Riphean Section Research Articles

Riphean of the Kamsko-Belsk aulacogen: Hiatuses and lithogeochemistry of clastic rocks

Research subject. Sandstones and clay rocks of the Lower, Middle, and Upper Riphean (the Kyrpino, Serafimovo, and Abdulino groups, respectively) of the Kamsko-Belsk aulacogen. Changes in the characteristic values of the ratios of a number of oxides and some trace elements that are the indicators of rock composition in paleowatersheds are considered, taking the hiatuses of different nature and duration into account. Methods. An analysis of the average values of K2O/Al2O3 etc., Th/Sc, La/Co and (La/Yb)N, characteristic of sandstones and fine-grained clastic rocks (shales, fine-grained clayey siltstones and mudstones) of the main lithostratigraphic units of the Riphean section, as well as an analysis of their changes given the position below and above the break surface. Results. Throughout the entire Riphean, regardless of breaks in sedimentation associated with certain rearrangements in the recharge and sedimentation areas, the lithogeochemical characteristics of both sandstones and fine-grained clastic rocks (K2O/Al2O3, TiO2/Al2O3, Th/Sc, La/Co, etc.) were found to experience no signi ficant changes. Conclusions. The hiatuses presented in the Riphean section of the Kamsko-Belsk aulacogene were unlikely to have any significant effect on the average values of a number of ratios–indicators of rock composition in paleowatersheds (K2O/Al2O3, SiO2/Al2O3, TiO2/Al2O3, Th/Sc, La/Co, and (La/Yb)N) in sandstones and clay rocks occurring below and above the hiatus surfaces. This suggests that there were no cardinal changes in the composition of rocks in the paleowatersheds surrounding such a large negative structure as the Kamsko-Belsk aulacogene during the Riphean. Similarly, no fundamental changes occurred in the pathways of clastics entering the sedimentation area associated with the named structure.

Geodynamic control on the petroleum habitat of the Uralian Foreland

A comparative study of the Uralian Foreland in the Timan-Pechora and Volga Urals basins reveals considerable differences in structure and petroleum habitat. These differences were in large measure controlled by their geodynamic settings. A typical scenario of the continental marginal basin which subsequently was modified in the foredeep basin took place in the Timan-Pechora basin. The sedimentary fill of the basin is made up of the Phanerozoic deposits only. The principal oil source rock is provided by the prolific Domanik Suite broadly developed in is eastern part of the basin. The mature Domanik source rocks charged the majority of oil and gas fields of the Timan-Pechora Basin. In the Volga Urals basin the sedimentary fill includes two overlapping foredeep sequence – Riphean and Paleozoic, of which the Riphean one is much thicker. The Riphean depocenters were subjected to structural inversion during the docking of the Magnitogorsk island arc to the Uralian margin in the Middle Devonian-Frasnian. The inversion was accompanied by the thickening of the Riphean section due to the reactivation of the pre-existing Precambrian deformation and caused the regional uplift in the Southern Uralian Foreland. This uplift led to shifting of the Domanik troughs to the westerly outboard part of the Volga-Uralian Basin. The Domanik source rock remained immature over much of the Southern Uralian Foreland which is interpreted to be a result of the inversion of the Riphean depocenters and related regional uplift. It is proposed that the enormous clustering of large oil deposits in the Paleozoic section of the south-eastern part of the Volga Uralian Basin was to a much extent produced by the charging from the multi-kilometer thick Riphean section of the Southern Uralian Foreland.

AGE RESTRICTIONS OF THE UPPER PRECAMBRIAN SUBPOLAR AND POLAR URALS: ON THE QUESTION OF THE TIMING OF THE FORMATION OF THE TIMAN PASSIVE MARGIN

An analysis of U-Pb datings of detrital zircons from Upper Precambrian basal deposits of the Subpolar and Polar Urals is given. The presented material, taking into account the available data for other regions of the north of the Urals, as well as Timan, indicates that, in contrast to the Southern Urals, where the complete Riphean section is known, the Timan-North Urals Upper Precambrian begins with deposits bordering in age between the Middle and Late Riphean, most likely Late Riphean. This indicates the Late Riphean time of the Timan passive margin. It has been suggested that in Pre-Late Riphean times, the northeast of Baltica was located in the inner part of the fragment of Columbia, which avoided destruction until the entry of Baltica into Rodinia.

FORECAST OF OIL AND GAS ACCUMULATIONS ZONES IN THE RIFEAN OIL AND GAS COMPLEX IN THE WEST OF THE CENTRAL-TUNGUSSKA OIL AND GAS BEARING REGION

Based on the data of deep drilling and regional seismic surveys, the correlation of Riphean sections was performed and a map of the distribution of Riphean formations on the pre-Vendian surface was constructed. Three perspective zones for the search for oil and gas deposits have been identified.

Age and Stratigraphic Position of Sedimentary Sequences of the Bagrush Mountains (Southern Urals) Based on the Results of U–Pb Dating (LA–ICP–MS) of Detrital Zircons

In order to determine the provenance and age of terrigenous-carbonate stratified units exposed in the region of Bagrush Mountains, Bashkirian uplift, U–Pb dating of detrital zircons from these units has been carried out. A comparison of the data results with analogous data on five stratigraphic levels within the Riphean section of the Bashkirian uplift has shown that the studied sequence should be, most likely, correlated to the uppermost Lower Riphean or the lowermost Middle Riphean.

To reconstruction of the river’s categories which formed sedimentary basins of Riphean basins in the region of the Eastern European platform and modern south Urals juncture

Information on the distribution of trace and rare-earth elements in the bottom sediments of estuarine zones of various river categories (large rivers, rivers draining water collections, composed of sedimentary or magmatic and metamorphic rocks, etc.) of silt and pelitic dimensions can be considered as the data, reflecting characteristic features of the suspention, transported to marine basins from various catchment provinces. In certain situations, the composition of the surface bottom sediments of the marine basin is determined by the suspension composition of any large river flowing into it (the Caspian Sea and the Volga River, the Laptev Sea and the Lena River, etc.). The similarity and difference in the rare-earth elements (REE) systematics of bottom sediments of estuarine zones and suspended particulate matter of rivers of various categories is quite well expressed in the pair diagrams (La/ Yb)N–(Eu/Sm)N, (La/Yb)N–Th, etc. A comparison of the features of the distribution of REE and Th in the Riphean clay rocks of the Southern Urals and in bottom sediments of the estuaries of various categories of recent rivers has been performed. It has been concluded that clay rocks forming a typical Riphean section are formed due to the erosion of mainly or predominantly sedimentary and/or metasedimentary complexes that existed during the PreRiphean or the Riphean in the east part of the East European platform and have not survived to date.

Early Riphean Billyakh Group of the Anabar Uplift, North Siberia: C–O Isotopic Geochemistry and Pb–Pb Age of Dolomites

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.

Paleomagnetism of the upper Riphean deposits in the Turukhansk and Olenek uplifts and Uda Pre-Sayan region and the neoproterozoic drift of the Siberian Platform

A series of new paleomagnetic results have been recently obtained for the Neoproterozoic of Siberia. Nevertheless, the Neoproterozoic segment of the Apparent Polar Wonder Path (APWP) for this craton and its paleogeography are still unclear. A definite solution of these tasks will certainly take many years of dedicated efforts. However, even now we may take an important step in this direction by establishing the general displacement trend of the Neoproterozoic paleomagnetic poles of the Siberian Platform. For doing this, we need to obtain several high-quality paleomagnetic results whose age corresponds to the least complete (the sparsest) part of the Neoproterozoic paleomagnetic record. In the scope of this task, we carried out paleomagnetic studies of the Late Riphean sedimentary and intrusive rocks from the key Late Riphean sections of the Turukhansk region (Sukhaya Tunguska, Derevnya, and Miroedikha formations), Olenek Uplift (Upper Khaipakh subformation), and Uda Pre-Sayan region (Tagul Formation of the Karagas Group, Nersa intrusion). Based on the results of these investigations, together with the previous data, we suggest two new alternative models of the Neoproterozoic segment for the Siberian APWP and analyze some tectonic implications of these models.

Geochemical and Lu/Hf isotopic (LA-ICP-MS) signature of detrital zircons from sandstones of the basal levels of the Riphean stratotype

This paper presents the results of selective study of trace elements (29 analyses) and the Lu/Hf isotopic system (41 analyses) in preliminarily dated (U-Pb) detrital zircons (dZrs) from sandstones of the Ai Formation of the Burzyan Group of the Bashkirian Anticlinorium, which compose the basal horizons of the typical Riphean section of the Southern Urals. The statistically processed trace-element patterns of dZrs showed that “diorites” were dominant over “syenites” among the source rocks of dZrs. The rock types estimated by trace-element patterns for the cores and rims of two large grains (“diorite” and “syenite”) coincided. The analysis of the Lu/Hf isotopic system of dZrs revealed a wide dispersion of the ɛHf value from +7.1 to −20.1 at the T DM C model age of the substrate from 2.25 to 3.95 Ga. Four grains (in one case with the core and rim studied) from the population of the large transparent cherry zircons (TCZ) are characterized by the “syenitic” rock type and extremely ancient T DM C values of 3.22, 3.45, 3.64, 3.66, and 3.75 Ga at ages of zircons of 2486, 2784, 2873, 1977, and 1984 Ma, respectively. Two “dioritic” grains from the TCZ population have significantly distinct parameters: 2.37 and 2.51 Ga at 2049 and 2057 Ma, respectively. It is evident that this specific population of dZrs was formed with a significant contribution of very ancient crustal material, which became active under “syenitic” magmatism and provided the T DM C value of >3.5 Ga. Numerous juvenile dZrs form a compact cluster, which correspond to the rocks of the southern part of the Volga-Sarmatian orogen (age of 2.1–2.0 Ga, T DM C = 2.1–2.4 Ga). The complexes of the entire the Volga-Uralia, the Volga-Sarmatian orogen, and adjacent areas could be the provenance areas for the Ai sandstones in contrast to the northeastern areas of the East-European Platform with dominant “granitic” source rocks and T DM C values lower than 3.5 Ga.

Siderite microconcretions in the glauconite-bearing clayey-silty rocks of the Khaipakh Formation (Middle Riphean, Olenek Uplift)

Siderite microconcretions in the glauconite-bearing clayey-silty rock member of the lower sub-formation of the Khaipakh Formation (Middle Riphean, Olenek Uplift) are scrutinized for the first time. In two Khorbusuonka River sections located with a spacing of 12 km, the microconcretions occur as lenses and interlayers. Together with glauconitites, they serve as a distinct marker horizon of this stratigraphic interval. Their structures, morphologies, diffraction characteristics, chemical compositions, and isotope data are considered. They were examined comprehensively with modern investigation methods (X-ray diffraction, scanning electron microscopy with the application of local microprobe analysis, and others). Structural-morphological types of microconcretions are identified. Calculation of the chemical composition of carbonates based on numerous microprobe analyses made it possible to reveal different degrees of mineral heterogeneity in each type in terms of the distribution of macro- and microlevel isomorphous trace-elements and to refine the character of their secondary alterations at different stages of lithogenesis. The results of oxygen and hydrogen isotopic studies are presented for algal dolomites and limestones from the Middle and Lower Riphean sections in the Olenek Uplift (Debengda, Arymass, and Kyutingda formations). They demonstrated that siderites are similar to limestones and dolomites in terms of the oxygen isotopic composition (δ18O = 17.6–24.8, δ18Oav = 20.0 ± 2.4‰), but are marked by low δ13C values (from −6.3 to −12.0‰ (δ13Cav = −8.6 ± 2.1‰), suggesting the formation of microconcretions during early diagenesis. Siderite microconcretions were formed in the clayey-silty sediment slightly after glauconite, whose grains could serve as crystallization centers and (or) be entrapped during the growth of separate microcrystals. The role of catalyzers during the formation of both glauconite and siderite was played by bacterial communities, whose poorly preserved remnants have been detected not only in the studied member of the lower Khaipakh subformation, but also above and below the section. Separate types of microconcretion could be formed during the replacement of oncolites by siderite.

Peculiarities of the structure and evolution of the Riphean Ai volcanic complex, South Urals

The Ai volcanic complex is a part of the Ai Formation, which begins the stratotypical Riphean section in the South Urals and lies on the Archean Taratash metamorphic complex. New geochemical and isotope data were obtained for the volcanic rocks. The dominant porphyritic plagioclase and pyroxene trachibasalts associated with dacites are characterized by higher contents of alkalis and titanium, which is typical of rift volcanism. However, other geochemical data, e.g., decreased Ni contents, are beyond of this scheme. The U-Pb (SHRIMP) age of zircons from dacites is 1415 ± 11 Ma.

Contribution of Grenvillian events to the formation of most complete Riphean sedimentary successions in Northern Eurasia

The Grenvillian orogeny (∼1250 to 980 Ma) was one of the most significant Riphean events. It determined the formation of many structures observable now in North and South America, northwestern Europe, South Africa, Western Australia, Antarctica, and other regions. Nevertheless, its reflections in the most complete and relatively well investigated Upper Precambrian sedimentary sections of northern Eurasia such as the Riphean stratotype (Bashkir meganticlinorium) and hypostratotype (Uchur-Maya region) still remain unknown. This is primarily true of the petrographic and chemical compositions of terrigenous rocks. This work is dedicated to the analysis of peculiar features in variations of the whole-rock chemical composition of sandstones and fine-grained clastic rocks (shales, mudstones, fine-grained clayey siltstones) that constitute Middle-Upper Riphean boundary layers of the Bashkir meganticlinorium, Kama-Belaya aulacogen, and Uchur-Maya region. The analysis reveals no tendency for the decrease in the degree of the chemical and, consequently, mineralogical maturity in the upward direction through the Middle-Upper Riphean sections in the above-mentioned regions. The whole-rock compositions of fine-grained clastic rocks associated with sandstones correspond mostly to that of “common” Upper Precambrian clayey rocks. The formation of practically the entire Yurmatinian-Karatavian succession in the Bashkir meganticlinorium proceeded under relatively stable TDM and ɛNd(T) values. The period of 1250 to 980 Ma in the central and eastern parts of the Siberian Platform was marked by repeated rifting episodes alternating with accumulation of mature platformal sediments, although repercussions of Grenvillian collisional processes are missing from this region as well. The performed analysis provides grounds for the conclusion that contribution of the Grenvillian events to the formation of most complete Riphean successions in northern Eurasia was insignificant.

The New Data on Stratigraphy of the Riphean Stratotype in the Southern Urals, Russia

A recent series of U-Pb age determinations of zircons (SHRIMP, IDTIMS) from volcanic flows of several levels permitted to refine stratigraphy of the Riphean of Bashkirian megaanticlinorium (Urals, Russia), and provide a better correlation of this straton with the International and Chinese scales of the Proterozoic. The Upper Proterozoic deposits of the Riphean and Vendian of the General Stratigraphic Scale of Russia (GSSR), widely exposed in the Bashkirian Megaanticlinorium BMA) of the Southern Urals, can be approxi- mately correlated with the Meso- and Neoproterozoic of the International Stratigraphic scale (ISS) (Stratigraphic schemes of the Urals; 1993, The Stratotype of the Riphean, 1983; The Geologic Time Scale, 2012), or Sinian to Changcheng succession of stratigraphic units of China. In the last years, after the International excursion to BMA, with participation of the Chinese and Korean stratigraphers (Field trip Guide…, 2006), the work of dating of volcanic rocks of the Riphean section was strongly activated owing to a considerable progress in technique of isotope research. The work was stimulated by understanding that the International Scheme of division of Meso- and Neoproterozoic into systems/periods of equal duration (200 Ma) contradicts to traditional principles of stratigraphy, with elaboration of a system of stratotypes and GSSPs in real sections, with application of methods of chemostratigraphy, event stratigraphy, micropaleontology and other approaches (Bleeker, 2004). The under- standing that the existing chronometric stratigraphic scale of the Precambrian needs an up-to-date chronostrati- graphic revision, based on global events, becomes now stronger (Geologic Time Scale…, 2012; Grazhdankin & Maslov, 2013), though the Meso- and Neoproterozoic scale in ISS is still unreformed. Volcanic eruptions are developed at three levels of the Riphean section of BMA and their dating is of a spe-

Regional structural features of Lower Riphean sections in connection with the problem of petroleum resource potential of deeply buried complexes in the southwestern Perm region

Regional structural features of Lower Riphean sections in connection with the problem of petroleum resource potential of deeply buried complexes in the southwestern Perm region

Stratigraphic setting and age of the complex Tappania-bearing Proterozoic fossil biota of Siberia

A microfossil assemblage has been found in the widely spaced (300 km apart) Baikit and Katanga areas (inner Siberian Platform) in stratigraphically coeval strata in the lower part of the petroliferous Kamo Group (Riphean). It comprises complex protists Tappania plana Yin; acritarchs Valeria lophostriata Jank., Satka sp., Lophosphaeridium sp., Pulvinosphaeridium sp., and Miroedichia sp.; porous vesicles of Tasmanites sp. and Osculosphaera sp.; vesicles with anastomosing processes, and spiral filaments of Obruchevella and Glomovertella. Siberian fossil microbiota is considerably more diverse than biota with Tappania protists known in Australia and China. A taxonomically similar fossil biota found 1500 km northeast of the well-studied Riphean section in the Olenek uplift suggests that the host rocks, too, are stratigraphically coeval. This permits refining the age of remains of the first complex eukaryotes from the inner platform and is an additional correlation criterion for the lower Kamo Group.

Clarkes of concentrations of trace elements in the Riphean fine-grained terrigenous rocks of the Uchur-Maya region and the Yenisei Range

The clarkes of concentrations (Kc) of a wide range of trace elements (Li, Be, B, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Ag, Cd, In, Sb, Cs, Ba, REE, Hf, Ta, Hg, Tl, Pb, Bi, Th, and U) were analyzed for fine-gained terrigenous rocks (mudstones, metapelites) from the reference Riphean sections of the Uchur-Maya region and the Yenisei Range. It was established that the shales and mudstones of the Uchur and Aimchan groups in the Riphean hypostratotype section are characterized by moderate (2.5 5) geochemical specialization for Li, B, and Zn. At the same time, the similar rocks of the Lakhanda and Ui groups do not exhibit any distinct geochemical specialization, although they are notably enriched in HREE. The metapelites from the basal formations of the Riphean sedimentary successions in the Yenisei Range are distinctly specialized for B and slightly for Li, Rb, Be, Nb, Ta, Th, Ge, and Cd. In addition, moderate specialization for Cu is characteristic of the metapelites from the Korda and Lopatino formations; for Bi, Sb, Hg, and V, for their analogs from the Potoskui Formation; and, for Hg and Cs, for the similar rocks from the Lopatino Formation. The metapelites of the Lower Riphean Korda Formation from the central zone of the Yenisei Range have elevated contents of significantly more elements (Li, Be, Sc, V, Cr, Co, Ni, Zn, As, Rb, Y, Zr, Nb, Sb, Ag, In, Hf, Hg, and others) than their counterparts from its eastern near-platform part. The mudstones of the ore-bearing (Pb, Zn) Gorevo Formation are characterized by elevated concentrations of several ore elements such as Pb, Cd, As, Sb, and Bi. The elevated Kc values of the rare lithophile and of several ore elements in the metapelites of the Yenisei Range are determined by the high geochemical differentiation of the Early Precambrian blocks constituting the western margin of the Siberian Craton, which were eroded in the Riphean, and the syn-sedimentary riftogenic and intraplate magmatism. On the contrary, the fine-grained and terrigenous rocks from the basal part of the Riphean section in the Uchur-Maya region are compositionally closer to the immature Late Archean substrates or their Early Proterozoic analogs.

Platforms: Shorikha formation of the Turukhansk uplift, Siberia

the shorikha basin characterized by a thick (>800 m) sequence of carbonate sediments in the upper part of the riphean section in the turukhansk uplift of siberia illustrates the dynamics of taxonomic diversity and abundance of stromatolite buildups related to changes in the structure of a carbonate platform (replacement of the relatively deep ramp by a spacious shallow shelf) and fluctuations of the relative sea level. it is shown that abundance and taxonomic diversity of stromatolite buildups were maximal in deep zones of the outer ramp being reduced to almost zero values in shallow settings of the carbonate shelf. the appearance of columnar stromatolites coincided in this basin only with brief sea-level rises (second-order transgressions). stromatolite-forming microbial communities represented an active producer of carbonate sediments for the entire basin, while the distribution of stromatolites through ramps was determined by geochemical barriers. disappearance of stromatolites from shallow carbonate shelves was controlled by rapid reduction in carbonate-production rates.

Geochemical features of the Riphean fine-grained terrigenous rocks of the Southern Urals, Uchur-Maya region, and Yenisei Range: Estimation of the maturity of the pre-Riphean continental crust and its evolution within 1.65–0.6 Ga

The geochemical features of basal fine-grained terrigenous rocks from the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Range were compared in order to estimate the maturity of the continental crust that was formed by the beginning of the Riphean. It was shown that initial shales from the base of the Riphean sequence of the Yenisei Range and fine-grained aluminosiliciclastic rocks from the base of the Riphean sections of the Southern Urals were formed by the erosion of a rather mature continental crust. In contrast, fine-grained terrigenous rocks from the base of the Riphean of the Uchur-Maya region were derived from immature Late Archean protoliths or their Early Proterozoic analogs. The fine-grained terrigenous rocks of the three sedimentary megasequences show different variations in the (La/Yb)N ratio. In the Southern Urals, this ratio is high (12–15) in the Burzyan Group and decreases upsection to 6–10. In the shales of the Uchur-Maya region, the (La/Yb)N ratio decreases upsection, and the La/Sc ratio shows a sympathetic behavior. This is due to a decrease in the proportion of “primitive” tonalite-trondhjemite associations of the Archean granite-greenstone terranes in the provenance area with time and the appearance of intra-plate (riftogenic?) granitoids and significant amounts of basic and ultrabasic rocks. The latter marks the onset of large rift-forming events in the Uchur-Maya region at the beginning of the Late Riphean. The (La/Yb)N of the studied rocks from the Yenisei Range are mostly similar to the PAAS ratio, but higher values were found in the Upper Vorogovka and Chingasan groups, which was related to the contribution of strongly LREE-enriched granitoids and rift felsic and alkali basaltic volcanic associations to the formation of the terrigenous material. A comparison of Rb, Sr, Y, Zr, Ba, Hf, Th, U, Cr, and Ni contents and Zr/Y, (La/Yb)N, Ni/Co, Cr/Th, Cr/Sc, and La/Th ratios in the fine-grained terrigenous rocks of the Riphean megasequences of the Southern Urals, Uchur-Maya region, and the Yenisei Range with those in the model geochemical objects (PAAS, UCPR1, UCAR2, and others) showed that, in terms of most of the parameters, the Riphean fine-grained terrigenous rocks from the three regions are similar to each other, PAAS, and Proterozoic cratonic shales. This indicates a fairly high general maturity of the protoliths that were eroded during the Riphean in the eastern East European craton and in the southeastern and southwestern parts of the Siberian craton.

Rb-Sr and K-Ar indicators of the transformation of precambrian clayey rocks in the East European Platform

Rb-Sr and K-Ar characteristics of Vendian and Upper Riphean sections in various structural units of the East European Platform are studied. It is shown that Neoproterozoic clayey rocks of the platform underwent postsedimentary transformations, primarily owing to processes of K accumulation (illitization). Their intensity decreases with depth and isotopic signatures of provenance rocks are partly retained in Riphean rocks. Stages of the most active transformations approximately 400 and 600 Ma ago are manifested in the Vendian clayey rocks. Events of approximately 1000 Ma ago are recorded in Riphean rocks of the East European Platform. The least altered rocks of the Pachelma aulacogen can retain ancient hydrocarbon pools.

The age and origin of volcanics in the Riphean section of the Siberian craton ( western Baikal area)

In the western Baikal area, the structural position, composition, and age of volcanic rocks in the section of the Riphean margin of the Siberian craton were studied. The age of these rocks, earlier assigned to the Khotskaya Formation, is estimated at 274±3 Ma (concordia constructed over 11 zircon grains, SHRIMP-II). The geochemical and isotope compositions of volcanics evidence that they resulted from the melting of mantle source of EM-I type contaminated by crustal material. The intrusion of volcanics into the upper crustal horizons might have been caused by the evolution of the Permian active margin of the Siberian continent, which took place on the background of the closure of the Mongolo-Okhotsk ocean. Based on the results of studies, a new subvolcanic complex of Early Permian age has been recognized in the region, which includes the above-mentioned volcanics and earlier described porphyrite dikes of close age in the Sharyzhalgai uplift. The data obtained disprove the concept that the studied volcanics are of Riphean age; therefore, the available stratigraphic charts of the Siberian Precambrian must be revised.