Home – Home – Jornals – Russian Geology and Geophysics 2001 number 7

A new Cambrian section has been exposed by Lemok BH-1, drilled on the left bank of the Yenisei River downstream from the mouth of the Angara River. Its lower part is saliferous, and the upper one is made up of variegated deposits. They are similar to the rocks of the Usol'e and Evenk Formations (Siberian Platform), respectively. These parts of the section are separated by a sequence of sulfate-carbonate and carbonate deposits containing shell debris and fragments of algal colonies. Trilobites Binodaspis cf. paula Suvorova and Bonnaria sp. as well as olenellids have been found in the depth range of 2804.4--2805.9 m. They indicate the Botomian age of the host rock.
We performed correlation of the sections of deep boreholes drilled on the left bank of the Yenisei River: Tyiskaya BH-1, Lemok BH-1, Eloguiskaya reference borehole, and Malokhetskaya BH-11. The correlation provides a basis for an independent scheme of Cambrian stratigraphy for the region under consideration. The Lower Cambrian includes four formations: Usol'e, Tyya, Averina, and Antsiferovka. The last three formations are proposed by us. They correspond to the Atdabanian, Botomian, and Toyonian, respectively. The Elogui Formation was formerly proposed for the lower Middle Cambrian (Amgaian). It is overlain by the Evenk Formation.
We have demonstrated that in the Early and early Middle Cambrian, the left bank of the Yeniser River was occupied by a vast carbonate platform (barrier reef). It separated the salt deposition basin of the Siberian Platform (continent) in the east from the oceanic water area open to the west. Volcanic rocks typical of back-arc basins have been found close to the margin of the Siberian continent in the Vezdekhodnaya area.

The periphery of the Siberian craton stores a record of the ancient history of our planet, of which the most important events are consolidation of continental crust, disintegration of continents, and opening of oceans. The southwestern surroundings of the craton include the Early Precambrian blocks of the Yenisei Range (in the center) and the northern blocks of East Sayan composed of fold and thrust belts or marginal basement uplifts.
The South Yenisei Range (Angara-Kan block) involves the greatest portion of the metamorphic and igneous complexes of the southwestern craton surroundings. The region underwent a complex tectonic evolution, as evidenced by a close spatial association of various lithological units and high metamorphic grades of rocks.
Collisions are the best evident events of the history of the South Yenisei Range, which are responsible for high-grade metamorphism of high-pressure granulite to amphibolite facies, crustal thickening, and large-scale emplacement of the Tarak granitoids. The orogeny gave way to rifting of the thickened crust associated with formation of metamorphic core complexes. As a result, several blocks of high-grade rocks were exposed by crustal extension and brought close to less metamorphic blocks. The removal of pressure was accompanied by magmatic underplating in the lower crust, which induced rheomorphic melting and homogenization of metamorphic complexes with a granite-like lithology and formation of porphyroblastic granitoid plutons.

We report generalized paleomagnetic data on the Vendian-Cambrian island-arc complexes of the southwestern framing of the Siberian Platform. They allow restoration of the arrangement of fragments of paleoisland arcs in the modern tectonic collage of the Altai-Sayan folded area (ASFA) at two stages: Vendian-Cambrian and Middle-Late Cambrian. Analysis of these data allowed us to establish the major periods of tectonic evolution of the Siberian marginal structure from the time of formation of the island arcs to the stage of collision and attachment of terrains to the Siberian continent.
Unlike the existing reconstructions of the structure and development of the ASFA, this reconstruction is based on new paleomagnetic data. It agrees with the available paleobiogeographic general conclusions and all geological data with minimum assumptions in interpreting the history of the geodynamic development of the region.
The constructed model confirms the earlier conclusions about the leading role of strike-slip faults in the formation of the Central Asian structure.

A similarity-based physical model of faulting in zones of lithospheric extension simulated formation of fault systems in extended viscoelastic ductile material and their gradual coalescence into a single major suture. This process includes several stages that are marked by specific structural reorganization in fault systems and recorded in variations of their parameters (density, length, fractal dimensions, etc.). Structural reorganizations are evident in plots showing variations in fractal dimensions of fault systems, longest suture lengths, and fault density associated with deformation increase. The fractal dimensions of fault systems correlate with deformation, and their contrasting changes correspond to structural reorganizations. In nature, the fractal dimensions of fault systems can be easily estimated from the known hierarchy of faults, which allows further estimation of the relative degree of deformation and the structural evolution stage of major fault zones.

In the northwestern Sayan region, within the Kan block - a fragment of the granite-greenstone province on the southwestern margin of the Siberian craton, - two greenstone belts, Idar and Kan, have been revealed. Study of the geologic sections and composition of the strata of the Kan greenstone belt (GSB) and reconstruction of the protoliths of metamorphic rocks by the wide spectrum of their trace, radioactive, and rare-earth elements have shown that rocks of tholeiite-basalt and andesite-dacite-rhyodacite associations are predominant in the metasedimentary-volcanogenic section. The overlapping metaterrigenous sediments include graywackes and aluminous pelites. The revealed types of rock associations and specific features of the trace-element composition of metasedimentary-volcanogenic rocks are typical of ancient greenstone belts. The complexes of the Kan GSB might have formed in the setting of an ensialic island arc, whose evolution was hindered by a zone of back-arc spreading. Results of the first isotope U-Pb and Ar-Ar dating of minerals show that the volcanic rocks of the Kan GSB accumulated in the Early Proterozoic and underwent intense metamorphism in the Early Cambrian. The trondjemites of the Upper Kan Massif also formed in the Early Cambrian.

We present original precision data on the chemical and trace-element (Li, Rb, Cs, Ba, Sr, Pb, Zn, Sn, Co, Ni, Cr, V, Cu, Sc, Mo, W, Nb, Zr, Hf, Th, U, REE, and Y) compositions of igneous rocks of the Tundak sill (Northern Transbaikalia) - picroteshenites, meso- and leucoteshenites, and analcime syenites. These rocks are shown to have formed through intrachamber crystallization differentiation of the same teshenite magma within the sill. The K-Ar age of the teshenites is 118.83 3.56 Ma.
The Sr isotope characteristics of the sill rocks and their hosted minerals disagree with the isochron model. We have established that the dispersion of Sr isotope compositions is caused mainly by postmagmatic alterations of these rocks rather than by the isotopic heterogeneity of the initial melt. The (87Sr/86Sr)₀ value of this melt (0.7053-0.7056) does not rule out the presence of admixed assimilated crustal matter in it.
Geological and geochemical analyses showed a great similarity of the North Transbaikalian and Late Cretaceous West Transbaikalian teshenites. We have also established that the study teshenites are close in age and composition to trachybasalts that effused in both regions at the earlier stages of the Late Mesozoic taphrogenic volcanism. We propose a genetic model for the formation of basaltoid melts of both types (teshenite and trachybasaltic) from the same mantle source. The model implies that the compositional differences between these types are related to the long evolution of parental magmas and admits mixing of deeply differentiated and primitive melts in intermediate magma chambers.

Three different genetic types of deposits have been analyzed to reveal similarities and differences of Cu-Ni sulfide mineralization. Comparison is performed by the example of the deposits related to the Archean komatiites of Western Australia, Phanerozoic (P-T) komatiites of Western Vietnam, and Phanerozoic (P-T) komatiite-like melts of the Norilsk district of the Siberian Platform. Comparison is made according to a number of indications and parameters of hugh genetic importance such as: geodynamic and geologotectonic setting, relation of the deposits to high-Mg melts, Ni/Cu propotions in ores and character of sulfide mineralization, facies of ore-bearing melts, specific conditions of deposition, etc. Taking into account literature and our own data, we suppose that the sulfide melt in the Archean komatiites is effused nearly simultaneously with their lavas at a small thickness of the Earth's crust, whereas the deposits in the Phanerozoic komatiite-like hypabyssal Norilsk ore-bearing intrusions are evidently dominated by sulfurization-metasomatic genesis. Unlike these, the Phanerozoic Ban Phuc deposit, related to a komatiite-basalt volcanoplutonic complex in northwestern Vietnam, exhibits an intermediate variant of genesis: It includes the appearance of massive veined ores at the magmatic stage of formation of this deposit, whereas the streaky and vein-disseminated ores seemed to appear during sulfurization -- at fluid-magmatic and hydrothermal stages. The close genesis of the considered types of deposits of Pt-Cu-Ni ore formation is mainly due to a deep-seated source of sulfur.

Skarn and exsolution spinels from the contacts of intrusions of a trap complex with carbonate--salt-bearing deposits of the Siberian Platform cover have been studied using scanning and translucent electron microscopy and electron microprobe. Spinel at dolerite-dolomite contacts occurs within exoskarn zones of magnesian skarns formed at the magmatic stage and in the endoskarn zone of metasomatic column formed at the postmagmatic stage. The compositions of skarn spinels deviate from the stoichiometric ones. These spinels are solid solutions of the system MgAl₂O₄-FeAl₂O₄-MgFe₂O₄-FeFe₂O₄ with negligible concentrations of manganese, zinc, and titanium. In calciphyre, spinels are represented by the series MgAl₂O₄-MgFe₂O₄-FeFe₂O₄; in the spinel-forsterite zone, by MgAl₂O₄-FeAl₂O₄-FeFe₂O₄; and in the spinel-fassaite zone, by MgAl₂O₄-FeAl₂O₄. Among exsolution spinels there are four groups differing in composition, form, and time and mechanism of crystallization. Under exsolution of magnetite solid solution, first nonstoichiometric spinels of the series MgAl₂O₄-FeAl₂O₄-MgFe₂O₄-ZnFe₂O₄ formed by the mechanism of heterogeneous nucleation, which disintegrated into pleonaste, magnesioferrite, and franklinite. Then lamellae of magnesian-ferruginous Al-spinel and hercynite were successively generated by the mechanism of homogeneous nucleation. Spinel was a source of alumina for phlogopite, clinochlore, hydrotalcite, and other minerals of postskarn associations.

Study of solid natural hydrocarbons is of great theoretical and practical importance. On the one hand, their industrial accumulations may be considered a nontraditional raw-material source for chemical industry. On the other hand, they permit us to solve problems on formation of by-products of naftidogenesis. In this paper we describe hard bitumens ("pyrobitumens") localized within dolerite sills and report evidence of the destructive-sublimation character of their formation simultaneous with sill genesis in the Minusa intermontane trough.

The process of formation of colloidal and metacolloidal minerals, which can give rise to monodisperse systems, is considered in the context of supramolecular crystallization. We present data on synthesis of monodisperse particles: oxides, hydroxides, sulfides, etc. The general regularities of supramolecular crystallization under experimental conditions and in nature are analyzed.

The paper presents an attempt to describe water exchange in Lake Baikal proceeding from interpretation of tracer (freon-12) contents and temperature of water. Disturbance of the metastable equilibrium of the upper water layer in the cold season at reverse stratification is suggested as a controlling mechanism of deep-water renewal. A system of one-dimensional equations of substance (for the tracer) and heat budget is proposed. The solution to an inverse problem for turbulence coefficients and water exchange flows shows a considerable role of turbulence in the upper water column. A function is proposed to describe the depth-dependent distribution of downwelling flows in the upper layer of water.

Instant variations in velocity and attenuation of seismic waves in a friable medium subjected to dynamic loading have been studied by new experimental techniques using a powerful seismic vibrator. The half-space below the operating vibrator baseplate was scanned by high-frequency elastic waves, and the recorded fluctuations were exposed to a stroboscopic analysis. The pulses of seismic velocities and attenuation are synchronous with the external load pulses but are out of phase with the latter and with each other. Instant variations in the studied parameters depend on the magnitude and absolute rate of deformation, which always reduces elastic-wave velocities. Some weak seismic boundaries have shown a high sensitivity to the presence of dynamic disturbance in the medium.