FINAL STAGES OF THE METAMORPHIC EVOLUTION OF THE TANAéLV BELT, KOLA PENINSULA, BALTIC SHIELD: EVIDENCE FROM U-PB DATING ON ZIRCON, TITANITE, AND RUTILE

Abstract

Field and isotopic evidence suggests the following sequence of metamorphic events for the Tanaélv belt, as determined at the Yavrozero area. The peak of high-temperature amphibolite (and, presumably, granulite) metamorphism related to the main deformation stage is constrained between 1945 ± 8 Ma (magmatic crystallization age of anorthosites) and 1928 ± 10 Ma (age of a garnet-pyroxene dike cutting across foliated anorthosites). Metamorphism of a garnet pyroxenite dike to high-temperature amphibolite facies suggests that high-temperature transformations continued after 1928 Ma. The exhumation-related decompression and cooling stage, which was marked by abundant zircon crystallization in the rocks of the Tanaélv belt and LGB, began at 1920-1916 Ma. The exhumation was followed by lower grade amphibolite metamorphism between 1909 and 1898 Ma, which was recorded in the formation of high-U zircons. Rock cooling to 450° C, as defined by the rutile age, occurred at 1880-1870 Ma. Age data on titanite and rutile having different closure temperatures of the U-Pb system made it possible to calculate the cooling gradient of the rocks from 700 to 450°C at ∼6°C/Ma.