Home – Home – Jornals – Russian Geology and Geophysics 2001 number 8

We present new U-Pb isotope dates for zircons from intrusive rocks of Kuznetsk Alatau, Gorny Altai, Rudny Altai, and Kalba and generalize the earlier published results of geochronological studies in these regions (U-Pb and Rb-Sr dates). Comparison of radiological ages with geological data permitted us to refine the scale of intrusive-magmatism events and recognize seven major epochs corresponding to a successive change of geodynamic settings: V

The age of ophiolite plagiogranites from the Khan-Taishiri ophiolite complex, western Mongolia, was determined by the U-Pb zircon method as 568.1 4.0 Ma. This complex is a rare fragment of the ancient Central Asian blocks where real geological relationships within large tectonic slabs making up the ridge have been established between all ophiolite sequences but ultrabasic rocks: pyroxenite and gabbro from banded sequence, basic rocks of dike complex, dolerite sills, basic pillow lava, and deep-water clay-siliceous sediments.
The Neoproterozoic age of the Khan-Taishiri ophiolite complex suggests the presence of a Vendian island-arc system within the Central Asian Mobile Belt. The model for this system was described for the first time by L. P. Zonenshain and O. Tomurtogoo on geologic objects of western Mongolia from Dzavhan River to northern slopes of the Mongolian Altai Ridge (Dzavhan, Gobi-ALtai, Khan-Taishiri, Naran, Ulan-Shandian, and Kuroi subzones). This system obviously included the Daribi ophiolites of western Mongolia; the Agardag and Shishkhid ophiolites of Tuva; the Dzhida and Ol'khon fragments of island-arc system; the belt of subductional layered massifs of Northern Baikal and eastern frame of the Muya Block; the Naran, Eastern Sangilen, Southern Khamar-Daban, OL'lhon, and Bodaibo fragments of sedimentary basin; the Dzavhan, Hovsgol-Bokson, Western Baikal, and Baikal-Patom shelf fragments. Ocean lithosphere was represented by Bayan-Khongor ophiolites.

Study of melt inclusions in quartz phenocrysts from quartz porphyries has clarified the composition of acid magmas of the Salair ore field. We have established that low-alkali rhyodacite melts of tholeiitic series actively participated in the formation of igneous complexes hosting barite-polymetallic deposits. The high temperatures of acid melts (1050 - 1165 ^oC) evidence their relation with basaltic igneous systems. The Salair rhyodacite melts are shown to be geochemically similar to acid magmas of pyrite deposits in Tuva and the Southern Urals. A comparative analysis of the bulk chemical compositions of quartz porphyrites and melt inclusions in quartz phenocrysts has revealed that low-alkali rhyolites and rhyodacites of tholeiitic series are significantly metamorphosed to high-alkali andesite-dacites enriched in Na and Mg and depleted in Ca, with SiO₂, TiO₂, and Al₂O₃ being their "inert components". A petrochemical analysis of rocks and melt inclusions has shown that some of the igneous complexes of the Salair ore field might have formed in the setting of a developing back-arc basin.

The ranges of temperatures and

An attempt is made to study pressure-heterogeneous high-alumina metapelites of andalusite-sillimanite and kyanite-sillimanite facies series for clearing up the PT-evolution of metamorphism related to thrusting tectonics. As a result of detailed research with the use of geothermobarometry and THERMOCALC-based analysis of mineral equilibria, PT-conditions were estimated and PT-paths of evolution were plotted for moderate and elevated pressures. The obtained data on the change of PT-conditions of metamorphism suggest that the kyanite substitution for andalusite and other mineral transformations were caused by an increase in pressure under low geothermal gradient, described by a model for tectonic thickening of the crust in the zone of the Panimba overthrust. The absence of a marked increase in temperature on thrusting is explained by specific behavior of stationary geotherms for different kinds of rocks that have different thermophysical and heat-producing properties.

A detailed petrologo-geochemical study of basic and ultrabasic rocks of the Biryusa block has shown that they resulted from metamorphism of weakly differentiated rocks of lhersolite-olivine websterite series and tholeiitic basalts. The temperatures of formation of igneous pyroxenes of ultrabasic rocks are estimated at 1100–1200 ^oC. The subsequent high-pressure metamorphism at 800– ^oC and 16-20 kbar produced garnet parageneses in the most ferruginous derivates of ultrabasic magma and in basic rocks.

Convection in the upper mantle has been investigated in the Boussinesq approximation with due regard to four solid-phase transitions above the "perovskite" transition at a depth of 670-700 km for systems of two types: (1) without basaltic crust (some slowly spreading ridges) and (2) with basaltic crust 7 to 20 km thick. Numerical calculations have been carried out on the basis of the method of control volume for a 2D region, 700 2500-5000 km² in size. Development of melting and its extent were estimated on the basis of parameterization of water-free lherzolite according to McKenzie (1984) and Niu and Batiza (1991). Mantle rocks melt in systems with initial temperatures exceeding 500 ^oC at a depth of 100 km, 800 ^oC at 300 km, and 1700 ^oC at the upper-lower mantle boundary. At a temperature at the lower boundary below 2000 ^oC, melting proceeds at depths of 24-290 km with the degrees of melting of ~0.05-0.8 15-20 Ma after the beginning of convection and exists for about 20-70 Ma. At a temperature higher than 2000 ^oC, the magmatic system passes through two stages: nonstationary, with the degree of melting of about 0.5-0.6 at depths of ~30-290 km, and quasi-stationary, with the degree of melting of 0.06-0.4 at depths of ~60-100 km. The horizontal extent of melting zones varies from 150 to 1000 km. A periodical structure forms in the melting zone, and the maximum of melting degree considerably shifts toward the upper boundary of the upper mantle. The velocity of mantle matter near the upper boundary reaches ~14-24 cm/year before melting, then gradually decreases to 3-9 cm/year by the climax of the magmatic system and to 1.5-2 cm/year by its degeneration. There is no linear relationship between the degree of melting and velocity of the Earth's crust over ascending flows. The thermal flow near the surface of the Earth's crust reaches ~60-145 mW/m² at maximum melting and decreases to ~40-60 mW/m² on degeneration of the magmatic system.

The thermodynamics of admixture extraction in magmatic melts with the presence of surfactants is considered. A theory for hydrodynamic transfer of the admixtures in a surfactant field is developed. It is demonstrated that admixtures move to zones of increased surfactant concentrations. Equations describing the self-conforming particle motion and surfactant diffusion have been deduced. It is suggested that the extraction process is unstable, which results in a dramatic reduction in the characteristic diffusion times of extraction.

The Angara fault zone that extends from the Baikal Rift into the Siberian Platform through the most densely populated regions of Siberia has been investigated using geological and structural data. A database including 243 sites of mass measurements of faults and joints has been compiled for the region of the Irkutsk reservoir and the city of Irkutsk. A tectonophysical analysis showed that the Angara fault is expressed on the surface as a broad zone in which the sedimentary cover is cut into triangular, quadrangular, and pentagonal plate-shaped fault blocks. The shallow crust structure in the region formed in three major stages of tectonic activity of various styles in response to deformation in the neighboring Sayan-Baikal mobile belt. During the latest stage of activity, intense motions occurred on the Angara fault within the rift as well as along its subsidiary fractures on the craton (region of Irkutsk). Therefore, the existing construction engineering standards call for reassessment.

The effect of vibrations (storm microseisms) in Lake Baikal on seismicity in its shore zone is investigated by comparing the number of earthquakes that occur in winter in the absence of waves and vibration to that during the time of strongest storms. Observations over ten recent years show that the number of small and medium earthquakes (6 K 13) in "quiet" time is nearly twice as high as that during storms, which indicates the influence of storm vibrations on the seismicity. The observed vibrations have been analyzed in terms of their spectra, attenuation, amplitude, and radius of influence.

A solution to a forward and an inverse problems for the transient response of coaxial cylindrical media (multistring constructions in boreholes) is based on the spectral approach using the Fourier integral. The inverse problem is solved by calculus of variations. The results of mathematical modeling of the transient process provided a basis for interpretation of real log data.

The paper presents an analysis of the nonpotential part of the Earth's electromagnetic field to account for a number of contradictions in the Earth's electromagnetic field theory.

Numerous attempts to improve classification of oil and gas resources made for the last 40 years have ignored the theoretical grounds. This fact has no excuse and brings about adverse results. Analysis of some of these results is attempted, and proposals for joint geological and mathematical studies on the basic problems of petroleum geology are put forward.