The Yudomian of Siberia,Vendian and Ediacaran systems of the International stratigraphic scale

In Russia, the terminal Neoproterozoic formally includes the Vendian of western part of the East European platform and the concurrent Yudoma Group of Siberia. As is shown in this work, the designated subdivisions correspond in the stratotypes only to the upper, Yudomian Series of the Vendian. In the Siberian platform, the Ust-Yudoma and Aim horizons of the Yudomian are tightly interrelated. The lower of them, bearing remains of Ediacaran Fauna, represents the Ediacarian Stage, whereas the upper one containing small-shelled fossils (SSF) corresponds to the Nemakit-Daldynian Stage divided into the trisulcatus and antiqua superregional zones. In more complete sections of the platform periphery, sediments of these subdivisions conformably rest on siliciclastic succession that should be ranked as basal subdivision of the Yudomian. The succession is concurrent to the Laplandian Stage of the East European platform. According to geochronological dates obtained recently, the Yudomian Series spans interval of 600–540 Ma. In the East European platform, the Upper Vendian (Yudomian) begins with the Laplandian basal tillites of synonymous stage. In the west of the platform, tillites are dated at 600 Ma like the Upper Vendian base in Siberia. The next Ediacarian Stage of the East European platform is stratigraphic equivalent of the Redkino Horizon, while summary range of the Kotlin and Rovno horizons is concurrent to that of the Nemakit-Daldynian Stage. The Vendian of Russia is conformably overlain by the Tommotian Stage of the Lower Cambrian. Intense pre-Vendian events constrained distribution areas of the Lower Vendian sediments in Russia. The Lower Vendian deposits of the East European platform are most representative and well studied in the central Urals, where they are attributed to the Serebryanka Group. In Siberia, separate subdivisions representing the Lower Vendian are the Maastakh Formation of the Olenek Uplift, two lower members of the Ushakovka Formation in the Baikal region, and the Taseeva Group of the Yenisei Range. Chronological interval of the Lower Vendian corresponds to 650–600 Ma. The Marinoan Glaciation dated in Australia at 650–635 Ma is concurrent to basal part of the pre-Yudomian interval of the Vendian in Russia, whereas the Laplandian Tillite and Gaskiers Glaciation (600–580 Ma) correspond to onset of the Yudomian Epoch. The new Ediacaran System (Knoll et al., 2004) legalized in the International Neoproterozoic scale is close in range to the entire Vendian (635–544 Ma), although without basal beds (Marinoan Tillite) it deprives the terminal Neoproterozoic of its original sense. Inferiority of the system consists also in its indivisibility into stages. Hence, it is clear that the Vendian System subdivided in detail in Russia should be retained in the rank of terminal system of the Precambrian, one of the basic in general scale of the Neoproterozoic.     

The Upper Riphean of the Yenisei Range

The Yenisei Range and the adjacent territories in the east are subdivided into (1) the Mid-Angara intracratonic depression; (2) the Yenisei pericratonic trough; and 3) a marginal oceanic block, the Isakovka-Predivinsk area. The lower part of the Riphean succession is subdivided into two principally different sedimentary complexes — the Lower Sukhoi Pit Subgroup and the Upper Sukhoi Pit Subgroup (the Pogoryui-Alad'in interval of the succession). The fundamental nature of the events that separate these two complexes and the characteristic, rhythmically bedded structure of the Upper Sukhoi Pit Subgroup allow the latter to be ranked a separate straton, the Bol'shoi Pit Group. Its lower boundary is associated with the Grenvillian events commencing with the emplacement of the Teya granite-gneiss domes and other intrusive complexes dated at 1100–1000 Ma. In the sedimentation record these events are manifested as a sudden change from the slate complex, for which we keep the name Sukhoi Pit Group, to the rhythmically bedded succession of the Bol'shoi Pit Group. The latter is interpreted as a product of uproofing of an elevated hinterland to the west. Insofar as the amplitude of this elevated area decreases progressively toward the Mid-Angara trough, the Bol'shoi Pit erosional unconformity and the associated interval of nondeposition are absent from the area. In the west of the Yenisei Range, in contrast, there is a major stratigraphic gap in the sequence, which is associated with the aforementioned events. The hypothesis on intensive events separating the deposition of the Bol'shoi Pit Group of the Kerpylian Horizon and the Tungusik Group of the Lakhandinian Horizon is not supported by the new data. The change from carbonate facies into siliciclastics in the west was misinterpreted as an erosional unconformity, with basal deposits corresponding to the lower boundary of the Tungusik Group. The occurrence of the Upper Tungusik deposits overlying much older rocks is a result of the pre-Bol'shoi Pit erosion and the gradual expansion of the Tungusik transgression. Thus, there are no grounds to argue for significant pre-Lakhandinian events in the region. Hence, the Kerpylian and Lakhandinian in the Yenisei Range, as well as in other parts of the Siberian Craton, constitute two parts of a larger supraregional straton, which corresponds to the lower half of the Upper Riphean and is designated here the Mayanian. The fundamentally different nature of the events associated with the next, Baikalian stage of the development allows its tripartite subdivision in the region. Deposition of the Lower Baikalian (the Oslyanka Group) was preceded by the crustal extension at the junction between the continental and oceanic blocks and, possibly, the formation of one of the Yenisei Range ophiolite complexes, followed by the emplacement of the Tatarka-Ayakhta batholiths at around 850 Ma. Fragments of both complexes are found as clasts in the basal conglomerates of the Middle Baikalian Chingasan Horizon. The specific character of the pre-Baikalian events determines their apparently poor expression in the sedimentation (weaker metamorphism of the Oslyanka deposits compared with the Tungusik Group). Even the activity leading to the formation of the Tatarka-Ayakhta granites cannot be regarded as a full-scale orogenic process. Collisional events separating the Lower and Middle Baikalian are manifested as the erosional unconformity at the base of the Chingasan Group and the emplacement of the Glushikha granites (760–730 Ma). The Middle Baikalian age of the Chingasan deposits is constrained by the data from paleontology, historical geology, and geochronology. Furthermore, the presence of glacial deposits renders this straton as a global stratigraphic marker. Further expansion of transgression in the Upper Baikalian is linked to another important event, but additional paleontological and geochronological information is needed to date the Upper Baikalian (Chapa Group) more accurately. The Baikalian events synchronously manifested themselves in all structural-facies zones of the Yenisei Range and are coeval to structural complexes from adjacent areas of the Siberian Craton. The tripartite Baikalian, therefore, has a potential for being included into the General Scale of the upper Upper Riphean.     

Geochemistry,Tectonic Settings,and Age of Metavolcanic Rocks of the Isakovskii Terrane,Yenisei Range: Indicators of the Early Evolution of the Paleo-Asian Ocean

The geodynamic nature of the Late Neoproterozoic island-arc dacites (691 ± 8.8 Ma) and rift basalts (572 ± 6.5 Ma) of the Kiselikhinskaya Formation, Kutukasskaya Group, in the Isakovskii terrane is inferred from geochemical data and U–Pb zircon (SHRIMP-II) dates. The volcanic rocks were produced during the late evolutionary history of the Yenisei Range, starting at the origin of oceanic crustal fragments and their accretion to the Siberian craton to the postaccretionary crustal extension and the onset of the Caledonian orogenesis. The reproduced sequence of geological processes marks the early evolution of the Paleo- Asian Ocean in its junction zone with the Siberian craton. The data refine the composition and age of volcanic rocks in the trans-Angara part of the Yenisei Range and specifics of the Neoproterozoic evolution of the Sayan–Yenisei accretionary belt.     

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, UC_PR1, UC_AR2, 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.     

C and Sr isotope chemostratigraphy of Vendian-Lower Cambrian carbonate sequences in the central Siberian Platform

We propose a detailed δ¹³C curve for the Vendian and Lower Cambrian (Tommotian) strata of the central Siberian Platform. Two positive carbon isotope excursions identified near the base of the Yuryakh Formation (up to 5.5‰) and in the lower Bilir Formation (up to 5‰) are assigned to the lowermost and middle Tommotian, respectively. This correlation is supported by paleontological data, specific ⁸⁷Sr/⁸⁶Sr values (0.70845-0.70856), and similar C isotope record in coeval Early Cambrian basins. The documented minor vertical oscillations (a few meters) of these isotope excursions relative to the formation boundaries in remote boreholes is presumably caused by the spatiotemporal migration of facies. A high-amplitude negative δ¹³C excursion (-8 to -11‰) in the upper Nepa Regional Stage putatively corresponds to the global Shuram-Wonoka negative carbon isotope excursion (Middle Ediacaran). Carbonates of the lower Nepa Regional Stage (Besyuryakh Formation) demonstrate positive δ¹³C values (up to 5‰) and minimum ⁸⁷Sr/⁸⁶Sr ratios of 0.70796-0.70832. The C and Sr isotope record of the Nepa Regional Stage provides its robust correlation with the Dal’nyaya Taiga and Zhuya Groups of the Patom Foredeep. Micropaleontological data herein reported and glacial diamictites documented at the base of the Vendian sedimentary cover both in the central Patom Foredeep and on its periphery suggest a full stratigraphic volume of the Ediacaran System in the most stratigraphically complete sections of the central Siberian Platform.     

Postglacial Deposits of the Dal’nyaya Taiga Group: Early Vendian in the Ura Uplift,Siberia. Communication 1. Barakun Formation

Analysis of modern paleontological, isotope-geochemical, and paleotectonic data on Vendian rocks in the Patom Basin (Siberia) is presented. The paper discusses depositional settings and features of the specifics of sedimentogenesis of the Barakun carbonate–terrigenous formation, which matches the lower strata of postglacial deposits of the Dal’nyaya Taiga Group. It is shown that formation of the transgressive Barakun Formation was governed by relatively deep-water (below the storm wave base) distal settings of a low-angle (about 1°) homoclinal ramp. Its formation was related to a more intense sagging of the northeastern part of the Ura Uplift. The formation of breccias and intrusions was fostered by high seismic activity of the paleobasin recorded in plastic deformations along with liquefaction and fluidization of sedimentary beds. The constant seismic destabilization of bottom beds provoked small but frequent landslides without the formation of turbidite flows. The continuous dislocation of sediments toward the depocenter actively governed specifics of the ramp architecture. The unusual behavior of sediments during their long-term thixotropic residence could be related to a high content of organic matter, processes of methanogenesis and, probably, dissemination of gas hydrate compounds.     

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.     

Accretionary Tectonics of Rock Complexes in the Western Margin of the Siberian Craton

The geological, geochemical, and isotope-geochronological evidence of the events at the final stage of the Neoproterozoic history of the Yenisei Range is considered (beginning from the formation of fragments of the oceanic crust in the region and their accretion to the Siberian Craton until the postaccretionary stage of crustal tension and onset of the Caledonian orogeny). Based on an analysis of new data on the petrogeochemical composition, age, and geodynamic nature of the formation of contrasting rocks in the composition of tectonic mélange of the Near-Yenisei (Prieniseiskaya) regional shear zone, we have found the chronological sequence of events that marks the early stages of the Paleoasian Ocean evolution in the zone of its junction with the Siberian Craton. These events are documented by the continental marginal, ophiolitic, and island-arc geological complexes, each of which has different geochemical features. The most ancient structures are represented by fragments of oceanic crust and island arcs from the Isakovka terrane (700–620 Ma). The age of glaucophane-schist metamorphic units that formed in the paleosubduction zone corresponds to the time interval of 640–620 Ma. The formation of high-pressure tectonites in the suture zone, about 600 Ma in age, marks the finishing stage of accretion of the Isakovka block to the western margin of the Siberian Craton. The final events in the early history of the Asian Paleoocean were related to the formation of Late Vendian riftogenic amygdaloidal basalts (572 ± 6.5 Ma) and intrusion of postcollisional leucogranites of the Osinovka massif (550–540 Ma), which intruded earlier fragments of the oceanic crust in the Isakovka terrane. These data allow us to refine the Late Precambrian stratigraphic scheme in the northwestern Trans-Angarian part of the Yenisei Range and the evolutionary features of the Sayan–Yenisei accretionary belt. The revealed Late Neoproterozoic landmarks of the evolution of the Isakovka terrane are attributed to the terminal phase of the breakup of Rodinia, separation of the Siberian Craton, and opening of the Paleoasian Ocean.     

Sources of clastic material of the Neoproterozoic metasedimentary rocks of the Baikal–Patom belt,Northern Transbaikalia: Evidence from Sm-Nd isotope data

First systematic data on the variations of ε_Nd(T) in the Neoproterozoic sedimentary sequence of the Baikal–Patom fold belt (Northern Transbaikalia) are reported. The range of obtained εNd(T) is–19.4…–2.0. It was established that the rocks of the Ballaganach and most part of the Dal’nyaya Taiga groups are characterized by ε_Nd(T) from–19.4 to–16.3. Upsection, beginning from the Khomolkha Formation, the rocks show a sharp change in the initial Nd isotope composition (ε_Nd(T)–8.3…–2.0). The results of Sm-Nd study of metasedimentary rocks of the BPB, as well as the comparison of their Sm-Nd characteristics with those of the inferred source areas suggest that the input of terrigenous material at the early stage of sedimentation in the Baikal–Patom belt was mainly related to the destruction of the Early Proterozoic crust of the Siberian craton. Owing to a change of sedimentation setting from passive continental margin to the “foreland” basin at the late stage, the terrigenous material of the Siberian craton was diluted by clastics from juvenile Neoproterozoic crust of the Baikal–Muya belt.     

The first magnetostratigraphic data on the stratotype of the Lopata Formation,Northeastern Yenisei Ridge: Problems of its age and paleogeography of the Siberian Platform at the Proterozoic–Phanerozoic boundary

New paleomagnetic and magnetostratigraphic data are presented for the stratotype of the Upper Riphean Lopata Formation (Teya River, Yenisei Ridge). The paleomagnetic pole calculated is significantly distinct from the Phanerozoic and Riphean poles of the Siberian Platform and is similar to the Late Vendian–Early Cambrian poles of the Madagascar Group. The stratigraphic range studied is characterized by an anomalously high frequency of geomagnetic inversions (15 zones of magnetic polarity), which is comparable with the inversion frequency of the Late Vendian sections of Baltica. These data, along with previous paleontological findings, indicate an age of the Lopata Formation of 555–540 Ma.     

Neoproterozoic Chingasan Group in the Yenisei Ridge: new data on age and deposition environments

Trachybasalt-alkali trachyte volcanism in the Yenisei Ridge was found out to be synchronous with deposition of coarse tilloids and flysch of the Chivida Formation of the Neoproterozoic Chingasan Group. New 703±4 Ma ⁴⁰Ar/³⁹Ar biotite and titan-augite ages of subalkaline basalts in the Chivida Formation indicated that they erupted in the Late Neoproterozoic. According to microfossil evidence, the Chingasan sediments correlate with Late Neoproterozoic strata in the type sections of the southern Siberian craton. The Chingasan deposition apparently lasted no longer than 30 Myr judging by the isotope ages obtained for the underlying Upper Vorogovka Group and subalkaline basalts in the Chivida Formation. The fault-parallel position of grabens and coarse grain sizes and variable thicknesses of their lithological complexes, as well as syndepositional trachybasalt-alkali trachyte volcanism provided evidence that the volcanosedimentary rocks of the Chingasan Group formed in an environment of active rifting.     

Ore-bearing Coaly Shales of the Lena Auriferous Region

Ore-bearing coaly shales of the Lena auriferous region (Patom Highland) are distinctly controlled by lithostratigraphic and structural factors. They are confined to carbonaceous and carbon-bearing (carbonate–terrigenous and terrigenous) lithologic varieties of primarily Au-rich black shales. However, the location of ore-bearing shales within these units is governed by structural factors. Gold mineralization and synore alterations are related to the tectonometasomatic reworking of substance of the geochemically favorable protoliths.     

Neoproterozoic ice sheets of the Siberian Platform: U-Pb-LA-ICP-MS ages of detrital zircons from the Bol’shoi Patom formation and the geotectonic position of its provenance

The centers of many Neoproterozoic and Phanerozoic glaciations were located on cratons and crystalline shields not necessarily, as some researchers believe, associated with rifts. An example is the Vendian Bol’shoi Patom glacial formation (Lena River, Yakutia). The U-Pb ages of detrital zircons from this formation imply that it was deposited by glaciers located on the pre-Riphean crystalline shields of the Siberian Platform. Glaciation in this region occurred after cessation of rifting and could not be related to the latter. The Bol’shoi Patom Formation is an element of the Middle Siberian glacial horizon that was deposited during the Marino Glaciation and is traceable along the southern periphery of the Siberian Platform from the Aldan Shield to the Baikal and Sayany regions. This indicates that the Marinoan Glaciation in Siberia involved at least the entire southern part of the Siberian Platform. Rifting was not the single, or even a principal, factor responsible for glaciations, although it could accompany and enhance them simultaneously providing space for sedimentation and conditions for preserving glacial deposits from erosion.     

Heavy metal distribution in the surface layer of bottom sediments of the Kara Sea

This paper addresses the distribution of heavy metals (Co, Ni, Cu, Zn, Cd, Sn, Sb, Pb, and Bi) as well as Si, Al, Fe, and Mn in the surface (0–2 cm) layer of bottom sediments of the Kara Sea. The contents of these elements are determined in each of the previously distinguished facies-genetic types of terrigenous sediments: fluvial, glacial, estuarine, shallow water–marine, “background” marine, and relict sediments. It is shown that these types reflect the modern conditions of accumulation of river discharge material, which forms fans of two greatest Siberian rivers, Ob and Yenisei. The main stages are distinguished in heavy metal accumulation. The first stage is related to the avalanche sedimentation of terrigenous sediments in the estuary and characterized by the elevated contents of Co, Ni, Cu, Zn, Cd, Sb, and Bi. The second stage reflects the mechanical differentiation of sedimentary material by waves and bottom currents in a shallow-water sea part adjacent to the estuarine zone, with accumulation of Pb- and Sn-bearing “heavy” ore minerals. The deepwater background terrigenous–marine sediments accumulate mainly Ni, Zn, and Cd, as well as Mn. The relict sediments differ in the high contents of Si, Mn, and Sn.     

Sequence stratigraphy in the upper cretaceous of the Basco-Cantabrian Basin (northern Spain)

The sedimentary cycles of the Cenomanian to Maastrichtian were investigated in the Basco-Cantabrian Basin (BCB) in northern Spain (Provinces of Alava, Vizcaya and Burgos). The depositional area was a distally steepened carbonate ramp which extended from Catalonia northwestwards to the Basque country. The investigated sediments range from calciturbidites and pelagic marls to marl-limestone alternations deposited on a distal carbonate ramp. Shallow marine limestones, marls and intertidal clastics and carbonates were deposited on the proximal part of the carbonate ramp. The establishment of a regional sequence analysis is based on the investigation of seismic profiles, well logs and outcrop sections. Examples of outcrop sections are interpreted in terms of sequence stratigraphy (unconformities of third- and second-order cycles, depositional geometries, systems tracts). The sequence stratigraphic interpretation of outcrop sections is based on facies analysis, interpretation of observed depositional geometries and correlation of unconformities and marine flooding surfaces through the basin. A biostratigraphic framework is established based on ammonites, inoceramids, planktonic and benthic foraminifera. As a result, a regional sequence stratigraphic cycle chart is presented and compared with published global cycle charts. The correlation of the regional cycle chart with published cycle charts is good. In the Cenomanian and Turonian, several sequence boundaries in the BCB are shifted by up to one biozone compared with the global chart. Some type 1 boundaries of the standard chart are only type 2 in the BCB. Important type 1 boundaries in the BCB are: top Geslinianum Zone with a 100 m lowstand wedge at the basis of the sequence (sequence boundary 92.2) ; base Petrocoriense Zone with a 250 m shallowing-upwards lowstand wedge at the basis (sequence boundary 89.2); and within the Syrtale Zone (sequence boundary 85.0).The Campanian-Maastrichtian sequence record is strongly disturbed by local compressive tectonics. Several sequences are recognizable and can be correlated with the global cycle chart. Correlation is hampered by the low biostratigraphic resolution in the western basin part. Subsidence analysis of several sections of the Upper Cretaceous of the BCB and its interpretation in the regional tectonic context leads to a discussion of the causes of the observed cyclicity. A regional eustatic curve is presented for the Upper Cretaceous of the BCB. Stage and substage names were used according Code-Committee (1977). Correspondence to: K.-U. Gräfe     

Structure,Age, and Settings of Formation of Ordovician Complexes of the Northwestern Frame of the Kokchetav Massif,Northern Kazakhstan

This work presents the data on the structure, geochronology, and formation settings of the Ordovician sedimentary and volcanogenic-sedimentary complexes of the Sterlitamak, Mariev, and Imanburluk structural and formational zones located in the western and northwestern frames of the Kokchetav massif (Northern Kazakhstan). In addition, the results of detailed stratigraphic, geochemical, and geochronological studies of the reference section of the Ordovician deposits of the Mariev Zone are given. The studied section is composed of carbonate, terrigenous, and less commonly volcanogenic-sedimentary deposits, confined to a wide stratigraphic interval from Tremadocian Stage of the Lower Ordovician to the lower Sandbian Stage of the Upper Ordovician. For the first time, the study of conodont assemblages made it possible to establish the Early to Middle Ordovician age of the most ancient limestone–dolomite sequence, which was previously conventionally attributed to the Cambrian. The above-lying tuffaceous–terrigenous Kupriyanovka Formation is now attributed to the Middle Ordovician. On the basis of compositional features of the lithoclastic tuffs composing the middle part of the formation, we assume that it was formed within the island arc zone. Limestones from the base of the youngest terrigenous–carbonate Kreshchenovka Formation are attributed to the lower part of the Sandbian Stage of the Upper Ordovician. The study of the geochronology of detrital zircons from terrigenous rocks of the limestone–dolomite sequence has shown that the Early Neoproterozoic quartzite–schist sequences of the Kokchetav massif were the most probable provenance area during its deposition. It was established that there was the change of sedimentation environments from closed lagoons to a relatively deep sea basin with normal salinity and intense circulation of water masses in the northwestern frame of the Kokchetav massif during the Ordovician. During this period of time, there was a sufficiently high level of erosion of provenance areas that resulted in the deposition of thick strata of terrigenous material. A general tendency of the deepening of sedimentation environments from the Early to Late Ordovician was interrupted by sea level rises in the Dapingian and early Darriwilian ages.     

Lower proterozoic metapelites in the northern Yenisei Range: Nature and age of the protolith and the behavior of material during collisional metamorphism

The variable P-T metamorphic conditions studied in the Fe-Al metapelites of the Karpinskii Range Formation are regarded as typical of collision-related metamorphism in the trans-Angara part of the Yenisei Range. Recently obtained geochronologic (SHRIMP-II U-Pb zircon dating) and geochemical data on the distribution of major and trace elements are used to reproduce the composition of the protolith, the facies conditions under which it was formed, the tectonic setting, and the age of the eroded rocks. The metapelites are determined to be redeposited and metamorphosed material of Precambrian kaolinite-type weathering crusts of predominantly kaolinite-illite-montmorillonite-quartz composition. The protolith of the rocks was formed via the erosion of Lower Proterozoic granite-gneiss complexes of the Siberian craton (dated mainly within the range of 1962–2043 Ma) and the subsequent accumulation of this material in a continent-marginal shallow-water basin in a humid climate and tectonically calm environment. These results are consistent with data of lithologic-facies analysis and geodynamic reconstructions of the Precambrian evolution of geological complexes in the Yenisei Range. Mass-transfer analysis with the use of the evaluated rock compositions and calculated chemical reactions indicates that the differences in the REE patterns of metapelites from distinct zones can be explained mostly by the chemical heterogeneity of the protolithic material and, to a lesser extent, by metamorphic reactions at a pressure increase.     

Pre-Riphean metapelites of the Yenisei Range: Chemical composition,sources of eroded material,and paleogeodynamics

The paper reports data on metapelites (gneisses and crystalline schists) from various Early Precambrian lithostratigraphic units of the Yenisei Range (Kan and Yenisei complexes, Garevka Unit, and Teya Group). A representative selection of 57 samples from the examined vertical section were analyzed for major oxides, LILE, transition elements, HFSE, REE, radioactive elements, and other trace and minor elements. The most important distinctive geochemical feature of metapelites composing most of the vertical section is their elevated concentrations of Al, which are higher than in the Post-Archean Australian Shale (PAAS), K, Rb, Ba, Ga, Sc, Pb, Th, Nb, Y, and REE. These data and the systematics of elements in certain discriminant diagrams testify that the metasedimentary complexes in the sedimentation basin and the rocks associations composing the erosion area (crystalline massifs of the ancient basement of the Siberian craton) were strongly geochemically differentiated and mature. The composition of the eroded crustal material was close to the average composition of the post-Archean continental crust and PAAS. The metapelites of the Kuzeeva Unit in the Kan Complex and Penchenginskaya Formation in the Teya Group were the only ones that could have basic rocks as an additional source of material, as also follows from the elevated Cr concentrations of the rocks and their REE systematics. The three types of REE patterns of the metapelites largely correspond to the composition of the rocks that composed the ancient drainage areas and the degrees of the differentiation and averaging of their terrigenous material. The first predominant type corresponds to PAAS and the averaged composition of the upper continental crust. The second and third types of the REE patterns with high and low La_N/Yb_N ratios, respectively, and with or without negative Eu anomalies provide evidence of the contribution of tonalite-trondhjemite-granodiorite (TTG) or basite sources typical of granite-greenstone provinces. In certain diagrams, the compositional fields of Early Precambrian metapelites in the Yenisei Range almost exactly coincide with the compositional fields of regional fine-grained Riphean terrigenous rocks. This suggests that the Late Precambrian sedimentary rock sequences inherited their geochemical features from more ancient rocks. The reproduced ancient geodynamic environments in which the Early Precambrian metaterrigenous complexes of the Yenisei Range were accumulated correspond to ensialic continental marginal basins. The rocks of the Kan and Yenisei complexes and the Garevka Unit were formed mostly on active continental margins and, less frequently, on passive margins. The protoliths of metasedimentary rocks of the Teya Group were accumulated in a subplatform environment at a passive margin.     

Ultramafic–mafic igneous complexes of the Precambrian East Siberian metallogenic province (southern framing of the Siberian craton): age,composition, origin,and ore potential

Based on new data on the age, mineralogy, and geochemistry of ultramafic–mafic complexes in the Precambrian structures of the southern periphery of the Siberian Platform, the East Siberian metallogenic (PGE–Cu–Ni) province is recognized. It includes the Yenisei Ridge, Precambrian Kan uplift, Alkhadyr terrane with the adjacent structures of the Biryusa block, and northern Baikal region (Yoko-Dovyren and other massifs of the Baikal–Patom basin). We have established that the U–Pb and Ar–Ar ages of ore-bearing complexes of dunite–peridotite– pyroxenite–gabbro association correspond to the Late Riphean (728–710 Ma). The mineralogical and geochemical similarity of ore-bearing complexes in different areas testifies to their genetic entity. All parental melts were similar in composition to picrites. The calculation results and the PGE enrichment of rocks and ores show high degrees of melting of the mantle source, which agrees with the plume model of formation of the ore-magmatic system. The recognized province is similar in the type of magmatism and time of its occurrence to the Franklin LIP in northern Canada. It is one of the highly promising ore districts of East Siberia.