Litologiâ i poleznye iskopaemye

The genesis of oolitic carbonates among the sulfate-carbonate-clayey deposits of the Lower Maeotian in the sections of Cape Kazantip was investigated through detailed analysis. Laboratory studies were conducted utilizing a range of analytical techniques, including carbonate chemical analysis, optical polarisation with computer support for photography and scanning electron microscopy (SEM), energy dispersive spectros- copy (EDS), isotopy, gas chromatography (GCM), electron paramagnetic resonance (EPR), infrared (IR) spectroscopy, and X-ray diffractometry. The chemical composition of the oolitic carbonates was found to be characterizing by the constant presence of dolomite (11.93–40.98%) and clay (2.42‒19.40%) compo- nents, as well as the most isotopically heavy values of carbonate carbon (2.74–5.40‰) among the enclos- ing carbonates. This indicates the oolites formation in saltwater of an extremely shallow coastal lagoon. The presence of gypsum in oolite cores and as cement in oolitic carbonates and oolitic conglogravelites has been established, and it has been determined that gypsumification of the sediments occurred at the stage of oolite formation. This process took place during a sharp fall in sea level and could be associated with the participation of sulphate ions from meteoric waters in the removal of sediments into the zone of meteoric-vadose conditions, possibly together with gas-fluid seepage. The occurrence of bottom gas-fluid seeps is corroborated by the detection of mineralized extracellular polymeric substance (EPS) in oolites, as well as the presence of bacterially induced halite, barite and high-Mn kutnogorite from the dolomite group. Among the carbonate minerals found in oolites, low and high-Mg calcites and Ca-dolomite have been identified; the distribution of the latter indicates the primary sedimentary genesis of dolomite micro- crystals in the oolite structure.

In order to determine the provenance areas and to solve paleotectonic and paleogeographic problems related to the stage of orogenic molasse formation in the Western Pre-Caucasia, sands and sandstones from Quaternary strata sections corresponding to two stratigraphic levels were studied: (i) the lower part of the Belorechenskaya Fm section (Gelazian) (sample MK-29) and (ii) the strata forming a series of contiguous terraces (Upper Neopleistocene) in the area of the southwestern outskirts of Maikop town (sample MK-30). U‒Th‒Pb isotope dating and morphological study of detrital zircons (dZr), as well as mineralogical analysis of garnet and tourmaline grains extracted from the heavy fraction of the samples have been performed. Density probability curves (DPC) characterizing the distribution of dZr ages from the studied samples were obtained. Together with previously published similar data for the Eo-Pleistocene sands of the middle part of succession of the Belorechenskaya Fm (sample K23-073) and the Holocene sands of the modern alluvium of the Belaya River (sample K22-032), this made it possible to obtain a summary characteristic for four successive stratigraphic levels of Quaternary deposits, demonstrating the change over time in the provenance signal in the orogenic molasse of the western Pre-Caucasia.The studied complex of clastic rocks participating in the Quaternary orogenic molasse contains well-defined signs of the “southern” (Caucasian) provenance signal, presented in the sets of U‒Pb isotope ages of dZr grains. In the case under consideration, the “southern” signal is represented by components characteristic only of it, associated with the erosion of: 1) Lower and Middle Jurassic volcanogenic-sedimentary, volcanogenic, subvolcanic and intrusive rocks of the Cimmerian structural level (Cimmerian provenance signal); 2) Early Paleozoic and Late Neoproterozoic igneous and metamorphic formations of the Cadomian complex (Cadomian provenance signal) as part of the Hercynian basement. A tendency of gradual decrease in the intensity of the Cimmerian and increase in the intensity of the Hercynian provenance signals in the sequence of stratigraphic units of the Quaternary age from the lower to the upper levels of the section has been established. This regularity characterizes the process of successive deepening of erosion of the Greater Caucasus orogen. In the beginning, at the early stages of formation of the western segment of the orogen in Gelasian, erosion affected rock complexes mainly of the Cimmerian structural stage. Later, starting from the Eo-Pleistocene and upto the present time, the main source of detrital material became the complexes of the Hercynian basement exposed on the surface.

Rift formations of the Ishlya graben, widespread on the western slope of the Southern Urals, are represented by alternating terrigenous rocks (carbonaceous shales, siltstones, siltstones) with a volcano-plutonic association (gabbrodolerites, basic effusives with a small amount of pyroclastic material). In the rocks of the Ishlya graben, complex rare earth mineralization was discovered, represented by allanite-(Ce), REE-containing epidote, monazite-(Ce), xenotime-(Y), chevkinite-(Ce), fergusonite-(Nb), rare earth fluorocarbonates (bastnaesite-(Ce), hydroxylbastnaesite-(Ce), parisite-(Ce), synchysite-(Ce)), which is characterized by a wide variety of morphological types of mineralization and the presence of complex associations. Based on the analysis of the chemical composition of metamorphic minerals, the P–T parameters of rock metamorphism were established (T = 250–600°C, P = 2–10 kbar), the chemical composition of the fluid phase and its temperature (CaCl₂ + NaCl; T = 180–408°C, for primary inclusions and FeCl₂; T = 121–248°C for secondary ones), as well as the hematization temperature (465–593°C) andre-equilibration of the ilmenite-titanomagnetite association (T = 501–576°C at oxygen fugacity from –23.15 to –21.25) were determined. It is shown that in the natural environment, the processes of rare earth mineral formation are diverse and multifactorial, with the chemistry of the local-scale mineral formation environment being of great importance. Based on the comparative analysis of the chemical composition of monazite and xenotime, it was established that neither the configuration of the normalized graphs nor the chemical composition of the minerals found in the rocks of the Ishlya graben and the Shatak complex correspond to analogs from alluvial deposits. Thus, it can be stated that the primary source of rare-earth phosphates from channel and alluvial deposits are the Riphean-Vendian metamorphosed rocks of the eastern subzone of the Bashkir meganticlinorium.