Pressure and temperature history of a low-pressure transitional granulite area, Turko, SW Finland

Abstract

The Turku granulite area forms the westernmost of several low-pressure granulite domains in the Svecofennian Schist Belt of southern Finland. The area shows a gradual transition from amphibolite to granulite facies metamorphism. Estimated peak metamorphic conditions, based on geothermobarometers applied to politic gneisses, metavolcanic and charnockitic rocks, are 650-680°C and 4.8 ± 0.5 kbar for the amphibolite domain, 675-725°C and 4.7 ± 0.5 kbar for the transition zone and 750 ± 50°C and 4.9 ± 0.5 kbar for the granulite domain. Prograde and retrograde mineral assemblages of pelitic gneisses indicate a 'clockwise' P-T path. On the basis of a comparison with other low-P granulite domains occurring in the same belt, namely the West Uusimaa Complex and the Sulkava area, we conclude that all three granulite domains were formed during the same thermal event in the late stage (1850-1800Ma) of the Svecofennian orogeny. The metamorphic evolution of the Svecofennides is consistent with moderate crustal thickening accompanied by additional heating through thinning of the mantle lithosphere.