U–Th–Pb ISOTOPIC DATING OF DETRITAL ZIRCONS AND GARNET AND TOURMALINE INDICATORS FROM MOLASSIC DEPOSITS OF THE BELORECHENSKAYA FORMATION (WESTERN PRE-CAUCASUS): GEOLOGICAL IMPLICATIONS

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 elastic 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 up to the present time, the main source of detrital material became the complexes of the Hercynian basement exposed on the surface.