Magnetostratigraphy of the Middle–Upper Cambrian Verkhnyaya Lena Group and Lower Ordovician Ust-Kut formation in the southern Siberian Platform

The available paleomagnetic data on the Verkhnyaya Lena Group from different areas of the southern Siberian Platform are revised. The group rests unconformably upon the Lower Cambrian strata and is overlain by Lower Ordovician rocks, which determines conditionally the age of its red-colored deposits. Paleomagnetic correlation of composite sections through the region using defined zones of normal and reversed magnetic polarity serves as a basis for development of the magnetostratigraphic scale for the Verkhnyaya Lena Group. The scale includes nine magnetic zones, which play the role of markers; seven of them are traceable in all the examined sections of the southern Siberian Platform. By the distribution of zones with normal (N) and reversed (R) polarity, the magnetostratigraphic scale is subdivided into three parts. Its lower part is represented by reversed polarity, which is characteristic of the second half of the Lower Cambrian. The middle part is characterized by frequently alternating zones with normal and reversed polarity corresponding to the Middle Cambrian. The upper part of the scale corresponds to the interval of reversed polarity characteristic of the Upper Cambrian and Lower Ordovician. The Middle–Upper Cambrian boundary is located near the last N–R reversal of the geomagnetic field in the Cambrian. The magnetostratigraphic scale includes nine orthozones united into three superzones, which are attributed to two hyperzones of magnetic polarity.     

Pb–Pb Age of Carbonate Rocks of the Kamo Group,Baikit Anteclise of the Siberian Platform

Doklady Earth Sciences - The first direct Pb–Pb dating of carbonate rocks of the Kamo Group has been carried out. The Pb–Pb age of carbonates of the lower units (the Madra, Jurubchen,...     

Evolution of Stromatoporoidea in the Ordovician–Silurian epicontinental basin of the Siberian Platform and Taimyr

The paper discusses the evolution of Stromatoporoidea in the epicontinental sedimentary basin of the Siberian Platform and Taimyr during the Ordovician and Silurian. Specimens of the oldest genus, Priscastroma, were found in the middle of Middle Ordovician sediments. This genus is represented by the species P. gemina Khrom., which has two forms, A and B. Tracing the emergence of new genera over time, we identified two distinct branches in stromatoporoid evolution.The ancestor of the first branch is P. gemina f. A, which gave rise to the genus Cystostroma. The latter is the ancestor of two subbranches with predominant horizontal skeletal elements. The subbranches differ only in tissue microstructure. The genera Stromatocerium, Dermatostroma, and Aulacera display dense fibrous microstructure, whereas the genus Rosenella and its descendants display dense microstructure. The genus Lophiostroma, with a lamellar–fibrous tissue, may be a dead branch of evolution.The ancestor of the second branch is P. gemina f. B, which gave rise to the genus Labechia and its descendants. This branch has a dense tissue, with predominant vertical skeletal elements.Ordovician stromatoporoids from Siberia were compared with those from other basins of the world. Comparison shows that all the Ordovician genera from the epicontinental basin of the Siberian Platform and Taimyr originated here. Thus, this basin was one of the centers of stromatoporoid origin.     

Geochemistry and the Sm–Nd model age of sedimentary rocks from the lower horizon of the cover in the northeastern part of the West Siberian Platform

The pre-Jurassic basement and lower (Jurassic) horizons of the sedimentary cover in Hole Borovaya 6 were studied. Analysis of rare and rare-earth elements shows that Jurassic sedimentary rocks were most likely formed at the expense of erosion and mixing of heterogeneous materials, namely acid sources of the Siberian Platform and Triassic riftogenic basaltoids. The variations of ¹⁴⁷Sm/¹⁴⁴Nd (0.1076–0.1250) and ¹⁴³Nd/¹⁴⁴Nd (0.512202–0.512437), as well as the Sm–Nd model ages of Jurassic sediments (1146–1362 Ma), provide certain evidence for participation of the Mesoproterozoic substrate in the formation of the rocks studied. The Sm–Nd model age of pre-Jurassic rocks (1281 Ma) is Mesoproterozoic as well. The Precambrian crystalline basement of the Siberian Platform is a likely source of these sedimentary rocks.     

Devonian ultramafic lamprophyre in the Irkineeva–Chadobets trough in the southwest of the Siberian Platform: Age,composition, and implications for diamond potential prediction

The results of geochronological, mineralogical, petrographical, and geochemical study of the Ilbokich ultramafic lamprophyre are reported. The specific features in the mineral and chemical compositions of the studied ultramafic lamprophyre indicate that it can be regarded as a variety similar to aillikite, while other differences dominated by K-feldspar can be referred to damtjernite. According to Rb–Sr analysis, ultramafic lamprophyre dikes intruded at the turn of the Early and Middle Devonian, about 392 Ma ago. This directly proves the existence of Early Paleozoic alkali–ultramafic magmatism in the northern part of the southwest Siberian Platform. A finding of Devonian alkali–ultramafic lamprophyre is of dual predictive importance. On the one hand, it is indicative of the low probability of finding large diamond-bearing deposits in close association with aillikite. On the other hand, it can be indicative of a possible large Devonian diamond province in the studied territory, where diamondiferous kimberlite is structurally separated from aillikite.     

Stratigraphic chart of Cambrian deposits in the Turukhansk–Irkutsk–Olekma region,Siberian Platform

The former stratigraphic chart of the Cambrian in the Turukhansk–Irkutsk–Olekma region of the Siberian Platform was compiled in 1986 and approved in 1988. Large amounts of data were obtained from new wells drilled in the western and northern parts of the region during the preceding 20 years. Modifications necessitated by new data entail an indefinitely continuous series of revised facies zonation, recognition of new facies areas, regions and zone, as well as definition of a regional early Mayan stratigraphic hiatus. A new local stratigraphic division is the Olenchima Member (base of the Evenk Formation) underlain by a hiatus. The term Kochumdek Formation is no longer used in the Bakhta region and its lithologically distinct subformations (Yasenga, Moktakon, Mara, and Abakun) were raised to formation rank. The Olenchima Formation is no longer used in the Baikit zone; but instead, the Litvintsevo Formation was established here by correlation.     

Native gold in complex Ti–Zr placers of the southern West Siberian Plain

Typomorphic features of native gold and its contents in complex Ti–Zr placers in the southern West Siberian Plain are reported. Three of the placers are of littoral-marine genesis, and two formed under conditions of an alluvial piedmont plain. Native gold from the studied Ti–Zr placers occurs mainly as flattened thin (?0.1 mm) particles which underwent mechanical action. It is marked by wide fineness variation and the abundance of a very fine (990–1000‰) variety. Most likely, this is chemically transformed clastogenic metal. The gold content of the productive bed (5–30 mg/m³ native gold and 8–140 ppb bulk gold) is consistent with the dispersion of heavy ore and accessory minerals during mechanical migration in water flows simultaneously with their concentration on geochemical barriers. The native-gold content of complex Ti–Zr placers shows a higher negative correlation with the primary source–placer distance than those of Ti and Zr minerals and a positive correlation with the degree of hydrodynamic reworking (gravity concentration) of transit terrigenous material. On the southern framing of the West Siberian Plain, some regions of northern Kazakhstan are promising for gold of complex Ti–Zr placers as well as fine- and thin-gold placers, gold-bearing weathering crusts, and primary gold deposits.     

Geochemical features and geodynamic setting of formation of the Lukinda dunite–troctolite–gabbro massif (southeastern framing of the Siberian Platform)

The Lukinda dunite–troctolite–gabbro massif in the Selenga–Stanovoy superterrane on the southeastern framing of the Siberian Platform was earlier considered Precambrian. The performed ⁴⁰Ar/³⁹Ar dating of the massif plagioclase yielded an Early Permian age (285 ± 7.5 Ma). The main specific petrochemical features of the intrusion rocks during their crystallization differentiation are an increase in SiO₂ and CaO contents and a decrease in FeO_tot content, with TiO₂ content remaining low and showing minor variations. A specific geochemical feature of the Lukinda massif ultrabasite–basites is a slight domination of LREE over HREE, with (La/Yb)_N= 1.0–8.2. The depletion of the massif rocks in LILE (except for Sr and Ba), REE, and HFSE suggests that the massif formed on an active continental margin.     

Stages of Tectono-Magmatic Activity in the Border Zone of the Siberian Platform and the Tannuola–Khamsara Segment of the CAFB (According to the Results of U–Pb Isotope Studies)

Doklady Earth Sciences - Based on the dating of metamorphic and igneous rocks of the northeastern part of the Khamsara terrane, the main stages of tectono-magmatic activity at the border of two...     

Structure of the Earth’s Crust in the Northeast Aldan Anteclise (Siberian Platform): Diamond Potential

Geotectonics - The gravitational and magnetic fields of the eastern Aldan anteclise, within the area bounded by longitudes 126° and 138° E and latitudes 58° and 64° N, are...     

Rb–Sr Age of Riphean Glauconites of the Kamo Group (Baikit Anteclise,Siberian Craton)

Doklady Earth Sciences - A mineralogical-geochemical study of globular phyllosilicates (GPS) of the glauconite-illite series of the Dolgokta Formation from the stratigraphic well Chunkinskaya Well...     

Proterozoic Granitoid Magmatism at the Southwestern Margin of the Siberian Platform in the Tectonic History of the Angara–Kan Block

Doklady Earth Sciences - The large massifs of ancient granitoids of the South Yenisei Ridge are divided into three complexes that differ in the geological, geochemical, and geochronological...     

“Cuboids” of pyrope found in Siberian Platform

Pseudocubical convex-faceted grains of pyrope are often found in upper parts of weathered crust, in sedimentary rocks and in modern alluvium, inside and outside the Yakutian diamond fields. They are never in the diamond pipes or in dense kimberlites where pyrope maintains its normal shape. The “cuboid” shape of pyrope grains is probably due to diagenetic and solubility effects, inasmuch as their optical properties and impurities are the same as those of the “normal” pyrope.     

Paleoproterozoic Age (2055–2050 Ma) of Volcanogenic–Terrigenous Rocks from the Bilyakchan Zone of the Siberian Platform and Okhotsk Massif Junction

Doklady Earth Sciences - The age of the Bilyakchan Group, representing the northern fragment of the Ulkan–Bilyakchan volcano-plutonic belt of the Okhotsk Massif and Siberian platform junction...     

The U–Pb age and Lu–Hf isotope composition of detrital zircon from metasedimentary rocks of the Onot greenstone belt (Sharyzhalgay uplift,southern Siberian craton)

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.     

Source Provenance and Paleogeographic Conditions during Deposition of Jurassic Continental Sediments in the Southern Part of the Siberian Platform (Implication of Sm–Nd and U–Pb Data)

The first Sm–Nd isotopic data and U–Pb (LA–ICP–MS) detrital zircon ages from sandstones of the Prisayan and Kuda Formations (the Irkutsk Basin, southern part of the Siberian Platform) have been obtained. They demonstrate that during accumulation of the sediments in the Irkutsk Basin, the contribution of local erosion sources decreased over time, while input from the Paleo–Transbaikalia sources increased. The change in provenance areas was triggered by tectonic rebuilding in Paleo–Transbaikalia caused by the closure of the Mongol–Okhotsk Ocean.     

Tectonic map of the Vendian–Lower Paleozoic structural stage of the Lena–Tunguska petroleum province,Siberian Platform

A new version of the tectonic map for the Vendian–Lower Paleozoic structural stage of the Lena–Tunguska petroleum province is presented. The map is based on the electronic structure map at a scale of 1:1,000,000, which was the first to be compiled for the uppermost Vendian within the study area. However, no tectonic zonation was made for the deep-buried Meso-Cenozoic Yenisei–Khatanga and Vilyui depressions. The principles and methodology of mapping are given. The study presents spatial, morphological, and quantitative characteristics of the major and large structures.     

The Composition Features of Volatile Components in Diamonds from the Placers in the Northeastern Part of the Siberian Platform by Gas Chromatography–Mass Spectrometry

The primary data are presented on gas chromatography–mass spectrometry of volatile material in diamonds from the placers in the northeast of the Siberian Platform. The new data obtained testify to the crucial role of hydrocarbons and their derivates in the processes of diamond formation within the Earth’s mantle. It was shown that the registered variations in the composition of volatile components in the treated diamonds were caused by a combination of processes including the transformation of redox conditions of the crystallization of diamonds.

     

The results of geochronological and isotope–geochemical study of zircons from tuff of the Sylvitsa Group (western slope of the Middle Urals): The origin of ash layers in Vendian rocks of the East European Platform

Doklady Earth Sciences - New LA-ICP-MS data on the U–Pb isotope age, Hf isotope characteristics, and concentrations of trace elements in zircons from volcanic tuff of the Upper Vendian...     

Distribution and Formation of Mississippi Valley–Type Lead–Zinc Deposits on the Eastern Margin of the Siberian Platform (Results of GIS Analysis of the Global Crust Model)

Doklady Earth Sciences - Recent studies of the lithosphere based on the gravity data from the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) were used for the first time to...