Stratotype of the Lower Riphean, the Burzyan Group of the Southern Urals: Lithostratigraphy, paleontology, geochronology, Sr- and C-isotopic characteristics of its carbonate rocks |
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Authors: | M A Semikhatov A B Kuznetsov A V Maslov I M Gorokhov G V Ovchinnikova |
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Institution: | 1. Geological Institute, Russian Academy of Sciences, Moscow, Russia 2. Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia 3. Institute of Geology and Geochemistry, Uralian Division, Russian Academy of Sciences, Yekaterinburg, Russia
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Abstract: | 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‰. |
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