FINE-GRAINED ALUMINOSILICICLASTIC ROCKS OF THE MIDDLE RIPHEAN STRATOTYPE SECTION IN THE SOUTHERN URALS: FORMATION CONDITIONS, COMPOSITION AND PROVENANCE EVOLUTION

Abstract

General trends of the formation of Middle Riphean fine-grained aluminosiliciclastic rocks in the Bashkir Meganticlinorium are considered. It is shown that Yurmatinian shales do not contain any significant pyroclastic admixture. Judging from the relatively constant Th/Cr ratio throughout the Yurmatinian section, the tectonic regime in the study territory during the early Middle Riphean is suggested to be rather stable. The main paleoclimatic indices and indicators of the pelitic material maturity (CIA, CIW, IVC, PIA, and Ce/Y) suggest that paleodrainage systems in the early Middle Riphean were dominated by humid climate that gave way to the arid or semiarid type in the middle Yurmatinian. The low Mo/Mn ratio and some other indicators of redox conditions in shales from all Yurmatinian lithostratigraphic units show that no explicit reducing conditions existed in the basin during the early Middle Riphean. The shales were characterized by the increase in K2O/Al2O3 ratio, gradual enrichment in REE, and growth of LREE/HREE and LaN/YbN ratios toward the middle Yurmatinian, indicating the gain of an appreciable amount of slightly weathered arkosic aluminosiliciclastic material in the sedimentary basin about 1220–1200 Ma ago. The REE distribution and the UCC- and AUC-normalized shale compositions suggest that the eroded upper crust was compositionally close to the UCC. The occurrence of mafic and ultramafic rocks is also inferred. Data points of Yurmatinian shales plotted in the Cr–Ni, Eu/Eu*–GdN/YbN, and (La/YB)N–YbN diagrams are localized between the fields of Upper Archean and post-Archean rocks or within the latter field. Hence, post-Archean igneous and metamorphic complexes prevailed in paleodrainage systems of the early Middle Riphean. This is also confirmed by the model Nd ages.