PECULIARITIES OF THE LATE ARCHEAN METAMORPHISM OF ALUMINOUS GNEISSES IN CHUPINSKY COMPLEX OF BELOMORIAN REGION

摘要

The Late Archean metamorphism of aluminous gneisses have been studied in Chupinsky nappe on the area of Verkhne-Pulongskoje Lake, where any later metamorphic transformations, typical for Belomorian mobile belt, aren't known. The metamorphic conditions were determined on the base of garnet and biotite compositions. Thus, two stages of metamorphism have been distinguished. Mineral parageneses of the first stage have been mapped in central part of the nappe, they corresponds to T = 570-700 deg C. The second stage processes have formed the actual appearance of rocks, they made migmatization and recrystallization of gneisses. The processes had regressive character and were the most intensive in the margin parts of the nappe where deformations were the most intensive also, Parameters of metamorphism decreased during the second stage from T 700 deg C and P = 10-11 kbar down to T =540 deg C and P = 5.5 kbar. Both stages did not exceed level of the high-temperature amphibolite facies. It means that in the Late Archean the granulite facies metamorphism had not development over the whole region, as it is believed in some works (Volodichev, 1990; Lobach-Zhuchenko e. a., 1993; Drugova, 1996, 1997, 1999).