Primary composition and P-T conditions of metamorphism of meta-sediment strata of the Neoproterozoic age? eastern zone of the Kharbey block (PolarUrals)

Abstract

Introduction: The article focuses on the primary composition and the T-P conditions for the metamorphism of meta-sediment strata of indefinite genesis found in the Eastern part of the Kharbeisk block of the Central Ural tectonic zone of the Polar Urals as part of the rocks of the Kharbeiyan amphibolite-gneiss complex.

Methodology: Petrographic and mineralogical features of the rocks were studied using a polarizing microscope and an electron microscope. The primary composition of the metamorphites was determined by means of identification diagrams. Probable provenance areas of metaterrigenous formations and thegeodynamic conditions for the formation of the rocks were determined by means of an analysis of the contents of petrogenic components, rare elements and rare earth elements. The T-P conditions for the metamorphism were determined using classical and multi-equilibrium thermobarometry.

Results and discussion: The petrographic study demonstrated that the meta-sediment strata is represented by interbedded black shales, crystalline schists, and marbles. Black shales are carbonaceous-siliceous and carbonaceous-terrigenous rocks. The protolytes of crystalline schists were clay rocks and greywackes, which were apparently formed in active continental margins due to the drainage of the rocks of the basic composition with oversubduction geochemical marks, possibly including metabasalts of the Khanmeikhoyan suite and continental acidic formations. In terms of the composition of their distributive provinces the analysed rocks are different from metaterrigenous formations of the Parikvasshorsk and Verkhnekharbeisk suites. The crystalline schists demonstrated Barrovian metamorphism garnet-staurolite-kyanite. The T-P parameters of metamorphism for the kyanite zone were calculated using the TWQ software based on the mineral association Ky+Pl+Bt+Ms+Grt+Qz+Ilm+Rt and correspond to the mean temperatures and increased pressures: Т – 530–550° С, Р – 7 kbar. The thermodynamic metamorphism conditions in the garnet zone were determined by means of classical thermobarometry (Gr-Bt geothermometers, GBMP geobarometer). They appeared to be lower: Т – 460–500° С, Р – 5.6–6.5 kbar. Based on the nature of metamorphism, the analysed strata correlates with the Parikvasshorsk and Khanmeikhoyan suites transformed in the Devonian-Carboniferous.

Conclusions: Based on the obtained results we can conclude that the meta-sediment strata in the Eastern part of the Kharbeiyan block is an independent suite. It is deposited higher than the Khanmeikhoyan suite and proves the assumption that the rocks of the Kharbeiyan block formed to the East of the Khadatinsk-Khanmeiysn thrust are of metamorphic and tectonic origin, which is different from that of the rocks in the Western zone. This can be explained by the obduction and collision processes taking place during the formation of the Ural orogen.