The Baikalian and Vendian sequences in the Lower Angara area (southwestern Siberian Platform)

In the section of Agaleevskaya BH-4 drilled in the lower reaches of the Angara River, Vendian and Baikalian sediment sequences have been recognized within Neoproterozoic strata. The Vendian sequence is formed by terrigenous-carbonate deposits of the Tetere, Soba, and Katanga Formations of the Danilovo Horizon, referred to as the Upper Vendian Nemakit-Daldyn Stage, as well as by terrigenous deposits of the Taseeva Group. The Baikalian Horizon is composed of the Brus Formation, earlier recognized only on the Chadobets uplift, and is separated from the Vendian deposits by a stratigraphic gap. In the Brus Formation, a microfossil complex similar to earlier described biotas of the Siberian Baikalian strata was found. The underlying deposits of the Terina Formation contain microfossils lacking below the basement of the Lakhanda Horizon (Neoproterozoic).     

Age of blastomilonites of the Yenisei regional shear zone as evidence of the Vendian accretionary-collision events at the western margin of the Siberian Craton

The age of the latest impulse of dynamometamorphic structural-compositional transformations (603–615 Ma) was established from the ⁴⁰Ar-³⁹Ar age of micas from balstomilonites of the Garevka metamorphic complex of the Yenisei Ridge confined to the Priyenisei shear zone. The age of these tectonic-thermal events marks the final stage of the Neoproterozoic evolution of the Yenisei Ridge related to the accretion of the Isakovka terrane to the western margin of the Siberian Craton. These results are in agreement with the age of metamorphic rocks of northwestern Prisayany, which are incorporated into the Sayany-Yenisei accretionary belt along with Yenisei Ridge. The conformable data on two regions indicate a global Vendian accretionary-collision event, which led to the formation of the Sayany-Yenisei accretionary belt, lateral thickening of the continental crust to the southwestern margin of the Siberian Craton, and its subsequent tectonic reworking. The synchronous Vendian orogenesis and consequence of the tectonic-thermal events along the periphery of the large Precambrian cratons of the Baltic and Siberia confirm their close spatial relation about 800 Ma ago, which agrees with contemporary paleomagnetic reconstructions.     

Geochemistry of the Paleoproterozoic metaterrigenous rocks of the Biryusa Block,southwestern Siberian Craton

Major and trace element compositions of the Paleoproterozoic metaterrigenous rocks (Neroi Group) formed in a large sedimentation basin in the southwestern Siberian Craton (Biryusa Block) were determined to reconstruct the protoliths of metasediments, degree of their recycling, and maturity of source rocks. Primary rocks from the lower part of the sequence (Alkhadyr Formation) are represented by both petrogenic (“first cycle”) and recycled sediments of the graywacke to siltstone and aluminous pelite series. Protoliths of the micaceous and carbonaceous schists from the upper part of the sequence (Tumanshet Formation) correspond to silty pelites and pelites. As the micaceous schists of the Alkhadyr Formation, these rocks have K₂O/Al₂O₃ < 0.3 and elevated Th concentrations, indicating the contribution of recycling in the formation of the fine-grained rocks. Distribution of trace and rare earth elements (REE) in metaterrigenous rocks of the Neroi Group testifies to the predominance of felsic rocks in the source area, while the prominent Eu minimum indicates the presence of granitoids—the products of crustal melting. Rocks of the Alkhadyr Formation also show elevated contents of Cr, Co, Ni, Sc, and Fe, indicating the development of mafic rocks in the source area. Comparison of the trace element contents and their ratios in rocks of the Neroi Group with those in the Archean (3.5–2.5 Ga) and Paleoproterozoic (2.5–1.6 Ga) upper continental crust made it possible to establish that metasedimentary rocks of the Neroi Group were formed by the erosion of sufficiently mature (geochemically differentiated) protoliths, which are similar to the Paleoproterozic crust. Judging from the Sm-Nd isotope data, one of the components of source areas for the terrigenous rocks of the Neroi Group were Archean rocks similar to basement rocks of the Biryusa block with the Nd model ages within 2.8–2.6 Ga. The second component in the source area could be juvenile Paleoproterozoic crust (Nd model age ∼1.9 Ga), which was probably represented by the metavolcanic associations of grabens surrounding the Biryusa block. The minimum Nd model ages for metaterrigenous rocks of the Neroi Group define the lowermost sedimentation boundary at 1.9 Ga.     

The Siberian lithosphere traverse: mantle terranes and the assembly of the Siberian Craton

The kimberlite fields scattered across the NE part of the Siberian Craton have been used to map the subcontinental lithospheric mantle (SCLM), as it existed during Devonian to Late Jurassic time, along a 1000-km traverse NE–SW across the Archean Magan and Anabar provinces and into the Proterozoic Olenek Province. 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe (major elements) and proton microprobe (trace elements). These data, and radiometric ages on the kimberlites, have been used to estimate the position of the local (paleo)geotherm and the thickness of the lithosphere, and to map the detailed distribution of specific rock types and mantle processes in space and time. A low geotherm, corresponding approximately to the 35 mW/m² conductive model of Pollack and Chapman [Tectonophysics 38, 279–296, 1977], characterised the Devonian lithosphere beneath the Magan and Anabar crustal provinces. The Devonian geotherm beneath the northern part of the area was higher, rising to near a 40 mW/m² conductive model. Areas intruded by Mesozoic kimberlites are generally characterised by this higher, but still ‘cratonic' geotherm. Lithosphere thickness at the time of kimberlite intrusion varied from ca. 190 to ca. 240 km beneath the Archean Magan and Anabar provinces, but was less (150–180 km) beneath the Proterozoic Olenek Province already in Devonian time. Thinner Devonian lithosphere (140 km) in parts of this area may be related to Riphean rifting. Near the northern end of the traverse, differences in geotherm, lithosphere thickness and composition between the Devonian Toluopka area and the nearby Mesozoic kimberlite fields suggest thinning of the lithosphere by ca. 50–60 km, related to Devonian rifting and Triassic magmatism. A major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data (extended from outcrops in the Anabar Shield) represent major lithospheric sutures, which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock-type distribution between 100 and 250 km depth. The presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the Magan and Anabar provinces is concordant with their Archean surface geology. The lack of harzburgitic garnets, and the chemistry of the lherzolitic garnets, beneath most of the other fields are consistent with the Proterozoic surface rocks. Mantle sections for different terranes within the Archean portion of the craton show pronounced differences in bulk composition, rock-type distribution, metasomatic overprint and lithospheric thickness. These observations suggest that individual crustal terranes, of both Archean and Proterozoic age, had developed their own lithospheric roots, and that these differences were preserved during the Proterozoic assembly of the craton. Data from kimberlite fields near the main Archean–Proterozoic suture (the Billyakh Shear Zone) suggest that reworking and mixing of Archean and Proterozoic mantle was limited to a zone less than 100 km wide.     

Sources and formation conditions of Early Proterozoic granitoids from the southwestern margin of the Siberian craton

Three stages of Early Proterozoic granitoid magmatism were distinguished in the southwestern margin of the Siberian craton: (1) syncollisional, including the formation of migmatites and granites in the border zone of the Tarak massif; (2) postorogenic, postcollisional, comprising numerous granitoid plutons of diverse composition; and (3) intraplate, corresponding to the development of potassic granitoids in the Podporog massif. Rocks of three petrological and geochemical types (S, I, and A) were found in the granitoid massifs. The S-type granites are characterized by the presence of aluminous minerals (garnet and cordierite), and their trace element distribution patterns and Nd isotopic parameters are similar to those of the country paragneisses and migmatites. Their formation was related to melting under varying H₂O activity of aluminous and garnet—biotite gneisses at P ≥ 5 kbar and T < 850°C with a variable degree of melt separation from the residual phases. The I-type tonalites and dioritoids show low relative iron content, high concentrations of CaO and Sr, fractionated REE distribution patterns with (La/Yb)_n = 11–42, and variable depletion of heavy REE. Their parental melts were derived at T ≥ 850°C and P > 10 and P < 10 kbar, respectively. According to isotopic data, their formation was related to melting of a Late Archean crustal (tonalite-diorite-gneiss) source with a contribution of juvenile material ranging from 25–55% (tonalites of the Podporog massif) to 50–70% (dioritoids of the Uda pluton). The most common A-type granitoids show high relative iron content; high concentration of high-field-strength elements, Th, and light and heavy REE; and a distinct negative Eu anomaly. Their primary melts were derived at low H₂O activity and T ≥ 950°C. The Nd isotopic composition of the granitoids suggests contributions to the magma formation processes from ancient (Early and Late Archean) crustal (tonalite-diorite-gneiss) sources and a juvenile mantle material. The contribution of the latter increases from 0–35% in the granites of the Podporog and Tarak massifs to 40–50% for the rocks of the Uda and Shumikha plutons. The main factors responsible for the diversity of petrological and geochemical types of granitoids in collisional environments are the existence of various fertile sources in the section of the thickened crust of the collisional orogen, variations in magma generation conditions $(\alpha _{H_2 O} , T, and P)$ during sequential stages of granite formation, and the varying fraction of juvenile mantle material in the source region of granitoid melts.     

Grenville tectonic events and evolution of the Yenisei Ridge at the western margin of the Siberian Craton

Geological, petrologic, geochemical, and isotopic geochronological evidence for Grenville events at the western margin of the Siberian Craton are considered. These events were related to assembly of the Rodinia supercontinent. Multiple manifestations of riftogenic and within-plate magmatism at the final stage of orogenic evolution gave rise to breakdown of Rodinia and the formation of the Paleoasian ocean. The results allowed us to develop a new concept on the Precambrian geological evolution of the Yenisei Ridge and the processes that created its tectonic structure. The chronological sequence of events in the history of the Transangarian Yenisei Ridge is based on geological evidence and isotopic dating of Precambrian complexes variable in geodynamic nature. Four tectonic stages dated at 1.4?1.1, 1.1?0.9, 0.90?0.85, and 0.8?0.6 Ga were controlled by collision and extension recognized from large regional linear crustal structural elements. The evolution of the Transangarian Yenisei Ridge, which lasted for ～650 Ma, corresponds in duration to supercontinental cycles that begin from rifting and breakdown of the predated supercontinent and was completed by orogeny and the formation of a new supercontinent. The regional geodynamic history correlates with the synchronous sequence and similar style of tectonothermal events at the periphery of the large Precambrian Laurentia and Baltica cratons. This is evidenced by paleocontinental reconstructions, which confirm close spatiotemporal links of Siberia with cratons in the northern Atlantic 1400?600 Ma ago and indicate incorporation of the Siberian Craton into the ancient Nuna and Rodinia supercontinents.     

Geology of southwestern margin of Siberian Platform in connection with the lower Cambrian oil and gas prospects

Accumulated data reveal the generally uniform sedimentation trend of the Upper Silurian marine basin of Tuva, which appears to have been shallow throughout its history, although reconstruction of the relief of the trough fringe is difficult. The climate of the region was evidently warm, dry and semi-arid. The observed geologic development trend - transgression followed by regression - was not accompanied by a consistent trend of changes in mineral associations of the terrigenous rocks.--IGR Staff     

Duration of the first biozone in the Siberian hypostratotype of the Vendian

The first biozone (Anabarites trisulcatus Assemblage Zone) in the Siberian hypostratotype of the Vendian (northwestern slope of the Olenek Uplift) is represented by the Turkut Formation of the Khorbusuonka Group and most of the Syhargalakh Formation of the Kessyusa Group. The lower part of the Kessyusa Group in some of the sections includes stratiform breccia coeval with the middle part of the Syhargalakh Formation. The breccia is shown to be the alteration product of tuff breccia, which is widely distributed in the region and occurs as diatremes. A U-Pb zircon date of 543.9 ± 0.24 Ma for tuff breccia provides the best constraint on the age of the boundary between the Anabarites trisulcatus and Purella antiqua Assemblage Zones. The first appearance of small skeletal fossils Cambrotubulus decurvatus (which define the base of the Anabarites trisulcatus Assemblage Zone) is 1.4 m above the lower boundary of the Turkut Formation. Ichnofabric in the underlying Khatyspyt Formation is globally distributed in the strata 553-551 million years old, always predating the first appearance of small skeletal fossils of the Anabarites trisulcatus Assemblage Zone. The base of the Anabarites trisulcatus Assemblage Zone is therefore younger than 553-551 Ma, whereas the duration of the assemblage zone does not exceed six million years.     

Tectonic evolution of the Siberian Platform during the Vendian and Phanerozoic

This paper presents characteristics of the structural regions surrounding the Siberian Platform and discusses the Vendian-present time evolution of the Siberian Paleocontinent with the Siberian Craton making up its nucleus. It shows that the paleocontinent underwent significant intraplate compressional deformations with vertical movements and formation of inversion structural features within broad areas. Such epochs of deformation took place at the Riphean-Vendian time boundary, during the Late Paleozoic, Late Triassic, Early Cretaceous, and during the Late Cenozoic. The principal rifting events took place during the Middle-Late Devonian. The paper presents paleotectonic reconstructions of East Siberia at several key time intervals.     

Sedimentation settings of the Vendian Vanavara formation,the Siberian platform

A model of sedimentation settings is elaborated for siliciclastic deposits of the Vendian Vanavara Formation, the Katanga saddle, inner areas of the Siberian platform. Four lithologic complexes are distinguished in the formation. The lower complex is composed of proluvial continental deposits exemplifying a dejection cone of ephemeral streams. Its eroded surface is overlain by second complex largely represented by sandstones of coastal zone, which grade upward into siltstones and shales of deeper sedimentation settings (third complex). Sea transgression advanced in northeastern direction. The fourth complex resting with scouring on the third one was deposited in settings of a spacious shallow-water sea zone: in a tidal flat, sand shoals and islands. Sedimentological data are used to correlate more precisely the Vendian siliciclastic deposits of the Katanga saddle and northeastern Nepa-Botuoba anteclise, and to verify subdivision of the Vanavara Formation into subformations and character of its boundary with the overlying Oskoba Formation.     

Microfossils and biofacies of the Vendian fossil biota in the southern Siberian Platform

Based on the study of the litho- and biofacies of the Vendian Nepa Horizon in the central area of the Siberian Platform inland, a paleoecological model for the Vendian microbiota has been developed. The sedimentation environments of the Katanga saddle have been reconstructed, and three sedimentary systems have been recognized: (1) lower continental, formed by the deposits of proluvial fans and riverbeds of temporary streams; (2) middle transgressive, made up of littoral sand facies in the lower part and of fine-clastic shelf strata in the upper part; and (3) upper, of sea highstand, composed of alternating sand bank facies and fine-clastic lagoon deposits. Four biofacies have been recognized in the fine-terrigenous deposits of the Nepa Horizon: (1) Appendisphaera, represented by a Doushantuo-Pertatataka acanthomorph assemblage; (2) Transitional, with a great diversity of plankton and benthic (including complex) taxa; (3) Vanavarataenia, dominated by Vanavarataenia complex benthic algae; and (4) Oscillatoriopsis, represented by taxonomically poor biotas with morphologically simple (mainly prokaryotic) remains. These biofacies are confined to the following sedimentation environments: Appendisphaera is widespread in the distal open-sea areas; the Transitional biofacies is localized in the distal environments of the semi-isolated inner basin; Vanavarataenia occurs in the proximal areas; and Oscillatoriopsis is typical of the shallow-water environments, both extended (corresponding to the highstand period) and local.