Abstract
The metamorphic grade in west Uusimaa, SW Finland, increases from low- to intermediate-amphibolite facies in the west (Island of Kemiö) up to granulite facies in the middle and eastern parts. Mineral assemblages in metapelitic rocks indicate metamorphic conditions of 700–825°C, 3–5 kbar for the high-amphibolite and granulite facies region, which is in agreement with independent estimates. These metamorphic conditions are used to evaluate garnet-biotite thermometry. Zoning in garnet, Fe enrichment and Mg depletion towards rims bordered by biotite or cordierite, and a distinct Fe depletion/Mg enrichment of biotites in contact with garnet are caused by FeMg exchange of these minerals during retrogression (550°C). The general prograde metamorphic trend from medium- to high-grade in West Uusimaa is reflected by a systematic change in the composition of biotites. Thermometry and biotite chemistry are used to indicate that the granulite facies metamorphism in West Uusimaa is probably caused by a thermal event.
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