Early Cambrian magmatism in the northeastern Siberian Craton (Olenek Uplift)

The Vendian–Lower Cambrian tectonomagmatic activation took place in the northeastern Siberian Craton, within the Olenek Uplift and in the Kharaulakh segment of the Verkhoyansk fold-and-thrust belt (the lower reaches of the Lena River). The Early Paleozoic volcanic activity in the Olenek Uplift is expressed in the form of basitic diatremes, small basaltic covers, and doleritic dikes and sills intruding and covering the Upper Vendian carbonate deposits. The material specificity of the Lower Cambrian basites and their mantle sources, jointly with the Vendian–Cambrian sedimentation history, gives reason to consider the Lower Cambrian riftogenesis and the associated magmatism as a consequence of the plume–lithosphere interaction in the northeastern Siberian Craton.     

Isotopic-geochemical features of rocks of the Middle-Upper Cambrian Verkhnyaya Lena formation in the Siberian Craton

Isotopic compositions of carbon and oxygen in carbonates and sulfur in sulfates of the Verkhnyaya Lena Formation (ε₂–ε₃), which terminates the Cambrian section of the Irkutsk Amphitheater of the Siberian Craton, are studied. Sulfates of the Verkhnyaya Lena Formation are marked by unusually low δ³⁴S values (4.6–12.0‰) relative to sulfates of the underlying Angara Formation. This is likely caused by variations in the facies-paleogeographic sedimentation at the transition of the Angara and Verkhnyaya Lena formations, as well as associated variations in the water and salt alimentation budget in sedimentation basins, due to their isolation from open sea and intensification of the continental and underground discharge. The δ¹⁸O(PDB) value in carbonates decreases from ?4.4‰ at bottom to ?10.4‰ at top, reflecting variation in postsedimentary transformations and probable continuous freshening of sedimentation basin. Isotopic composition of carbon in most samples shows normal marine δ¹³C values (0 ± 1‰). Only in some samples, does the δ¹³C value increase up to ?3.8 and 2.2‰ due to specific features of postsedimentary processes. The Rb-Sr systems of the clayey component of marls from the 500-m-thick section of the Angara Formation and bottom of the Verkhnyaya Lena Formation record an age of 512 ± 10 Ma, which is close to the assumed stratigraphic age of the Verkhnyaya Lena Formation. The ⁸⁷Sr/⁸⁶Sr initial ratio is 0.7082 ± 0.0004.     

Neoproterozoic and Lower Cambrian rock complexes in central areas of the Siberian Craton: Their structure and petroleum prospects

The paper summarizes data on the geology, lithology, and geochemistry of petroliferous Riphean, Vendian, and Lower Cambrian rocks in the central parts of the Siberian Craton. The petrological-geological properties of these sediments have been assessed based on results of paleogeographic analysis of these rocks, discrimination of oil reservoirs and oil-source successions, determination of secondary alterations of the rocks, and sources of oil generation and regional migration of hydrocarbons into various traps in zones of possible oiland-gas accumulation.     

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.     

Paleomagnetism of the Ulkan trough (Southeastern Siberian Craton)

The first results of the paleomagnetic study of one of the key Paleoproterozoic objects of the Aldan-Stanovoy Shield (the Ulkan trough) in the Bilyakchan-Ulkan volcanoplutonic belt are presented. The volcanosedimentary rocks of the Elgetei Formation and the granites of the Ulkan Complex were studied. According to these data and their comparison with the apparent Paleoproterozoic polar wandering path in the Angara-Anabar province, the Ulkan trough was (1) located during the timing of the studied rocks at 18°–26° S and (2) subjected to rotation (relative to the Angara-Anabar block) at 70° ± 8° in the time interval of 1732–1720 Ma ago. Based on the combined interpretation of the paleomagnetic, geochronological, and geochemical data published previously, a paleogeodynamic model is proposed. According to this model, the Aldan-Stanovoy and Angara-Anabar provinces of the Siberian Craton became a single rigid block about 1720 Ma ago.     

Paleoproterozoic,High-Metamorphic,Metasedimentary Units of Siberian Craton

Abstract: Sensitive, high-resolution ion microprobe zircon U–Pb ages of Paleoproterozoic, high-grade, metasedimentary rocks from the south-western part of the Siberian Craton are reported. Early Precambrian, high-grade complexes, including garnet–biotite, hypersthene–biotite, and cordierite-bearing gneisses compose the Irkut terrane of the Sharyzhalgay Uplift. Protoliths of studied gneisses correspond to terrigenous sediments, ranging from greywacke to shale. The paragneiss model Nd ages of 2.4–3.1 Ga indicate Archean-to-Paleoproterozoic source provinces. Zircons from gneisses show core-rim textures in cathodoluminescence (CL) image. Round or irregular shaped cores indicate detrital origin. Structureless rims with low Th/U are metamorphic in origin. The three age groups of detrital cores are: ≥2.7, ~2.3, and 1.95–2 Ga. The ages of metamorphic rims range from 1.86 to 1.85 Ga; therefore, the sediments were deposited between 1.95 and 1.86 Ga and derived from Archean and Paleoproterozoic source rocks. It should be noted that Paleoproterozoic metasedimentary rocks of the Irkut Block are not unique. High-grade metaterrigenous sediments, with model Nd ages ranging from 2.3 to 2.5 Ga, are widely distributed within the Aldan and Anabar Shields of the Siberian Craton. The same situation is observed in the North China Craton, where metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 to 2.1 Ga (Wan et al., 2006). All of these sedimentary units were subjected to Late Paleoproterozoic metamorphism. In the Siberian Craton, the Paleoproterozoic sedimentary deposits are possibly marked passive margins of the Early Precambrian crustal blocks, and their high-grade metamorphism was related to the consolidation of the Siberian Craton.     

Banded iron formations of the calciphyre-metabasite-gneiss association in the East European Craton

The paper presents characteristics of the least studied iron formations of the East European Craton (Archean banded iron formations of the calciphyre-metabasite-gneiss association), a typical member of granulite complexes of the Ukrainian Shield, Belarussian-Baltic region, and Voronezh crystalline massif. They are mainly composed of diverse metasedimentary rocks: aluminous gneisses; silicate-magnetite, magnetite, and barren quartzites; eulysites; calciphyres; and marbles associated with metavolcanic rocks. Data on chemical compositions of the metasedimentary rocks are summarized for the first time and their possible primary mineral composition has been reconstructed using the MINLITH software. It is shown that they could be formed from a lithogenetic series of sediments linked by gradual transitions and geochemical commonness of sediments: from fine-grained terrigenous insufficiently mature sediments to chemogenic sediments depleted in terrigenous material (ferruginous-siliceous, ferruginous-siliceous-carbonate, siliceous-carbonate, and carbonate sediments). The inferred primary mineral assemblage indicates sedimentation in the central parts of large paleobasins in a reducing environment characterized by deficit of oxygen and excess of carbon dioxide. Lithological specifics of the banded iron formations in different regions presumably reflect different distances of sedimentation zones from submarine hydrothermal discharge sites and sources of terrigenous material. The banded iron formations at the present-day erosion section of basement represent metamorphosed fragments of the lateral-facies zoning of rocks of the Archean sedimentary basins (or a single basin) of the East European Craton. Unlike other Early Precambrian banded iron formations of the East European Craton, rocks of the calciphyre-metabasite-gneiss association are marked by a high Mn content.     

Proterozoic Dyke Swarms of the Siberian Craton and Their Geodynamic Implications

正We present a summary of late Paleoproterozoic to Neoproterozoic mafic magmatism in the Siberian craton which allows us distinguish following main pulses of mafic dyke emplacement:1)1860–1850 Ma mafic dykes are localized within the     

Late Cambrian nautiloids of the Malyy Karatau

The latest Cambrian deposits of the Malyy Karatau in Kazakhstan contain nautiloid cephalopods associated with trilobites. Four new species of nautiloids are described and assigned as follows: Ellesmeroceratidae — Tamdoceras n. gen; T. lisogorae n. sp.; T. logicameratum n. sp.; Protocycloceratidae — Chabactoceras n. gen.; C. balashovi n. sp. — C. R. Palmer.     

First occurrences of Lapworthella in Lower Cambrian of the Siberian Platform

Two new species of Lapworthella are described from abundant silicified specimens from the Lower Cambrian Tyuser Shaya strata along the Lena river in northern Kharaulakh. Based on shape and size of shell, shell structure and composition (calcium phosphate), and character and arrangement of sculptural elements, Stenothecopsis schodackensis Lochman (1956) and several species of Stenothecopsis described by Poulsen (1942) are placed in synonymy with Lapworthella. The family Lapworthellidae Missarzlaevskiy, fain. nov. is proposed to include Lapworthella and Stenothecopsis. The two genera are removed from Crustacea (Cobbold, 1921, 1935) and placed in the unassigned order Hyolithelminthes Fisher, 1962.—M. E. Taylor.     

Late Cambrian deformation in the Lesser Himalaya

The Tons Valley, situated in the central-easternmost part of the Himachal Lesser Himalaya, adjoining the Garhwal Himalaya, shows geological features suggestive of a strong pre-Tertiary deformational episode. The Paleoproterozoic Dharagad Group, overlain by the Mesoproterozoic Deoban and Neoproterozoic Simla groups rest as a thrust sheet over the Middle Cambrian Chilar Formation, which occurs as windows and also as tectonic slivers within the thrust sheet designated as the Dharagad Thrust Sheet (DTS). The mineral lineation, inclination of tectonic slivers and overturned beds suggest that the DTS was translated from the NE. The westernmost and southwesternmost leading edges of the DTS are exposed at Subathu and Morni WNW and WSW respectively of the Tons Valley. The position of the leading edges of the DTS vis-à-vis the windows in the Tons Valley suggest a minimum translation of about 50 km for the DTS. The Simla Group at Subathu and the Deoban at Morni, forming parts of the DTS, constitute basement for the Thanetian–Lutetian Subathu Formation of the Himalayan Foreland Basin (HFB). This stratigraphic relationship unambiguously demonstrates that the Simla and the Deoban Groups, forming leading edges of the allochthonous DTS, were already translated and emplaced at Subathu and Morni before the creation of the HFB in which the deposition commenced with the Subathu Formation in Thanetian. It implies that the DTS was translated from the NE to the present position at Subathu and Morni in pre-Thanetian time. There is no direct evidence to constrain the age of the thrusting.In view of regional regression in Late Cambrian, a distinct angular unconformity between the Cambrian and the overlying Ordovician, Early Paleozoic metamorphism and extensive development of Early Paleozoic granites and their rapid exhumation, a Late Cambrian age is suggested for the DTS thrusting. Not only the direction of movement of the DTS is same as that of the Tertiary thrust sheets but also Cambrian folds are co-axial with the Tertiary folds. This strange coincidence shows that similar kinematic field existed during two tectonic events. A ridge, like the present Central Crystalline Axis, was elevated between the Tethyan and Lesser Himalayan basins, which contributed zircons of the Early Cambrian age to both basins.     

Late Ordovician evolution of the intracratonic basins in north-west Gondwana

The Palaeozoic intracratonic basins in northwest Gondwana, i.e. the Amazonas, Parnaiba and Acera basins, probably opened during late Caradoc and Ashgill times. The fluviatile sedimentation later changed to littoral at the basinal margins. A transgression from the north-west region of Gondwana slowly overlapped the margins of the intracratonic basins. The transgression reached its maximum in the Rawtheyan (late Ashgill), as evidenced by fossiliferous shallow marine sediments in the Amazonas Basin. The Hirnantian glaciation in north Gondwana lowered the sea level, and in the Amazonas Basin a littoral sedimentation followed on shallow marine strata. From the opening of the basins onwards, a shallow sea probably existed close to the epicontinental basins in north-west Gondwana. The basins were connected via a narrow passage between the Guayana and Ivorian cratons.     

Lithochemical typification of Early Precambrian ferruginous-siliceous formations in the East European Craton

The first comparative paleolithochemical characteristics of Early Precambrian ferruginous-siliceous formations of the East European Craton confined to four stratigraphic levels—Lower Archean, Upper Archean (Lopian), Lower Karelian, and Upper Karelian—are presented. Using the MINLITH method and software package for lithochemical calculations, the possible primary composition of metasedimentary rocks is reconstructed and paleogeographic settings of sedimentation are suggested. It is shown that different age formations represented initially lithogenetic groups with different compositions and quantitative relationships between the major types of sedimentary rocks with gradual transitions and genetic affinity. They accumulated in paleotectonic and facies settings that were specific for each stage of iron ore sedimentation, resulting in the development of four genetic (Bug, Algoma, Okolovo, and Lake Superior) types of ferruginous-siliceous formations.