Age and geochemical constraints for partial melting of granulites in Estonia

摘要

The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone - 1824 + - 26, tonalite from the South Estonian Zone - 1788+ - 16 Ma and charnockite from the Tapa Zone -1761 + - 11 Ma) yield U-Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U-Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 deg C and 6 kbar) in the Estonian basement occurred at 1788-1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650-700 deg C at 1728 + - 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia.