Carbon, oxygen, strontium, and sulfur isotopic compositions in late Precambrian rocks of the Patom Complex, central Siberia: Communication 1. results, isotope stratigraphy, and dating problems

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Abstract
Results of the study of isotopic compositions of C, O, S, and Sr in late Precambrian sections of the Patom Complex and its analogues are presented. Total scatter in δ13C values is more than 21‰ (from −13.5 to 8.1‰). The sections strongly differ in thickness, but they have similar carbon isotope curves with two dramatic drops in δ13C from extremely high (>4‰) to extremely low (<−8‰) values. The lower negative excursion is associated with the glacial horizon (Dzhemkukan and Nichatka formations), while the upper excursion spans the entire Zhuya Group, which is as thick as 800–1000 m on the carbonate shelf (Nikol’skoe and Chencha formations along the Zhuya and Lena Rivers) and only 250–350 m at the carbonate platform (Torgo Formation, Chara River). Steady extremely high δ13C values (from 7 to 8‰) are typical of the glacial horizon underlying the Mariinsk Formation, as well as the Barakun and Valyukhta formations and their analogues, which separate negative excursions. The minimum 87Sr/86Sr ratios in limestones of the Kumukulakh (0.70725), Barakun (0.70727), Valyukhta (0.70769), Nikol’skoe (0.707904), Chencha (0.70786) and Torgo (0.70799) formations suggest the accumulation of sediments 660–580 Ma ago. Correspondingly, glacial diamictites of the Nichatka and Dzhemkukan (Bol’shoi Patom) formations can be correlated with the early stage of the Marinoan glaciation (635–665 Ma); the Zhuya Formation, with transgression that terminates the late stage of the same glaciation or the Gaskiers glaciation (580 Ma). Problems related to the genesis of carbonate rocks with extremely high and low δ13C values will be considered in the second communication.

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