The amphibolite‐granulite facies transition in West Uusimaa, S.W. Finland. A fluid inclusion study

Abstract This work presents the results of a fluid inclusion study of an amphibolite‐granulite facies transition in West Uusimaa, S.W. Finland. Early fluid‐inclusions in the granulite facies area are characteristically carbonic (CO2), in contrast to predominantly aqueous early inclusions in the amphibolite facies area. These early inclusions can be related to peak metamorphic conditions (750‐820°C and 3‐5 kbar for peak granulite facies metamorphism). Relatively young CO2 inclusions with low densities (<0.8g/cm3) indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift. N2‐CH4 inclusions, with compositions ranging between pure CH4 and pure N2 (Raman spectral analysis), were found in the whole area. They are probably syn‐ or even pre‐early inclusions. Only nearly critical homogenizing inclusions have been found (low density). Pressure estimates, based on densities of early fluid inclusions, show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric. Granulite facies metamorphism in West Uusimaa is a thermal event, probably induced by the influx of hot, CO2‐bearing fluids.