Stratotype of the Lower Riphean, the Burzyan Group of the Southern Urals: Lithostratigraphy, paleontology, geochronology, Sr- and C-isotopic characteristics of its carbonate rocks

The main objective of this work is the generalization of lithostratigraphic, biostratigraphic and isotopic-geochronological data characterizing carbonate rocks from type succession of the broadly acknowledged chronostratigraphic subdivision of the Lower Riphean, such as the Burzyan Group of the Southern Urals and its analogs. Using an original approach to investigation of the Rb-Sr and Pb-Pb isotopic systems in carbonates and strict criteria of their retentivity, we studied the least altered (“best”) samples of the Burzyan carbonates, which retain the 87Sr/86Sr ratio of the sedimentation environment. As long ago as 1550 ± 30 and 1430 ± 30 Ma, that ratio corresponded to 0.70460–0.70480 and 0.70456–0.70481. The results confirm the influx of the mantle material predominantly into the World Ocean of the Early Riphean. The influence of meteoric diagenesis was likely responsible for local declines of δ18O in the Burzyan carbonates down to the values of −2.5 to −1.5‰ V-PDB. In the “best” samples, this parameter ranges from −0.7 to 0‰, which is consistent with the assumption that δ18O values (0 ± 1‰) characterized the stasis of the carbonate carbon isotopic composition in oceanic water 2.06–1.25 Ga ago. C-isotopic data on carbonate from the Paleoproterozoic-Lower Riphean boundary formations of the Urals, India, North America and Siberia suggest that the mentioned stasis ended by the commencement of the Early Riphean ca. 1.6–1.5 Ga ago. In the least altered carbonates of the Early Riphean, the δ18O variation range corresponds to 4.0–4.5‰.