Geochemistry of C,O, and Sr isotopes and chemostratigraphy of neoproterozoic rocks in the northern Yenisei Ridge |
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| Authors: | B G Pokrovsky M I Bujakaite O V Kokin |
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| Institution: | 1.Geological Institute,Russian Academy of Sciences,Moscow,Russia |
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| Abstract: | Isotopic compositions of C, O, and Sr in carbonates, as well as Rb-Sr systems in the silicate material from Upper Precambrian
and Lower Cambrian rocks exposed by the Chapa River in the northern Yenisei Ridge, are studied. The Late Precambrian part
of the section includes the following formations (from the bottom to top): Lopatinskaya (hereafter, Lopatino), Vandadykskaya
(hereafter, Vandadyk) or Kar’ernaya, Chivida, Suvorovskaya (hereafter, Suvorovo), Pod”emskaya (hereafter, Podyom), and Nemchanka.
They are characterized by alternation of horizons with anomalously high and low δ13C values (such alternation is typical of the ∼700–550 Ma interval). The lower, relatively thin (20 m), positive excursion
(δ13C up to 4.3‰) was established in dolomites from the lower subformation of the Vandadyk (Kar’ernaya) Formation (hereafter,
lower Vandadyk subformation). The upper positive excursion (δ13C = 2.2 ± 0.6‰) was recorded in the 3-km-thick Nemchanka Formation enriched in terrigenous rocks. The lower negative excursion
stands out as uniform, moderately low δ13C values (−2 ± 1‰) and significant thickness. It comprises the upper part of the Vandadyk Formation, as well as Chivida and
Podyom formations. The upper negative excursion is related to a thin (∼20 m) marker carbonate horizon of the upper Nemchanka
subformation, in which δ13C values fall down to −8.3‰. The lower part of the Lebyazhinskaya (hereafter, Lebyazhino) Formation, which overlies the Nemchanka
Formation, shows a step-by-step increase in δ13C from −2.2 to 2.5‰ typical of the Vendianto-Cambrian (Nemakit-Daldyn Horizon/Stage) transitional sequences. The absence of
relationships between the carbon and oxygen isotope compositions and other parameters of postsedimentary alterations suggests
that the excursions characterized above could form at the sedimentation stage and coincide in general with δ13C fluctuations in seawater. The value of 87Sr/86Sr = 0.7076−0.7078 in limestones of the Podyom Formation points to their early Ediacaran age. Values of 87Sr/86Sr = 0.70841 and 0.70845 in dolomites of the lower Lebyazhino subformation correspond to the Early Cambrian. The Rb-Sr systems
of the clay material from the Vandadyk and Chivida formations are approximated by a straight line, parameters of which correspond
to the age of 695 ± 20 Ma (87Sr/86Sr0 = 0.7200 ± 0.0013) and probably characterize the epigenetic stage of older sedimentary rocks, which were subjected to very
rapid exhumation and “polar” sulfuric acid weathering in the course of glacioeustatic regression. |
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