Historical seismicity on the southern margin of the Siberian craton: new data

We discuss historical evidence for seismicity on the southern margin of the Siberian craton collected from old local newspapers. The reported earthquakes vary in magnitude from M = 2.5 to 4.5, and their macroseismic locations agree well with the regional tectonic framework. The new data prove seismic activity in the area and can be used in seismic risk assessment.     

Magmatism evolution and carbonatite-granite association in the neoproterozoic active continental margin of the Siberian craton: Thermochronological reconstructions

Neoproterozoic carbonatites and related igneous rocks, including A-type granites in the Tatarka-Ishimba suture zone of the Yenisey Ridge are confined to a horst-anticlinal structure that was formed in a transpression setting during the oblique collision between the Central Angara terrane and the Siberian craton. The carbonatites, associating mafic (including alkaline) dikes as well as the Srednetatarka nepheline syenites are the oldest igneous formations of the Tatarka active continental margin complex. Geochronological data indicate that magmatic evolution continued in the studied anticline for nearly 100 m.y. On the earliest stage carbonatites were formed and on the last stage — the emplacement of mantle-crustal A-type Tatarka granites took place. According to new U/Pb zircon studies, the earliest rocks in the Tatarka pluton are A-type leucogranites aged 646 ± 8 Ma. The younger ⁴⁰Ar/³⁹Ar ages of carbonatites obtained for phlogopites (647 ± 7 and 629 ± 6 Ma) are related to the last tectonic events in the studied region of the Tatarka-Ishimba suture zone, which are coeval with the formation of the A-type granitoids (646–629 Ma).     

Neoproterozoic evolution of Rodinia: constraints from new paleomagnetic data on the western margin of the Siberian craton

The paper summarizes paleomagnetic results obtained from the Neoproterozoic rocks of the western margin of the Siberian craton. On the basis of the obtained paleomagnetic poles and available paleomagnetic data for the Precambrian of Siberia, a new version of the Neoproterozoic segment of the apparent polar wandering path (APWP) is proposed for the craton and is compared with the Laurentian APWP. The superposition of these paths suggests that in the Neoproterozoic the southern margin of the Siberian craton (in modern coordinates) faced the Canadian margin of Laurentia. Most likely, in the end of the Mesoproterozoic and during the Neoproterozoic the Siberian craton and Laurentia were connected to form the supercontinent Rodinia. At 1 Ga the western margin of the Siberian craton was a northern (in modern coordinates) continuation of the western margin of Laurentia. The available paleomagnetic data on Laurentia and continental blocks of Eastern Gondwana (Australia, Antarctica, India, South China) and the proposed APWP trend allowed a new model for the breakup of this segment of Rodinia. Analysis of a total of the data available suggests that strike-slip movements on the background of the progressive opening of the oceanic basin between Siberia and Laurentia were predominant in the south of the Siberian craton during the Neoproterozoic. Similar kinematics is typical of the western margin of Laurentia, where strike-slip motions are probably associated with the progressive opening of the ocean basin between Laurentia and eastern Gondwana.     

A-type leucogranite magmatism in the evolution of continental crust on the western margin of the Siberian craton

We discuss problems of the origin, settings, and age of Neoproterozoic A-type leucogranites widespread in the Yenisei Ridge. Combined analysis of geological, petrological, and geochemical (including isotope) data shows that some granitoids (Glushikha complex) were formed at the postcollisional stage (750–720 Ma), and others (Tatarka complex), in an anorogenic environment (680–630 Ma). The anorogenic complex contains diverse igneous rocks, including alkaline varieties and carbonatites. Leucogranites form separate plutons within different igneous complexes. They have high contents of potassium (up to ultrapotassic composition in the Glushikha complex), iron, and fluorine and are depleted in europium. Postcollisional granitoids show the highest concentrations of Rb, Th, and U, extremely low concentrations of Ba and Sr, whereas anorogenic granitoids are rich in Ta, Nb, Y, Sm, and HREE. The obtained data point to the augmented mantle contribution to the formation of continental crust of the Yenisei Ridge between 750 and 630 Ma. We also report new results of U-Pb zircon dating, including SHRIMP and Ar-Ar data.     

华北克拉通南缘张士英岩体大陆动力学背景:来自地球化学、锆石U-Pb年龄和Hf同位素的证据

张士英岩体位于华北克拉通南缘,岩体主要由石英正长岩组成。石英正长岩的LA-ICP-MS锆石U-Pb年龄为122.8±1.5Ma。其Si O2含量为66.04%~67.80%,Na2O+K2O=9.03%~10.97%,K2O=4.40%~6.37%,K2O/Na2O1属于钾质长英质岩石。A/CNK=1.26~1.58,A/NK=1.63~1.79属于过铝质岩石系列。石英正长岩的Mg#变化范围在12.9~39.4。富集LREE亏损HREE,轻重稀土分异明显,(La/Yb)N=15.48~21.12,Eu呈弱的负异常(δEu=0.54~0.99)。富集Rb、K、Th、U等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素。张士英石英正长岩岩浆锆石εHf(t)集中在-17.6~-13.9,Hf两阶段模式年龄tDM2集中在1.7~1.9Ga。石英正长岩的岩浆Zr饱和温度高(936~998℃)。地球化学及同位素显示张士英石英正长岩源区主要为古老的壳源物质,并有少量年轻组分加入,这种年轻组分是幔源物质。岩体形成于拉张性构造环境下,拉张性的环境导致了幔源物质的上涌,底侵下地壳使其发生部分熔融。形成时代正好位于华北克拉通破坏峰期,张士英石英正长岩正是这一地质事件的响应。     

New data on biostratigraphy of the Vendian Nemakit-Daldynian stage in the southern Siberian platform

New paleontological finds in sediments of the Upper Vendian Nemakit-Dadynian Stage from different areas of the southern Siberian Platforms (Yenisei Range, Sayany region, central areas, Patom Highland) are discussed. The base of the Lower Cambrian Tommotian Stage (∼540 Ma) is established at the first appearance level of characteristic small-shelled fossils in the western part of the Patom Highland. This boundary coincides with the episode of a brief sea-level fall and enhanced terrigenous sedimentation in the basin with dominant carbonate sediment deposition. The base of the Purella antiqua Zone (544 Ma), which is registered by paleontological data in the Yenisei Range sections, is marked by the replacement of terrigenous sedimentation by the carbonate one. In the northwestern Yenisei Range, this boundary coincides, in addition, with a major hiatus and subsequent paleobasin extension. The base of the Nemakit-Daldynian Stage (base of the Anabarites trisulcatus Zone, ∼550 Ma) in most sections corresponds to the onset of sea transgression. In addition to small-shelled remains, sediments of this zone contain characteristic ichnofossils and Vendotaenia flora. It is shown that defined boundaries are traceable through the entire southern part of the Siberian Platform. This makes them useful for wider stratigraphic interpretations.     

Seaward dipping reflectors along the SW continental margin of India: Evidence for volcanic passive margin

Multi-channel seismic reflection profiles across the southwest continental margin of India (SWCMI) show presence of westerly dipping seismic reflectors beneath sedimentary strata along the western flank of the Laccadive Ridge — northernmost part of the Chagos-Laccadive Ridge system. Velocity structure, seismic character, 2D gravity model and geographic locations of the dipping reflectors suggest that these reflectors are volcanic in origin, which are interpreted as Seaward Dipping Reflectors (SDRs).     

Formation stages of the Early Precambrian crust in the Sharyzhalgai basement uplift,southwestern Siberian craton: Synthesis of Sm-Nd and U-Pb data

Newly obtained U-Pb and Sm-Nd isotopic data on Early Precambrian metamorphic and granitoid complexes in the southwestern margin of the Siberian craton (Sharyzhalgai basement uplift) are synthe-sized in order to elucidate the crustal evolution starting at the Paleoarchean (∼3.6 Ga) to Late Paleoproterozoic (∼1.85 Ga), evaluate the lateral extent of the Paleo-Mesoarchean crust, and identify major stages in its growth and recycling. Two crustal growth stages were determined in the Onot and Bulun granite-greenstone terranes: at 3.6–3.3 and 2.8–2.9 Ga. The earliest recycling processes (at ∼3.4 and 3.2 Ga) involved partial melting, metamorphism, and migmatization and produced a stable continental crust. Crustal growth in the Mesoarchean (∼2.8–2.9 Ga) due to basaltoid magmatism was associated with the recycling of the Paleoarchean crust, which served as a source of felsic melts and of detrital material for terrigenous sediments. The Archean crust of the Irkut granulite-gneiss terrane was formed by two pulses of intermediate-felsic and basic volcanism at ∼3.6-3.4 and ∼2.7 Ga. In the terminal Archean (at ∼2.55 Ga), the preexisting crust was involved in metamorphic and magmatic processes. Traces of recycling of the Paleoproterozoic crust are identified in the isotopic parameters of the intermediate-felsic granulites. Two discrete stages in the influx of juvenile material are identified in the Paleoarchean: at ∼2.0 and 1.88–1.85 Ga, with the latter stage associated with the large-scale recycling of the Archean crust during the origin of granitoids.     

Paleomagnetism of trap basalts in the northwestern Siberian craton,from core data

We present new paleomagnetic data for continental flood basalts (Siberian traps) obtained from cores of two boreholes in the northwestern Noril'sk area, within the Kharaelakh and Vologochan basins. Paleomagnetic measurements of lava and tuff samples from KhS-59 and SSV-19 boreholes allowed reconstructing and correlating the polarity patterns. Thus multiple paleomagnetic anomalies (PMA) have been discovered as brief polarity changes in narrow intervals of the magnetostratigraphic section above the principle reversal at the boundary between the Ivakin and Syverma Formations.The most prominent anomalies are observed at the bases of the Morongo and Mokulai Formations. The samples from the anomalous intervals differ from those of other intervals neither in rock magnetic properties, nor in mineralogy and magnetic grain sizes. Therefore, the revealed PMA record excursions of the geomagnetic field. Comparison of the results with the Meishan Global Stratotype Section and Point (GSSP) of the Permian–Triassic boundary implies a revision to the P–T position in the trap basalt succession of the Noril'sk area. Judging by the EMF behavior, basalts in this part of the trap province erupted for at least 500 kyr during an interval of stable normal polarity.     

Crust structure and composition in the southern Siberian craton (influence zone of Baikal rifting), from magnetotelluric data

The magnetotelluric (MT) profile traverses the southeastern edge of the Siberian craton and the adjacent Paleozoic Olkhon collision zone, both being within the influence area of the Baikal rifting. The processed MT data have been integrated with data on the crust structure and composition, as well as with magnetic, gravity, and seismic patterns. Large resistivity lows are interpreted with reference to new geothermal models of rifted crust in the Baikal region. The northwestern and southeastern flanks of the profile corresponding, respectively, to the craton and the collision zone differ markedly in the crust structure and composition and in the intensity of rifting-related processes, the difference showing up in the resistivity pattern. The high-grade metamorphic and granitic crust of the craton basement in the northwestern profile flank is highly resistive but it includes a conductor (less than 50 ohm · m) below 16–20 km and a nearly vertical conductive layer in the upper crust. The crust in the southeastern part, within the collision zone, is lithologically heterogeneous and heavily faulted. High resistivities are measured mainly in the upper crust composed of collisional plutonic and metamorphic complexes. Large and deep resistivity lows over the greatest part of the section are due to Cenozoic activity and rift-related transcrustal faults that vent mantle fluids constantly recharged from deeper mantle.     

Early evolution of the Paleoasian ocean: LA-ICP-MS dating of detrital zircon from Late Precambrian sequences of the southern margin of the Siberian craton

We present U–Pb (LA-ICP-MS) data on detrital zircon from the Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation, western Cisbaikalia (southern flank of the Siberian craton). The sources of clastic material for the studied sediments are interpreted. The youngest group of detrital zircon grains from the upper Baikal Group and Ushakovka Formation permits assigning these sediments to the Vendian. The lack of Mesoproterozoic detrital zircon in most of the analyzed samples confirms the hypothesis of a global (~ 1 Gyr) break in endogenic activity within the southern flank of the Siberian craton through the Precambrian. The abundance of Neoproterozoic zircon in sandstones from the upper horizons of the Baikal Group and the Ushakovka Formation might be due to the shrinkage of the ocean basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleoasian ocean.     

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.     

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.     

The isotope composition of Hf in zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Sharyzhalgai uplift (southern Siberian craton): Implications for the continental-crust growth

This paper presents results of U–Pb dating (SHRIMP-II) and Lu–Hf (LA–ICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388±11 and 3311±16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalite–trondhjemite–granodiorite (TTG) complexes are characterized mainly by positive values of εHf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average εHf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the Lu–Hf isotope composition of zircon from the Bulun rocks in the period 3.33–3.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the Lu–Hf isotope characteristics of zircon and the Sm–Nd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.45–3.60 and ~ 3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.     

The geodynamic evolution of the folded framing and the western margin of the Siberian craton in the Neoproterozoic: geological,structural, sedimentological,geochronological, and paleomagnetic data

The formation of the western margin of the Siberian craton in the Neoproterozoic is considered, with a focus on its transformation from a passive continental margin into an active one, accretion and collision processes, formation of island arcs and ophiolites, orogeny, and continent-marginal rifting. The evolution and correlation of sedimentary basins within fold-thrust belts of the Siberian Platform framing are considered. New structural and kinematic data on the Yenisei fault zone are discussed. On the basis of paleomagnetic data obtained for the structures in the zone of junction of the Siberian Platform and the West Siberian Plate, new models are proposed for the location of the Siberian craton relative to other paleocontinents and microcontinents in the Neoproterzoic. All these data provide a consistent evolution scheme for the western margin of the Siberian paleocontinent in the Neoproterozoic and constrain the position of the Siberian craton margin in Late Neoproterozoic (pre-Vendian) time.