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TRACE ELEMENTS IN THE ROCKS OF THE MOBILE BELTS OF THE PRECAMBRIAN BASEMENT OF THE VOLGA-URAL OIL AND GAS PROVINCE
Abstract
There are the structural-material complexes of the rocks in the crystalline basement of the Volga-Ural oil and gas province, which characterized by different composition and structural position. The biggest structure, which occupies a central position in the region, is the Tatar arch. The Tatar arch is a large megablock of the lithosphere (granite-gneiss core) bounded by mobile zones (graben-shaped deflections). They are associated with pre-metamorphic basic magmatism. The areas of the crust bounding and separating the granite-gneiss domes are zones of a repeated diaphtoresis process. In the Archean-Proterozoic, the rock complexes underwent a regional metamorphism of the granulite facies and at the regressive stage the amphibolite facies. Rocks of mobile belts were drilled by deep parametric wells. They are represented mainly by biotite-, amphibole-, pyroxene-containing gneisses, plagiogneisses, and rarely crystal shales. The layer of gneisses and plagiogneisses is heterogeneous in petrographic composition. It was probably formed by various rocks of predominantly basic composition. In the Archean period, they underwent a metamorphism of granulite facies. In the early Proterozoic, rocks underwent ultrametagenic process with partial granitization. Their modern morphology was formed as a result of subsequent diaphtoresis in the conditions of the amphibolite facies. The findings of relict pyroxene grains in amphibole and biotite crystals and relict phaneritic texture indicate that the original rocks were primary magmatogenic and sedimentary-volcanogenic rock complexes. These rock complexes formed from ancient ultrabasic-basic substrate. They are characterized by an appropriate composition of trace elements. Elevated concentrations of such elements as Cr, Ni, Zr, indicate the participation of ultrabasic rocks in the formation of the complex. Almost throughout the all section, the rocks are broken by a rare network of cracks, filled with red oxide material and black chlorite-containing clayey substance, which indicates the occurrence of hydrothermal processes.
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