The Sr isotope chemostratigraphy as a tool for solving stratigraphic problems of the Upper Proterozoic (Riphean and Vendian)

Abstract

Published and original data on the Sr isotopic characterization of carbonates from the Riphean and Vendian key sections of the Southern Urals, Siberia, Asia, Africa, Australia, and North America are considered in compliance with the suggested principles of reconstructing the Sr isotopic composition of the Proterozoic seawater. The suggested methodic approach is used to plot the reference curve of the 87Sr/86Sr variations in the Riphean and Vendian oceans. During the time span of 1600–1250 Ma, the 87Sr/86Sr variations were in a narrow range corresponding to 0.70456–0.70494, but approaching the date of about 1030 Ma, the 87Sr/86Sr ratio rose to 0.70601–0.70611 and then quickly declined to 0.70519–0.70523 near the date of 1000 Ma. In the second half of the late Riphean and in the Vendian, the ratio grew almost steadily from 0.70521–0.70535 to values of 0.70874–0.70885 characteristic of the Late Vendian time. The subsequent regular growth of that ratio in seawater lasted from 840 to 550 Ma, though there were short-term epochs when the ratio noticeably dropped to 0.70561–0.70575 at approximately 760 Ma and to 0.70533–0.70538 at 670–660 Ma. After the mid-Late Vendian maximum, it declined to 0.70812–0.70823 at the end of the Nemakit-Daldynian Age and decreased to 0.70806–0.70812 during the Tommotian Age of the Early Cambrian. As is shown, the Sr isotopic variations in the Riphean and Vendian oceans were interrelated with global tectonic events in geospheres and formation stages of the Rodinia and Gondwana supercontinents. The Baikalian Complex of Siberia is considered in the work as a case in point illustrating advantages of the expounded approach with respect to age substantiation of particular stratigraphic subdivisions.