GEOLOGY & GEOPHYSICS, 2001, V 42, N 8.

PETROLOGY, GEOCHEMISTRY, AND MINERALOGY

Major epochs of intrusive magmatism of Kuznetsk Alatau, Altai, and Kalba (from U-Pb isotope dates). 
Vladimirov A.G., Kozlov M. S., Shokal'skii S. P., Khalilov V. A., Rudnev S. N., 
Kruk N. N., Vystavnoi S. A., Borisov S. M., Berezikov Yu. K., Metsner A. N.,
Babin G. A., Mamlin A. N., Murzin O. M., Nazarov G. V., and Makarov V. A. 
1149
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-C1 -
accretion-subductional, related to the evolution of sea-marginal-island-arc 
systems of the Paleo-Asian ocean; C2-O1-accretion-collisional; D1-D31 and
D32-C1t - accretion-subductional, related to the evolution of active continental 
margin of the Andean type; C1v-C3 - accretion-collisional, related to the 
oblique collision of the Dzhungar microplate with the Siberian continent and 
closure of the Ob'-Zaisan paleoocean basin; P1-T2 -postcollisional, realized in 
the strike-slip fault-disjoining regime; and T3-J1 - within-plate
magmatism.Geochronology, U-Pb and Rb-Sr isotope dating, granitoid
magmatism, Kuznetsk Alatau, Gorny Altai, Rudny Altai, Kalba

Late Vendian age of the Khan-Taishiri ophiolite complex in western Mongolia.
Gibsher A.S., Khain E. V., Kotov A. B., Sal'nikova E. B., Kozakov I. K.,
Kovach V. P., Yakovleva S. Z., and Fedoseenko A. M.
1171 
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. Late
Vendian, U-Pb method, ophiolites, continental margin, island-arc system,
Khan-Taishiri Ridge, western Mongolia, Central Asia.

Formation of igneous complexes of the Salair pyrite-polymetallic ore field 
(West Siberia).
Lapukhov A. S., Simonov V. A., and Kovyazin S. V. 
1178
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-C) 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. Salair ore field, igneous crystals,
melt inclusions.

Tvolution of the compositions of ore minerals in high-potassium basalts and
associated rocks of Permo-Triassic volcanogenic series of the
Maimecha-Kotui province (northern Siberian platform).
Zolotukhin V. V., Vasiliev Yu. R., and Tomshin M. D. 
1188
The ranges of temperatures and -lgPO2 equilibrium for the pairs of coexisting
titanomagnetite and ilmenite in high-potassium basalts and associated
subalkalic basic rocks of the Maimecha-Kotui province were estimated using
an improved ilmenite-magnetite geothermometer - 700-1180 oC and 10<+>20,
respectively. Their highest values, approaching magmatic ones, have been
determined for olivine tholeiitic basalts (T = 1000-1320 -C, -lgPO2 =
8.2-12.6). These parameters for potassic basalts and subalkaline basalts
suggest they formed under the magmatic to postmagmatic conditions, the
latter being evidently predominant. In postmagmatic fluids, of major
importance were, possibly, increased activity of alkalies and, hence, O2
in the evolution of compositions of coexisting ore mineral phases
discussed in this work. High-potassium basalts, coexisting ilmenites and
titanomagnetites, T-PO2 conditions, Maimecha-Kotui province, Siberian
Platform

Metamorphic evolution of high-alumina metapelites near the Panimba 
overthrust (Yenisei Range): mineral associations, PT-conditions, and 
tectonic model.
Likhanov I. I., Polyanskii O. P., Reverdatto V. V., Kozlov P. S.,
Vershinin A. E.,  Krebs M., and Memmi I. 
1197 
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. High-alumina metapelites,
thermobarometry, PT-paths, tectonic model, Yenisei Range. 

Garnet ultramafites and associated metabasic rocks of the Biryusa block  
(southern Siberia). 
Kolotilina T. B. and Mekhonoshin A. S. 
1213
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-C. The subsequent high-pressure metamorphism at
800-900-C and 16-20 kbar produced garnet parageneses in the most ferruginous 
derivates of ultrabasic magma and in basic rocks. Garnet ultramafites, 
lherzolites, metabasic rocks,high-pressure metamorphism.

Dynamics of melting in the oceanic upper mantle.  
Perepechko Yu. V. and Sharapov V. N.
1229
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<$Etimes>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 -C at a depth of 100 km,
800 -C at 300 km, and 1700 -C at the upper-lower mantle boundary. At a 
temperature at the lower boundary below 2000 -C, 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 -C, 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. Upper
mantle, convection, melting.

Mass transfer between phases in a surfactant-containing magma.
Dorovskii V. N.
1241 
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.Extraction, magmatic melts, surfactants, hydrodynamics.

TECTONICS

Tectonophysics of the Angara fault zone (southern Siberian Platform).  
Seminskii K. Zh., Gladkov A. S., and Lunina O. V. 
1252
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. Faults, joints, blocks, tectonic activity, Baikal Rift,
Siberian Platform, tectonophysical analysis.

GEOPHYSICS

The effect of storm microseisms on seismicity in the shore zone of Lake 
Baikal.
Tabulevich V. N., Drennova N. N., Potapov V. A., and Chernykh E. N. 
1263
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. Seismicity, microseisms, Lake Baikal.

Numerical solution to a forward and an inverse problems in pulse 
electromagnetic wall-thickness measurements in boreholes.
Potapov A. P. and Kneller L. E.
1271 
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.
Forward and inverse problems, pulse EM wall-thickness measurements.

DISCUSSION

The nature of air nonpotential part of the Earth's electromagnetic field. 
Aksenov V. V. 
1277 
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. Earth's magnetic field, electric currents, flows 
in the core.

On improvement of classification of oil and gas resources. 
Voronin Yu. A. 
1281
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. Resources, oil,
gas, classification, the theory of classification, assessment.