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.     

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.     

New Geochemical Data on the Organic Matter in Rocks of the Lower and Middle Cambrian Kuonamka Complex,the Lena–Amga Interfluve Area,Southeastern Siberian Platform

Saturated hydrocarbon biomarkers were studied in bitumens from organic matter (OM) in the Lower and Middle Cambrian Kuonamka Complex in the Lena–Amga interfluve of East Siberia. Their contents and distribution were analyzed. It was established that OM of siliceous and carbonate rocks from the lower part of the sequence differs from OM of overlying mainly mixed siliceous–carbonate rocks in terms of distribution of alkanes, steranes, tricyclanes, hopanes, and ratios of their homologs. It was concluded that the peculiarities of molecular composition of OM in the rocks are related to the biochemistry of microorganism communities, the remains of which were accumulated in sediments of Cambrian sea. It is possible that the microbiota changed its composition in response to a sharp change of sedimentation settings, which follows from biomarker proxies. It is suggested that sediments in the lower part of the sequence were formed under conditions of H₂S contamination. Catagenesis of OM and contribution of the Lower and Middle Cambrian potentially oil-generating rocks in naphthide generation on the northern slope of the Aldan anteclise are discussed.     

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,...     

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.     

Structure and evolution of the Earth’s crust in the region of junction of the Central Asian Fold Belt and the Siberian Platform: Skovorodino–Tommot profile

An integrated geological and geophysical study was performed to investigate the region of junction of the eastern part of the Central Asian Fold Belt and the Siberian Platform in the Skovorodino–Tommot 3-DV reference profile line (52°–60° N, 122°–129° E), where the belt is separated from the Aldan–Stanovoi Shield of the Platform by a series of deep faults. The main results are as follows: Seismic, density, and geoelectric characteristics of rocks were obtained and used to determine (refine) the intracrustal boundaries of tectonic structures; large-block structure of the Earth’s crust, caused by mantle faults, and the difference between the layered structure of the crust for the shield and fold regions were established; and available paleomagnetic data were used to perform palinspastic reconstructions for 180 and 140 million years, the most productive metallogenic epoch in the region, coeval with collision processes at the closure of the Mongol-Okhotsk paleobasin.     

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.     

New Data on the Age of the Tonalite–Trondhjemite Orthogneisses of the Olekma Complex of the Central Part of the Chara–Olekma Geoblock,Aldan Shield

New U–Pb zircon (TIMS) results allow dating of protoliths of tonalite–trondhjemite orthogneisses of the Olekma Complex in the central part of the Chara–Olekma Geoblock (Aldan Shield) to 2825 ± 3 Ma and 2994 ± 3 Ma. Together with the results of previous geochronological studies, this proves that the Olekma Complex comprises heterochronous igneous rocks intensively reworked under amphibolite facies conditions and formed during different stages of geological evolution of the Aldan Shield.     

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.     

Catagenetic transformations of gravelly–sandy rocks of the Taseeva Group in the Irkineevo–Chadobetsk riftogenic trough (southern Siberian Platform)

The stagewise optical and electron-microscopic analysis of the texture and mineralogy of the Upper Riphean–Lower Vendian gravelly–sandy rocks (Taseeva Group) in the Irkineevo–Chadobetsk riftogenic trough revealed a wide spectrum of secondary transformations: gravitational corrosion of allothigenic minerals, their plastic deformation, intense fracturing, and authigenic mineral formation. Traces of low-temperature hydrothermal processes were recorded. The multistage nature of authigenic mineral formation is attributed to the periodic resumption of tectonic dislocations owing to reactivation of fluid dynamics and thermal impulses ΔT.     

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.

     

Formation of PGM-Cu-Ni deposits in the process of evolution of flood-basalt magmatism in the Noril’sk region

The timing of the emplacement of ore-bearing melts in the process of evolution of flood-basalt magmatism in the Noril’sk District is discussed. The current models of ore formation consider the emplacement of ore-bearing intrusions either under the conditions of a closed magmatic system as a product of a self-dependent magmatic event, or under the conditions of an open magmatic system, where intrusions are parts of the conduits feeding lava flows. In both cases, the composition of the initial magma, the content of volatile components therein, and the contribution of country rock assimilation are important for the development of a genetic model. The relationships between lavas and intrusions are exemplified in the South Maslov intrusion, which cuts through the rocks of the Nadezhdinsky Formation. No geological evidence for links of lavas to intrusions has been established. Substantial difference in geochemistry (Ti contents, Gd/Yb and La/Sm ratios, etc.) of the tuff and lava sequence on the northern shore of Lake Lama and the Maslov intrusions are demonstrated. It is concluded that the Noril’sk deposits were formed as products of emplacement of self-dependent portion of magma in the post-lower Nadezhdinsky time. The melt composition determined from melt inclusions in olivine corresponds to high-Mg tholeiitic basalt (up to 7–8 wt % MgO) containing up to 1 wt % H₂O and 0.3 wt % Cl and undersaturated with sulfur. The fluid regime of flood-basalt volcanism had no anomalous features—the fluid was aqueous-carbon dioxide. The melts of ore-bearing and barren intrusions had similar concentrations of volatile components. The distribution of major and trace elements in intrusive rocks of the contact zone with the lower part of the Nadezhdinsky Formation characterized by high (La/Sm)_N ratio in comparison with gabbroic rocks (2.8–2.3 and 1.3–1.6, respectively), indicates that contamination of the initial melt only took place in a narrow (1 m) contact zone or did not develop at all. New data on isotopic compositions of Sr (⁸⁷Sr/⁸⁶Sr)₂₅₁ = 0.7089 and Pb (²⁰⁶Pb/²⁰⁴Pb = 20.877–24.528 in anhydrite confirm that local assimilation did not play a substantial role in the formation of rock and ores. On the basis of chemical composition of ore-forming intrusions, their isotopic characteristics, and the composition of melt inclusions in olivine, it is suggested that the lower crustal rocks were a major source of ore-bearing magmas.     

Complexity in hydro-seismicity of the Koyna–Warna region,India

Koyna–Warna region in western India is known to be the largest case of the reservoir-triggered seismicity in the world with M6.3 earthquake in 1967. This region continues to be seismically active even after 45 years with occurrences of earthquakes up to M5.0. The porous crustal rocks of Koyna–Warna region respond to changes in the prevailing stress/strain regime. This crustal section is highly fractured and is being fed by rivers and reservoirs. It is also subjected to fluctuating plate boundary forces and significant gravity-induced stresses due to crustal inhomogeneities. These changes induce variations in the water level in bore wells before, during and after an earthquake, and their study can help in understanding the earthquake genesis in the region. The ongoing seismicity thus requires understanding of coupled hydrological and tectonic processes in the region. Water table fluctuations are a reflection of the ongoing hydro-tectonics of the region. The fractal dimension of water levels in the bore wells of the region can be used as measure of the nonlinear characteristics of porous rock, revealing the underlying complexity. In this paper, we present values of correlation dimensions of the water level data in the bore wells using the nonlinear time series methodology. The spatiotemporal changes in the fractal dimensions have also been determined. The results show that hydro-seismically the region behaves as a low-dimensional nonlinear dynamical system.     

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.     

Graphite of the Turgenevskoe and Tamginskoe deposits of the Lesozavodsk area in the Primor’e region

The regional carbonization of the Riphean metamorphic complexes is discussed using as an example the Tamginskoe and Turgenevskoe graphite deposits located in the northern part of the Khanka terrane. It is shown that the noble metal mineralization associates closely with the graphitization. Isotopic, X-ray, and thermal analyses and Raman spectroscopy were first used for investigating the structural state of the graphite with defining its two varieties. The first of them is represented by nanocrystalline fluidogenic graphite that was formed during gas condensate crystallization from deep-seated reduced ore-bearing fluid. The second variety (large-flake graphite) represents a product of metamorphic recrystallization of carbonaceous terrigenous protoliths. The recrystallization was accompanied by the granitization of the sedimentary protolith, mobilization, and the transfer of the carbonaceous and ore matter of the host rocks. It is inferred that the graphitization associated with noble metal mineralization is a polygenic process. The graphite of the first generation associates closely with amorphous diamond-like carbon. This unexpected find may bear genetic information useful for geological and geochemical reconstructions.     

Mineralogical–Geochemical Types and 190Pt–4He Age of Ferroan Platinum of Placers of the Anabar River Basin,Northeastern Part of the Siberian Platform

Doklady Earth Sciences - Typical features of platinum group minerals (PGMs) and the results of 190Pt–4He dating of the Fe–Pt solid solutions are presented for complex...     

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...     

Petrography of Devonian and Lower Carboniferous terrigenous deposits in the southwest of the West Siberian Plate (Vagai–Ishim and Tobol–Ubagan structures)

We present new data on the material composition of terrigenous deposits in the pre-Jurassic basement of the West Siberian Plate, which formed on the margin of the Kazakhstan paleocontinent. Studies of the sections of several wells drilled in the east of the Kurgan Region showed that the Middle–Upper Devonian and lower Lower Carboniferous continental and littoral deposits are graywackes rich in rock and plagioclase clastics. Quartz grains amount to no more than 3–5%. The rock clastics are mainly volcanics of basic and normal compositions, which were supplied, most likely, from acting volcanoes of the marginal belt. By genesis, these are predominantly tephroids. Along with allothigenic components of sandstones, we consider authigenic minerals, which permit treating the postsedimentational transformation of the Middle Devonian and Early Carboniferous rocks as early catagenesis. By the vitrinite reflectance, the Lower Visean sandstones correspond to gradations MC2 and MC3 (medium catagenesis).     

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.