Results of geological and geophysical studies of the Nerkayu complex in the Nether-Polar Urals

Abstract

Introduction: among researchers, there are currently conflicting views on the origin of the Nerkay complex, the history of metamorphic and post-metamorphic transformations, their number, sequence, and time of manifestation. Given the inaccessibility and poor exposure of the Nerkayu complex, the combination of geophysical and petrophysical methods represents a valuable research tool for studying high-pressure metamorphic formations.

Methodology: rock density was determined by hydrostatic weighing using Mettler Toledo laboratory balances on whole rock samples. The magnetic susceptibility of rocks in powder form was measured using a KAPPABRIGE KLY-2 device with a measurement accuracy of 0.2−1%. Petrographic description of the rocks was carried out in polarized sections using a Polam-215 polarizing microscope. Ore minerals were studied using a Tescan VEGA3 LMH scanning electron microscope. The chemical composition of the rocks was obtained by the method of wet chemistry and the complex method of wet chemistry and X-ray fluorescence analysis. The interpretation of geophysical material consisted of a comprehensive interpretation of gravitational and magnetic fields, as well as transformants, using the comparison method, i.e. using a map overlay using the results of petrophysical grouping of rocks.

Results, discussion and conclusions: six subgroups of rocks with different densities and magnetic properties have been identified. The primary petrophysical groups, represented by metabasites and crystalline schists, were delineated based on density parameters, material composition, and textural and structural characteristics. The crystalline schists are further divided into two categories based on material composition and density. The first category, formed by sedimentary rocks (pararocks), and the second, formed by major igneous formations (orthorocks). Although the rocks are completely or partially recrystallized, studies of metabasites and schists have shown that their petrophysical properties are primarily determined by the density of the primary composition (protolith). Differences in the magnetic properties of metabasites have allowed the delineation of the area of distribution of metasomatic processes (diaphthoresis). The local nature of the distribution of magnetite in para- and orthorocks allowed to identify non-magnetic and magnetic varieties among them.

The results of qualitative interpretation of geophysical data made it possible to divide the Nerkayu complex into two areas: northeast and southwest. In the northeastern part, which is most susceptible to post-metamorphic processes, the areas of unchanged or slightly altered metabasites in the east and ferromagnetic, difluorinated and metasomatically altered amphibolites in the west are mapped along the contours of local gravity and magnetic anomalies. In the southwestern part of the Nerkayu Complex, predominantly reduced gravity and magnetic values are observed over widely developed schists of sedimentary origin.The presented example of geological and geophysical research on the Nerkayu complex will assist specialists engaged in the reconstruction of the primary composition and geodynamic position of metamorphic complexes in the northern Urals region in their understanding of the relationship between the physical properties of rocks and geophysical fields and the deep structure of the Nerkayu complex.