The composition and conditions of the origin of Late Precambrian terrigenous rocks of the Karagass group (Biryusa Sayan Region)
We are presents the results of petrographic and geochemical studies of the Late Precambrian terrigenous rocks of the Shangulezh and Tagul suites of the Karagas group of the Biryusa Sayan Region (South-West of the Siberian Craton). The results of reconstruction of the sources clastic material and the degree of their chemical conversion are presented. The studied terrigenous deposits of the Shan-gulezh and Tagul suites of the Karagas group are represented by arkoses sandstones and siltstones, and are characterized by similar characteristics. For the majority of the studied rocks, mineralogical signs of epigenetic transformations at the stage of late catagenesis are recorded, expressed by fouling of potassium feldspar fragments by freshly formed autigenic potassium feldspar, regeneration of quartz clastic grains, and also by fouling of quartz fragments and potassium feldspar clay-micaceous clay-micaceous aggregate (ithlite-hydrite ). A characteristic petrochemical feature of most of the studied samples is sharply increased K2O concentrations relative to very low Na2O contents. The presence of fragments of granitoids, quartzite's, flints in the clastogenic component of the studied sandstones and siltstones indicates the basement rocks of the Siberian craton as the main feeding province for sedimentary strata of the Shangulezh and Tagul suites of the Karagas group. The set of accessory minerals in the heavy fraction of most of the studied samples, as well as the distribution pattern of rare and trace elements that are not mobile during epigenetic transformations, indicate the predominance of acidic rocks in the region in the sours area, and a distinct europium minimum in the REE spectra of the studied terrigenous rocks indicates the prevalence of granitoids in source of demolition. Taking into account the geological structure of the study area, it is assumed that one of the main rocks in the sources area for these sediments could be granitoids of the Sayan complex, which are widespread in the Cissayan marginal ledge of the Siberian craton basement. Similar petrographic and petro- and geochemical characteristics of the studied terrigenous rocks indicate that they accumulated in the same tectonic regime, in a single sedimentation basin, when clastic material came from the same feeding province.