Composition features of the Upper Carboniferous-Lower Permian rocks of the Losinoostrov Fm. and the role of extrabasin sources in their formation (Subpolar Urals)

Research subject. The Upper Carboniferous-Lower Permian rocks of the Losinoostrov Formation of the Belsk-Eletsk structural-formational zone, exposed on the left bank of the Kozhim river, near the mouth of the stream Northnichael of the western slope of the Subpolar Urals. Methods. Optical and electron microscopic methods (including cathodoluminescence), chemical silicate, carbonate and X-ray diffraction analyses. Results and conclusions. The studied rocks are assigned to three main groups: mixtolites (rocks of mixed composition), carbonatoliths, and silicytolites with varieties.  Mixtoliths consist of three and four main components: carbonate, silty, argillaceous and siliceous. The variable ratio of rock-forming components of mixtolites was clearly reflected in the discriminative modular diagram. Here, fields of predominantly siliceous and silty mixolites (cluster I), predominantly argillaceous (cluster III), as well as transitional mixolites (cluster II) are identified. Relatively pure and silty-argillaceous-siliceous limestones were distinguished among carbonatoliths. According to the formed elements, pure limestones are divided into bioclastic and peloid-micritic. Silicytolites are represented by (1) radiolarites and radiolarian spongoliths; (2) silty-argillaceous silicytolites and (3) cherts (secondary siliceous formations). The established quantitative ratios of rock-forming components showed that more than half of the studied mixtoliths are composed mainly of the material of extrabasinal origin. Relatively high contents of extrabasinal fine material in carbonatoliths indicate a periodic decrement of the hydrodynamic regime during the accumulation of the upper part of the formation. The studied deposits were probably formed in distal and proximal ramp settings.