Paragneisses of the Nyartinskiy metamorphic complex and the Mankhobeinskaya suite (Subpolar Urals): metamorphism, mineral and structural properties

The paper discusses mineral and structural properties of paragneisses of the Nyartinskiy complex and the Mankhobeyskaya suite. The PT-parameters of mineral equilibria of the second and third metamorphism stages have been estimated using the computer programs by the classical and multi-equilibrium mineral thermobarometry methods. By the microstructural parameters, the study rocks undergo three independent tectonic-metamorphic stages. The first stage is acknowledged by the presence of strongly compact and isoclinal folds in the paragenesis with first-generation foliation and by relict minerals that are absent in the overlying (Upper Riphaean) sediments. The second stage corresponds to the progressive metamorphism with principles of the epidote-amphibolite facies (metamorphism peak T=606-527 °C, P=8.8-7.9 kbar) and is accompanied by the development of asymmetric similar folds in the paragenesis with cleavage foliation. The third metamorphism stage (diaphthoresis) in paragneisses means the development of secondary changes. The calculated crystallization temperatures of chlorites correspond to the facies of green shales (T=285-232°C).