Thermodynamic conditions of metamorphism of upper proterozoic deposits of Polar Urals

The PT parameters of metamorphism of Upper Proterozoic deposits of the Nyarovey series of the Polar Urals were estimated for the first time on the basis of mineralogical geothermometers. Among the rocks of the Nyarovey series, two main groups of rocks are distinguished: quartz-epidote-chlorite-actinolite rocks (metabasalts) and micaceous-quartz schists (metapelites). The mineralogical analysis showed that the metabasalts are composed of the following rock-forming minerals - quartz, chlorite, albite, muscovite, amphibole, fluorapatite, epidote and accessory - zircon, titanite, rutile, magnetite, pyrite, chalcopyrite, galena. In the metapelites - quartz, chlorite, albite, muscovite, biotite, potassium feldspar, calcite, fluorapatite, epidote, allanite; accessory and ore - zircon, apatite, garnet, titanite, rutile, tourmaline, ilmenite, magnetite, sphalerite, pyrite, chalcopyrite. When calculating geothermometric parameters, a plagioclase-amphibole geothermometer for metabasites and a muscovite geothermometer for metapelites were used. Our estimation of pressure and temperature values in the metabasites and metapelites did not reveal any strong differences. It was established that the values of temperature and pressure were within 350-500 °C and 3-5 Kbar. At the same time, the temperature range turned out to be higher than assumed in previous studies. The metamorphic rocks of the Nyarovey series were formed under the conditions of the epidote-muscovite-chlorite subfacies of the greenschist facies and the lower part of the epidote-amphibolite facies, under medium and elevated temperatures and low pressures.