PHYSICAL AND CHEMICAL CONDITIONS OF ROCKS METAMORPHISM OF YEREMKINSKY STRATUM (KOCHKAR ANTICLINORIUM)

Abstract

The article describes the geological structure of the Yeremkinsky stratum, developed in the frame of the Varlamovsky, Borisovsky, Yeremkinsky and Sanarsky granite-migmatite domes (Kochkarsky anticlinorium). The stratum includes biotite-garnet-stavrolite plagiogneisses, crystalline schists, amphibolites, graphite-mica quartzites. Staurolite, kyanite, sillimanite, andalusite, muscovite are noted in association with garnet and biotite in plagiogneisses and schists. The contacts with the underlying formations are unknown, and with the overlapping marbles of the Kuchinsky stratum and quartzites, the quartzitic sandstones of the Alexandrovsky stratum are tectonic. Pb-Pb datings obtained from zircons selected from the rocks of the Yeremkin stratum range from 523 - 1350 Ma. Its Lower Middle Riphean age is accepted in accordance with the South Ural serial legend and is currently not reliably characterized by radiological data. Based on a study of garnet-biotite, garnet-staurolite parageneses, comprehensively studied experimentally and widely represented in the rocks of the Yeremkin stratum, the temperature and pressure of regional metamorphism (T = 640 - 690 °C, P = 3.5 - 4.6 kbar) and diaphtoresis (T = 550 - 580 °C, P = 5.9 - 6.2 kbar) were obtained, which correspond to the conditions of the cummingtonite-amphibolite and epidote(almandine)- amphibolite facies. Consideration of the petrochemical features of amphibolites and graphitic quartzites made it possible to reconstruct the paleogeodynamic and paleogeographic conditions of their formation. Taking into account the obtained physicochemical parameters of metamorphism, it can be assumed that the rocks of the Yeremkin stratum were formed in the lower-middle Riphea in relatively deep-water conditions in the rift-continental geodynamic setting and later they experienced a diaftoresis in the epidote-amphibolite and almandine-amphibolite facies boundary conditions. This is due to the formation of the Late Riphean-Vendian granite-gneiss domes (Borisovsky, Varlamovsky, Yeremkin and Sanarsky), as well as the thermal effects of the Late Paleozoic granitoids of the Sanarsky massif.