Bulletin of the Geological Society of Finland (Dec 1999)
Structural analysis and metamorphism of Palaeoproterozoic metapelites in the Seinäjoki-Ilmajoki area, western Finland
Abstract
The Palaeoproterozoic Svecofennian bedrock of the Seinäjoki-Ilmajoki area, western Finland, is largely composed of porphyroblastic metapelites. In the area, the regional metamorphic grade increases towards the southwest. Over a distance of 15 km, andalusite mica schists gradually grade into migmatitic garnet cordierite-sillimanite mica gneisses with a facies-series of the andalusite-sillimanite type. Five regional metamorphic zones are present: andalusite, sillimanite-muscovite, sillimanite-K-feldspar, cordierite-K-feldspar and garnet-cordierite-K-feldspar. The primary layering (S0) of the mica schists is deformed by an isoclinal fold phase (F2), which is synchronous with the main metamorphic phase and the growth of micas. S1 is very weak and subject to interpretation. The S2 schistosity is deformed by intense late-metamorphic F3 and F3b folds, which have formed under slightly different metamorphic conditions: practically no metamorphic micas have grown parallel to axial planes while within F3b folds there are a few granitic veins parallel to these planes. The F3 and F3b folds probably belong to one phase. S2 dominates in the mica schists while S3 and S3b dominate in the mica gneisses. The metapelites are also deformed by younger minor fold phases (F4 and F5). A composite schistosity (S0±S1±S2±S3, or S3b) commonly occurs in the metapelites. The peak of regional metamorphism has been associated with the intrusion of 1.89-1.88 Ga old tonalite plutons. Geothermometric estimates for regional metamorphism are c. 730 °C at an assumed pressure of 5 kbar. Neosomes in the high-grade mica gneisses occur as patches rather than as elongated, narrow veins. Garnet coexists with cordierite, but the minerals are rarely in equilibrium. Muscovitization and the formation of retrogressive andalusite did not occur in the high-grade mica gneisses, but there is minor kyanite indicating that the crust probably underwent near-isobaric cooling. The area of highest-grade regional metamorphic conditions contains a post-kinematic mangeritic quartz monzonite (1.87 Ga), which has a narrow granulite-grade contact aureole. The chemical composition of biotites in the metapelites varies as a function of metamorphic zoning, but the chemical composition of metapelites remains mostly unchanged.
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