U-Pb age and Sr isotope signature of cap limestones from the Neoproterozoic Tsagaan Oloom Formation, Dzabkhan River Basin, Western Mongolia

摘要

The U-Pb and Pb-Pb methods were used for determining age of cap limestones from the Neoproterozoic Tsagaan Oloom Formation corresponding to the lower part of the sedimentary cover in the Dzabkahn microcontinent of Central Asia. The weighted average age value appeared to be equal to 632 ± 14 Ma (MSWD = 0. 11, probability 0. 74). This value allows the following assumptions: (1) the lower boundary of the Tsagaan Oloom Formation corresponds to the beginning of the Ediacaran; (2) Dzabkhan tillites are correlative with glacial sediments of the Marinoan Epoch. The low ~(238)U/~(204)Pb and ~(232)Th/~(238)U ratios observed in initial Pb sources of limestones from the Tsagaan Oloom Formation indicate that the Dzabkhan paleobasin received at its early development stages a bulk of material from eroded upper Riphean juvenile rocks. The ~(87)Sr/~(86)Sr ratio in fractions of Tsagaan Oloom limestones enriched with primary carbonate material and satisfying geochemical criteria of Rb-Sr systems retentivity (Mn/Sr < 0. 20 and Fe/Sr < 1) varies from 0. 70676 to 0. 70691 and reflects this ratio in the World Ocean approximately 630 Ma ago.