I. A. Vishnevskaya

Sr isotope signatures in the Vendian Khorbusuonka group of the Olenek uplift (Northeastern Siberian Platform)

Owing to the comprehensive knowledge [1–3],unique paleontological characteristic, and availabilityof geochronological dates [4], the Vendian section ofthe Olenek uplift is one of the best reference successions for both regional and global correlation of UpperPrecambrian deposits. At the same time, no data areavailable on the Sr isotope chemostratigraphy, whichprevents adequate assessment of age constraints for theVendian in the Siberian hypostratotype. The publisheddata on the C isotope composition in these rocks [2, 3]provide an ambiguous age interpretation of sedimentatogenesis. In this connection, we studied the distribution of Sr isotopes in Vendian carbonate rocks of theOlenek uplift to assess their age more adequately ascompared with other isotopic–chemostratigraphicmethods.A collection of >200 samples of carbonate rocksfrom the Maastakh, Khatyspyt, and Turkut formationsof the Khorbusuonka Group taken from three sectionsin the middle reaches of the Khorbusuonka Riverbasin served as material for this study (Fig. 1). The carbonate rocks of the Maastakh and Turkut formationsare represented by biolaminitic, stromatolitic, oolitic,pisolitic, and oncolitic dolomites and their massiveand detrital varieties. The Khatyspyt Formation iscomposed of bedded and detrital, frequently, bituminous, and, locally, clayey limestones. According to thesedimentological analysis [1, 3, 5], each formationreflects a complete transgresssive–regressive sedimentation cycle being bounded at the base and top by erosional surfaces and hiatuses. Finds of Late Vendianbiota of the Ediacaran and Miaohen types and ichnofossils in the Khatyspyt Formation and also smallshelled fossils characteristic of the uppermost VendianNemakit–Daldynian Horizon in the Turkut Formation provide very important paleontological data [1, 5](Fig. 1). The oldest age of the overlying KesyusyaGroup, the middle part of which is marked by the firstfinds of Early Cambrian skeletal remains and ichnofossils [1], is estimated by the SHRIMP U–Pbmethod on zircons from tuff breccias of the uppermostKhorbusuonka Group to be 543 ± 0.3 Ma [4] (Fig. 1).At the first stage, the least altered samples wereselected macroscopically for assessing the degree ofpreservation of the

Sr isotopic characteristic of neoproterozoic carbonate sediments from the southern Yenisei Ridge

This paper presents the first Sr isotopic data for the Late Precambrian carbonate rocks of the southern Yenisei Ridge. Their geochemical study allowed estimation of the degree of secondary alterations and gave the possibility to reveal rocks with a less disturbed Rb-Sr isotopic system. The Sr isotopic data indicated Neoproterozoic sedimentation of the rocks about 1070–750 Ma ago. Sr and C isotopic data showed that carbonate rocks of the Sukhoi Pit, Tungusik, and Shirokino groups are Late Riphean and could be comparable with sedimentary sequences of three Precambrian key sections of the Northern Eurasia: the subsequent Derevnino, Burovaya, and Shorikha formations from the Turukhansk Uplift, the Lakhanda Group from the Uchur-Maya region, and the Karatav Group from the South Urals. All studied carbonate rocks are older than 750 Ma and, according to the International Stratigraphic Chart, accumulated prior to global glaciations in the Cryogenian. This is evident from sedimentological study indicating the absence of tillite horizons in the studied sections. δ13C values in the sections vary from +0.4 up to +5.3‰, which testifies to the absence of periods of great cold.

Main stages of tectonomagmatic activity of the Tuva–Mongolian microcontinent in the Precambrian: data on the U–Pb age of zircons

The U–Pb age of zircons from Ediacaran sandstones of the cover of the Tuva–Mongolian microcontinent and the rocks of its Early Precambrian basement (Gargan block) was analyzed by the LA–ICP–MS method. The major stages of tectonomagmatic activity of this block include the Neoarchean, Paleoproterozoic (no younger than 2 Ga), and Neoproterozoic. Comparison of the age of zircons from Ediacaran terrigenous rocks of the Tuva–Mongolian microcontinent and sandstones of the reference sections of the Ediacaran shelf of the Siberian platform undeniably indicates their independent accumulation.

The Vendian Vorogovka Group of Yenisei Ridge: Chemostratigraphy and results of U–Pb dating of detrital zircons

Application of the method of isotope stratigraphy showed that the 87Sr/86Sr ratio in the least altered carbonate rocks of the Severnaya Rechka and Sukhaya Rechka formations in the Vorogovka Group of Yenisei Ridge ranges from 0.70813 to 0.70828, and δ13C varies from –0.7 to +1.8‰. Comparison of these data with similar data for the Late Precambrian sequences from other regions provides evidence for the Vendian age of the Vorogovka Group. This is supported by the results of LA ICP-MS U–Pb dating of detrital zircons from sandstone of the Severnaya Rechka Formation, which is the basal level of the group: the mid-weight age of the youngest zircon population is 584 ± 3 Ma.

Sedimentary complexes of the cover of the Dzabkhan continental block: Different sedimentary basins and source areas

The geochemical and Sm–Nd isotope characteristics of Late Precambrian and Early Cambrian sandstones previously related to the sedimentary cover of the Dzabkhan continental block are reported. It is established that the Riphean and Vendian sedimentary rocks of the Ul’zitgol’skaya and Tsaganolomskaya Formations were accumulated within the Dzabkhan continental block as a result of recycling of the terrigenous deposits formed at the expense of destruction of basement rocks and younger granite. The formation of terrigenous rocks of the Bayangol’skaya Formation after a gap in sedimentation occurred in the sedimentary basin, where only the Late Riphean formations of the juvenile crust, probably of the Dzabkhan–Mandal block were the sources, without the contribution of the ancient crustal material. The Tsaganolomskaya and Bayangol’skaya Formations were formed in different sedimentary basins and cannot be related to the same complex.

Nd isotope systematics of the Vendian–Early Cambrian sedimentary ores in the northern segment of the Paleoasian Ocean

Isotope–geochemical studies of Mn, P, and Ba ores were performed in order to establish the influence of submarine hydrotherms on the formation of Early Cambrian sedimentary rocks of the southern environs of the Siberian Platform. Based on study of the geochemical and isotope (εNd) characteristics of the shallow-water Mn and Ba ores and phosphorites of southern environs of the Siberian Platform with similar ages, two types of sedimentary basins of the different geodynamic origins were distinguished: intraplate oceanic and those of the active continental margin, for which the sources of ore materials differ by the proportions of the mantle and contaminated crustal matter.

Geochemistry and Chemostratigraphy of Meso- to Neoproterozoic Sedimentary Rocks of the Yenisei Ridge (Siberia, Russia)

Here we report the results of a study of the sedimentary rocks of the Yenisei Ridge, located on the south-western margin of the Siberian Platform. We investigated geochemical data on Meso- to Neoproterozoic terrigenous rocks and the isotopic (Sr, C, O) characteristics of carbonate rocks (the Sukhoi Pit, Tungusik, and Shirokino groups). The data show that the Sukhoi Pit, Tungusik, and Shirokino groups were deposited more than 850 Myr ago. The absence of tillite deposits and the occurrence of positive δ13C values indicate that the deposition took place before the occurrence of the Cryogenian global glaciations. The distributions of rare and trace elements in these rocks indicate that they were formed by the erosion of very mature (highly differentiated) rocks of the Siberian Platform.