C, O Isotopic Composition of the Neoproterozoic Pre-, Syn-, and Post-Glacial Carbonates of the Longdor Uplift and the Western Slope of the Aldan Shield (South of the Siberian Platform)

Here we present new data on the C and O isotopic composition in the Neoproterozoic carbonates of the Ballaganakh and Dal’naya Taiga horizons of the southeastern margin of the Patom paleobasin. In the upper part of the Ballaganakh horizon, a negative δ13С anomaly was established. It is comparable in amplitude and stratigraphic position to the Trezona anomaly, which preceded the Marinoan glaciation. The carbonate layers in the glacial deposits of the Nichatka Fm. of the Dal’naya Taiga horizon have moderately positive δ13С values, and the dolomite horizon at the base of the postglacial sequence is characterized by moderately negative values δ13С which is typical for the cap-carbonate sequence associated with the end of the Marinoan glaciation. In the lower part of the postglacial sequence (the Barakun Fm.), there is a gradual increase in the proportion of calcite in the carbonate component of the rocks and a shift in the isotope composition corresponding to the difference in the fractionation coefficients of stable C and O isotopes for dolomite and calcite. This phenomenon, which is also recorded in the postglacial sequences of Namibia and Canada, indicates that the composition of carbonate minerals was determined by a global change in water chemistry, and not by postsedimentary substitution. The overlying carbonate deposits of the Dal’naya Taiga horizon in the epiplatform part of the Patom paleobasin (Sen Fm.) are depleted in 13С compared to their shelf analogs in the Ura uplift. It has been suggested that carbonate accumulation in the epiplatform and shelf facies of the Patom paleobasin occurred asynchronously. The reconstructed trend of variations in the C isotope composition for the Dal’naya Taiga horizon is characterized by positive and low-amplitude negative δ13С anomalies similar to those in the Doushanto Fm. of China.