area. Structural and metamorphic observations indicate that peak metamorphism was reached during the compressional late orogenic D3 stage ca. 1840-1830 Ma ago. Crustal thickening simultaneous with thinning of the mantle lithosphere may explain this kind of tectono-metamorphic evolution.
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