Vol 27, No 5 (2019)

The U and Pb contents and C, O, and Pb isotopic compositions are determined for dolomites of the Billyakh Group (6 and 15 samples of the Kotuikan and Yusmastakh formations, respectively), which composes the upper part of the Riphean section of the Anabar Uplift of North Siberia. The δ¹³С and δ¹⁸O values of dolomites vary from –3.0 to +0.8‰ PDB and from 23.7 to 27.3‰ SMOW, respectively, and are in agreement with those of carbonate sediments 1600–1400 Ma in age. On the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb plot, 18 data points of all samples of both formations occur along a straight line which corresponds to the age of 1513 ± 35 Ma. The Pb–Pb age of dolomites is a first direct geochronological characteristic of sedimentary rocks of the Anabar uplift cover and, along with the previously published Rb–Sr age of glauconite of the underlying Ust’-Il’ya Formation, allows us to ascribe the entire sedimentary sequence of the Billyakh Group to the Lower Riphean. These results indicate relatively fast (~10 m.y.) terrigenous-carbonate sedimentation of this group prior to intrusion of dolerites of the Kotui complex and origination of the Mesoproterozoic Kuonamka igneous province on the Siberian Craton. New data close a discussion on the age position of Riphean sediments of the Anabar Uplift.

The Pb–Pb isotope age and the Sr isotope signature of the Upper Riphean sedimentary limestones from the south of the Yenisei Range on the western margin of the Siberian Craton were obtained. The Pb–Pb age of carbonate deposits of the Dadykta Formation of the Tungusik Group in the Kamenka facial zone in the east of the Yenisei Range is 1020 ± 20 Ma. The Pb–Pb age of limestone of the Gorevka Formation of the Shirokaya Group in the Glushikha facies zone in the western part of the region is estimated at 1020 ± 70 Ma. The ⁸⁷Sr/⁸⁶Sr ratio in limestones of the Dadykta Formation lies within 0.70536–0.70590 and that of the Gorevka Formation lies within 0.70552–0.70578. This coincides with a decrease in the ⁸⁷Sr/⁸⁶Sr ratio in a paleo-ocean immediately after the culmination of the Grenville orogeny. The correlation of the Shirokaya Group with the Oslyanka Group of the Kamenka Zone in combination with other data has shown that the accumulation of the Tungusik, Shirokaya, and Oslyanka groups occurred within an interval of 1030–950 Ma. The isotope age of the Middle and Upper Riphean boundary at the base of the Tungusik Group of the Yenisei Range is estimated at 1030 Ma, which is consistent with the age of this boundary in the sections of the Turukhansk Uplift and the Uchur-Maya Region of Siberia. It was revealed that the greater part of the Upper Riphean (Neoproterozoic) in the sedimentary record of the Yenisei Range is missing. The observed facial zoning of the Riphean deposits of the Yenisei Range represents the configuration of the ancient sedimentary basin that existed at the western margin of the Siberian Craton long before the collisional events accompanied by granite intrusions and superimposed metamorphism in the Neoproterozoic.

This work presents the results of the mineralogical, crystallochemical, and geochronological study of globular phyllosilicates (GPS) of glauconite–illite series from the Bakeevo Formation. This formation is the basal member of the Vendian Asha Group in the Southern Urals. The monomineral GPS fractions representing the globules varying in size and density are composed of Al-glauconite and illite. The Rb–Sr dating of the GPS of the Bakeevo Formation in combination with the modeling of cation distribution in their structure and comparison of the model data with Mössbauer and IR spectroscopy data was performed for the first time. The Rb–Sr age of the Bakeevo glauconite is 642 ± 9 Ma. The mineralogical and geochemical characteristics in combination with model calculations show that the stratigraphic significance of this age is quite reasonable. The initial ⁸⁷Sr/⁸⁶Sr ratio in the glauconitization environment is 0.7080 ± 0.0009 and is consistent within the experimental error with this ratio in the Vendian seawater. Thus, the lower boundary of the Vendian in the Southern Urals has an age of about 640 Ma.

Similarity analysis of the taxonomic composition of the 13 major localities for Vendian macrofossils worldwide is performed using the Ochiai coefficient. There are two distinct subgroups among shallow-marine localities. The first subgroup includes Australia, the southeastern White Sea area, Podolia, India, and the Urals (degree of similarity 0.16–0.38). The second subgroup includes localities of Siberia, Northwest Canada, South China, South America, Southern Namibia, and the United States (degree of similarity up to 0.58). Such a division of the shallow-marine localities corresponds to the distribution of carbonate sediments and of the tubular problematic species Cloudina. This division is probably climate-related; i.e., localities of the second subgroup are associated with warm-water conditions (precipitation of carbonates, distribution of problematic remains with a carbonate skeleton), while the localities of the first subgroup are associated with cold-water conditions. Thus, the analysis performed indicates the location of the Australia, Baltica, and India cratons at high latitudes (which differs from the results demonstrated in some paleotectonic reconstructions based on paleomagnetic studies) and confirms the hypothesis that the Siberian Platform was located at tropical and subtropical latitudes. Accordingly, two paleobiogeographic realms are identified for the Late Vendian: the Subpolar Realm (Australia, Baltica, and India) and the Equatorial Realm (Siberia, Laurentia, Kalahari, South China, and Rio de la Plata).