GEOLOGY & GEOPHYSICS, 2001, V 42, N 7, July.
STRATIGRAPHY
Stratigraphic scheme of the Cambrian deposits in the Yenisei region (West
Siberia)
Yolkin E.A., Kashtanov V.A., Kontorovich A.E., Korovnikov I.V., Krinin V.A.,
Luchinina V.A., Saraev S.V., Tishchenko V.M., Filippov Yu.F., and Khomenko A.V.
1015
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. Stratigraphy, paleogeographic settings, Cambrian, West
Siberia
TECTONICS
A tectonic model of the Early Precambrian evolution of the South Yenisei Range
Popov N. V.
1028
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 etamorphic
And gneous 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. Lithology, magmatism, metamorphism, geodynamic model, surroundings
of the Siberian craton, Yenisei Range
Tectonic evolution of the southwestern framing of the Siberian Platform in
the Vendian-Cambrian according to paleomagnetic data.
Kungurtsev L. V., Berzin N. A., Kazanskii A. Yu., and Metelkin D. V.
1042
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. Paleoisland-arc system, strike-slip faults, paleomagnetic pole,
Altai-Sayan folded area, Siberian continent
Modeling of large faults in zones of lithospheric extension and numerical
constraints on deformation
Sherman S. I., Cheremnykh A. V., Bornyakov S. A., and Shishkina L. P.
1052
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. Ruptures, faults, fractures,
parameters, physical modeling, fractal dimension, extension,
deformation, structural changes
PETROLOGY, GEOCHEMISTRY, AND MINERALOGY
Structure, composition, and formation conditions of metasedimentary-
volcanogenic complexes of the Kan greenstone belt (northwestern Sayan region)
Nozhkin A. D., Turkina O. M., Bibikova E. V., and Ponomarchuk V. A.
1058
In the northwestern Sayan region, within the Kan block <197> a fragment of the
granite-greenstone province on the southwestern margin of the Siberian craton,
<197> 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. Greenstone belt, amphibolites, gneisses,
geochemistry, petrology, protoliths, geodynamics, East Sayan
Composition and age of teshenite magmatism of Northern Transbaikalia
Kazimirovskii M. E., Stupak F. M., and Dril' S. I.
1077
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. Continental rifting,
teshenites, trachybasalts, geochemistry, geochronology, Sr isotopy, Transbaikalia
Genetic aspect of Cu-Ni sulfide mineralization during komatiite and related magmatism
Zolotukhin V. V., Polyakov G. V., Polyakov P. A., and Glotov A. I.
1088
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.
Komatiites, picrites, Cu-Ni sulfide ores, genesis, genetic types
Composition and features of crystallization and transformation of spinels
at the contacts of dolerites with dolomites and rock salt
Mazurov M. P. and Titov A. T.
1108
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. Spinel, magnesian skarn, metasomatism,
dolerite, dolomite, rock salt
Veined pyrobitumen in dolerites of the Kuzmensky complex, the Minusa intermontane trough
Fedoseev G. S., Fadeeva V. P., and Melenevskii V. N.
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. Pyrobitumens, IR spectra, pyrolysis, dolerite sills, Minusa intermontane
trough
Supramolecular crystallization in mineral formation processes
Kalinin D. V. and Serdobintseva V. V.
1116
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.
Monodisperse systems, supramolecular crystallization, colloidal minerals
GEOPHYSICS
Deep-water renewal in Lake Baikal.
Kodenev G. G.
1125
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. Chemical tracers, renewal time, heat and mass
exchange
An experimental study of instant variations in velocity and attenuation of
seismic waves in a friable medium in situ under a pulse dynamic load
Geza N. I., Egorov G. V., Mkrtumyan Yu. V., and Yushin V. I.
1135
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.
Pulse transmission, vibration, ground, nonlinear medium, dynamic nonlinearity,
instant variations, seismic velocity, attenuation