Kyanite-talc schists from the Beloretsk eclogite-bearing metamorphic complex, the southern Urals

Abstract

In 1940, Ozhiganov [1] described the Beloretsk metamorphic complex (BMC) on the western slope of the southern Urals in the eastern part of the Bashkir Anticlinorium. At present, the protolith of this complex is correlated rather reliably with the Riphean unmetamorphosed rocks that occur in the western area [2]. The thermobaric conditions of metamorphism in the BMC were a matter of debate for a long time and were considered as low-pressure ones in [3‐5 and others]. We have repeatedly stated that the BMC rocks were formed under high-pressure conditions of the kyanite‐sillimanite metamorphic facies [6]. The character of the Late Vendian regional metamorphism of the BMC was specified in 2001‐2002 as a genotype of a new high-pressure zoisite‐omphacite metamorphic facies [7]. The recognition of the BMC as a high-pressure metamorphic facies was based on the occurrence of abundant crustal eclogites in this complex [8], the identification of zoisite (previously mistaken for andalusite during geological mapping), and the determination of phengite as a rock-forming mineral in crystalline paraschists. We suggested the presence of kyanite in these schists based on frequent findings of unrounded kyanite crystals in heavy concentrates from the BMC-eroding river channel sediments of streams and the occasional presence of kyanite-type pseudomorphs in thin sections [9]. We found samples of metamorphic paraschists with incompletely replaced kyanite only in 1999. Based on laboratory investigations, the micalike mineral from kyanitebearing schist was identified as talc. It is known that kyanite‐talc (kyanite‐garnet‐talc) schists are rather rare exotic rocks. At the same time, they are typical of eclogite-bearing high-pressure metamorphic complexes in some provinces [10].