Mineralogical indicators of the metamorphism of precambrian rocks in the crystalline basement of the volga-ural oil and gas province

Abstract

© SGEM2019. The crystalline basement of the East European Platform is composed of metamorphic rocks of the Archean-Proterozoic age. They are interesting as products of fluid-magmatic differentiation of the primary crust, which led to the formation of the early continental crust. The Volga-Ural segment of the crystalline basement is located in the east of the platform. It is covered by a sedimentary cover up to 1.5-2.0 km and was opened by the deep oil drilling. Based on the study of the obtained materials it was established that the crystalline basement within the Volga-Ural segment is composed of metamorphic complexes of granulite and amphibolite facies. The rocks have different genesis and age. They were formed in environments with different tectonic regimes and metamorphosed. Granite-gneiss domes located at the central part of the arch uplifts. Their formation is associated with the processes of metamorphic and metasomatic processing of the primary crust of the basic-ultrabasic composition. Within the crystalline basement of the Tatar arch granite-gneiss complexes are composed of highly metamorphosed, partially migmatised mafic and high-alumina rocks. Mafic rocks are metamorphic altered associations of igneous rocks of predominantly basic composition (Otradnenskaya series). The high-alumina rocks are composed of metamorphosed primary sedimentary rocks (Bolshecheremshanskaya series). The original nature of these rock complexes causes debate. Sensitive indicators of geological processes are the minerals of the garnet group. Their formation in the rocks of the crystalline basement is associated with the anatectic melting of the original rocks. All of the investigated garnets belong to the almandine series, but they are characterized by variability of the chemical composition. The different chemical composition of almandines notes to the different chemical composition of the substrate from which the almandines crystallized. Thus, the composition of almandine reflects the chemical characteristics of the original rocks and can be used for the reconstruction of their primary genesis. The data of mineralogical studies are confirmed by the results of carbon isotope analysis. The carbon presents in the rocks in the composition of rock-forming minerals and graphite. The predominance of the heavy carbon isotope indicates the magmatogenic origin of the rocks of the Otradnenskaya series, and the light one notes to the biogenic nature of carbon in the rocks of the Bolshecheremshanskaya series. The development of the earth's crust in its early stages has a very complex history. Its reconstruction is a fundamental geological problem.