Sm-Nd garnet and U-Pb monazite dating of high-grade metamorphism and crustal melting in the West Uusimaa area, southern Finland

摘要

The 100 km wide late Svecofennian granite-migmatite zone in southern Finland contains the Sulkava, the Turku and the West Uusimaa low-pressure, high-temperature granulite areas. In the West Uusimaa area the peak metamorphic conditions are estimated at T = 750-800 degrees C and P = 4-5 kbars. Detailed isotopic dating of different parts of migmatites (mesosomes and leucosomes) as well garnet-orthopyroxene gneisses was undertaken by conventional analysis of U-Pb on monazite and Sm-Nd on garnet. U-Pb monazite ages show that the West Uusimaa area underwent a granulite facies metamorphism at peak conditions between 1832 +/- 2 Ma and 1816 +/- 2 Ma. The area was then cooled down to 700-600 degrees C at 1.81-1.79 Ga according to Sm-Nd garnet-whole rock data. These results together with previous data show that all the three granulite areas in southern Finland share a coeval thermal event probably stemming from common or similar heat sources.