SR-ISOTOPE SIGNATURE AND PB-PB AGE OF THE BAKAL FORMATION LIMESTONES IN THE LOWER RIPHEAN TYPE SECTION, THE SOUTHERN URALS

Abstract

The progress in Rb-Sr, U-Pb, and Pb-Pb methods offered new possibilities in gaining direct information concerning the sedimentation environment of Precambrian carbonate rocks and their age. The application of this approach to Riphean type sections of eastern Siberia and the Southern Urals allowed us to obtain new Sr-chemostratigraphic and Pb-Pb geochronological evidence for 87Sr/86Sr ratio of Middle and Late Riphean seawater and isotopic age of the Middle-Late Riphean boundary. These results appreciably modified the previously existing notions. However, the knowledge of geochronology and geochemistry of the Lower Riphean still remains insufficient even in its type section in the Southern Urals. The present work partly fills this gap, giving both the Sr-chemostratigraphic and Pb-Pb geochronological characteristics of carbonate rocks from the Bakal Formation, i.e., the upper unit of the volcanosedimentary Burzyan Group of the Lower Riphean stratotype.