Geologiya I Geofizika, 2002, V 43, N 8, August.

PETROLOGY, GEOCHEMISTRY, AND MINERALOGY 

EARLY PROTEROZOIC POSTCOLLISIONAL GRANITOIDS IN SOUTHWESTERN SIBERIAN CRATON
V. I. Levitskii, A. I. Mel'nikov, L. Z. Reznitskii, E. V. Bibikova, T. I. 
Kirnozova, I. K. Kozakov, V. A. Makarov, and Yu. V. Plotkina
717

The basement structures of the southwestern Siberian craton include the cis-
Sayan and Angara-Kan marginal uplifts that are parts of the Yenisei Ridge. The 
postcollisional granitoids abundant within the cis-Sayan uplift were earlier 
assigned to the Sayan and Shumikha complexes, but there was no unanimity in 
interpretation of their geodynamic environments. Geochemical studies of the 
Sayan granitoids at the junction between the Biryusa block and the Urik-Iya 
graben fill and the Shumikha granitoids at the junction of the Onot greenstone 
belt and the Sharyzhalgai block of mafic gneisses have confirmed the 
postcollisional nature of both complexes. The Shumikha granitoids share some 
features of A-type granites, including those typical of the Primorsky rapakivi-
like complex in the western Baikal region. The Sayan and Shumikha granitoids
 were dated by the U-Pb zircon age method as 1858+-20 and 1871+-17 Ma, 
respectively. Interpreted in the context of 1.8-1.9 Ga granite magmatism on the 
craton periphery, the intrusion of the two complexes most likely postdated the 
collision responsible for the formation of an Early Proterozoic supercontinent 
between 2.0 and 1.9 Ga. Together with the Primorsky and Taraka complexes and the 
Akitkan volcanoplutonic belt, the Sayan and Shumikha intrusions record rather
the onset of postcollisional extension which failed to cause continent break-up 
and oceanic crust production than the formation of the Proterozoic supercontinent.
Granitoids, geochronology, zircon, major-element chemistry, trace-element 
chemistry, basement of Precambrian craton, Early Proterozoic, supercontinent 


STRUCTURAL CONTROL OF LOCATION AND WATER CHEMISTRY OF LAKES AND SPRINGS IN 
THE OLKHON REGION
O. A. Sklyarova, E. V. Sklyarov, and V. S. Fedorovskii 
732
 
The location and water chemistry in lakes and springs in the Olkhon region are 
structurally controlled by Cenozoic faults of two types: i) inherited NE faults 
that follow the Early Paleozoic syncollisional (late stage) dextral strike-slip 
faults; ii) NEN pull-apart faults produced by sinistral strike-slip movements at 
the early stage of Baikal rifting.Water in springs has a generally low salinity 
(0.2-0.5 g/l) and compositions with invariable cations (Ca-Mg) and variable 
anions (from HCO3 to SO4-HCO3). The lakes we studied are fresh-water (up to 1 
g/l), brackish (1-10 g/l), and saline (14-45 g/l). Fresh-water lakes have mostly
 Mg-Ca-Na or Mg-Na-Ca bicarbonate chemistry, the composition of brackish lakes 
is either HCO3-SO4 Na-Mg (Mg-Na) or SO4-HCO3-Cl, Na-Mg (Mg-Na), and that of 
saline lakes is dominated by SO4, Cl, Mg, and Na. The composition of limnic 
water is controlled by the chemistry of feeder groundwater and by the grade of 
water metamorphism. Springs and lakes of different salinities show regular 
association with the two types of faults: Low-mineralized springs and fresh-
water lakes are located within inherited faults, whereas most of brackish and 
saline lakes are attributed to pull aparts. There are two possible explanations 
for this regularity. (1) The water chemistry of springs and fresh-water lakes in 
inherited faults is closer to the meteoric water as the depth of their feeder 
channels is constrained by complex thrust and strike-slip geometry of older 
faults with shallow-dipping planes. Pull-apart faults associated with synrift 
sinistral strike-slip motions are discordant with pre-Cenozoic structures and 
drain deeper aquifers with compositions (Cl-Na) strongly different from meteoric 
water. Mixing of various groundwaters and their metamorphism may be responsible 
for the observed variations in total mineralization and chemistry of lake water. 
(2) The diversity of water compositions in lakes is rather controlled by 
metamorphism (most often cryogenic) of the feeder meteoric water, and pull-apart 
faults maintain long existence of relatively small lakes in which water 
evolution reaches a high grade.
Lakes, springs, tectonics, water composition, Olkhon region 


RELATIONSHIPS BETWEEN Ag AND Au IN Au-Ag NATURAL SYSTEM 
V. P. Samusikov
746
 
Native gold samples (Au-Ag system) in the range of fineness from 200 to 1000- 
received complex comparative study. It has been established that HNO3 etched 
samples with gold fineness of about 700- and less have a two-phase structure. 
Atomic absorption and microprobe analyses show that the phases are pure Au and 
Ag. Gold and silver particles are a few hundred nanometers in size. Distribution 
of particles is uniform, i.e., this is an ultradispersive statistically
 homogeneous mixture composed of native gold and native silver.
We have observed no obvious two-phase structure in the HNO3 etched samples with 
gold fineness in the range of 700 to 850- . The fineness of gold increases in 
the surface layer, and traces of silver are observed in the solution. At some 
points, the microprobe analysis of fineness (without etching) reveals a drastic 
change in fineness, which either increases or decreases relative to the average 
value (deviations up to 250-). We suggest that the samples of this group are 
similar to those from the previous group, but predominant gold prevents silver 
particles from contacts with HNO3. Therefore, the samples of this composition do 
not dissolve in HNO3.There is no evidence of phase heterogeneity in samples with 
a gold fineness of 850 to 1000- . We suppose that Au and Ag substitute 
isomorphically for each other in the crystal lattice and form a homogeneous 
solid solution. Possible reasons for the phase heterogeneity of native gold are 
discussed. We suggest that it is the result of microblock crystallization 
associated with high oversaturation of solutions.
Gold, silver, fineness, solid solution, phases 


TECTONICS 

NEOGEAN EVOLUTION OF THE YOUNG URAL-SIBERIAN PLATFORM 
V. S. Surkov
754

The Ural-Siberian young platform is reconsidered in terms of Yanshin's criteria 
as formed through the early and late Neogean evolution stages. The early Neogean 
stage (Riphean-Paleozoic, 1600-250 Ma) produced the northwestern segment of the 
Ural-Mongolia fold belt with new continental crust. At the late Neogean stage
(Mesozoic-Cenozoic, since 250 Ma), the young platform formed by amalgamation of 
the West Siberian and South Kara plates, the Urals, the Kazakhstan and Altai-
Sayan shields, and the Yenisei Ridge in response to Arctic-Atlantic rifting and 
orogeny in southern Asia. Young platforms differ in structural inheritance of 
their sedimentary cover from the basement and the existence of an intermediate 
Paleozoic structural stage.
Neogean, plate, shield, rifting, rift graben, tectonomagmatic cycle

STRATIGRAPHY 


STRATIGRAPHY, MICROFOSSILS, AND BIOFACIES IN THE REFERENCE SECTION OF 
CALLOVIAN AND UPPER JURASSIC FROM THE TYUMEN' SUPERDEEP WELL (West Siberia) 
B. L. Nikitenko, V. I. Il'ina, and L. A. Glinskikh
762

The Callovian and Upper Jurassic section in the Tyumen' superdeep well SD-6 
(West Siberia) drilled with almost continuous recovery of core (about 200 m) and 
characterized by rich assemblages of microfossils is a unique object for 
development and improvement of Jurassic regional biostratigraphic schemes of 
West Siberia based on microfossils. This paper presents results of 
micropaleontological and palynological research (foraminifers, 
microforaminifers, ostracods, dinoflagellate cysts, acritarchs, prasinophytes as 
well as spores and pollen grains of terrestrial plants) and bio- and 
lithostratigraphic features of the uppermost Middle and Upper Jurassic part of 
the reference section from the Tyumen' superdeep well. The Vasyugan, Georgievka, 
and Bazhenov Formations are described in detail, taking into consideration their
 micropaleontological and palynological characteristics. The strucural features 
of sand productive beds of <$Eroman J sub 2 sup 0> and J1 groups in the Vasyugan 
Formation and <$Eroman J sub 1 sup 0> group at the base of the Georgievka 
Formation are discussed. Comprehensive lithostratigraphic analysis of Callovian-
Upper Jurassic part of the section SD-6 and biostratigraphic data allow us to 
localize the boundaries of some formations, subformations, and beds in this well.
 The main regularities of stratigraphic and lateral distribution of Callovian 
and Upper Jurassic formations as well as their thickness in the Urengoi district 
and adjacent territories are revealed. Analysis of stratigraphic distribution of 
microfossil species allow a detailed zonal subdivision of reference section SD-
6, with this section being the most complete Jurassic succession in northern 
West Siberia. The Jurassic regional-scale biostratons were revealed in Siberia 
according to foraminifers (f-zones), some of them being proposed for West 
Siberia for the first time. Also for the first time, an almost continuous 
sequence of dinocyst-based biostratons (dinozones) for Callovian-Middle Volgian 
has been established in a single section. It may be the basis for the
 development of Jurassic zonal scale according to dinocysts for West Siberia.
Distinctive features of microbenthos communities and microphytoplankton 
associations reflecting biofacies have been studied. The main regularities of 
distribution of microbenthic communities and microphytoplankton associations 
depending on changes of transgressive-regressive events and paleoenvironments in 
the Callovian-Late Jurassic have been established.
Callovian, Upper Jurassic, stratigraphy, biofacies, community, foraminifers, 
microforaminifers, dinocysts, zonal scales, eustatic, Tyumen' superdeep well 
(SD-6), West Siberia 


GEOPHYSICS 


TERRESTRIAL HEAT FLOW IN ZONES OF ACTIVE FAULTS IN SOUTHERN EAST SIBERIA
S. V. Lysak 
791
 
Active faults and faults reactivated in Cenozoic or, less often, Mesozoic time 
are considered in relation to regional heat flows in the Baikal Rift and its 
surroundings. Abnormal heat flow and manifestations of thermal activity (hot 
springs, Cenozoic volcanoes and flood basalts) are most often associated with 
faults. Thermal activity of faults, controlled by faulting mechanism and by the
 interplay of conductive and convective transport of deep heat, is the highest 
in strike-slip and normal faults and the lowest in thrusts. Geothermal 
parameters as indicators of regional tectonothermal activity can thus serve as a 
proxy in tectonic implications.
Faults, normal faults, strike-slip faults, thrusts, heat flow, hot springs, heat 
flow anomalies, Baikal Rift Zone, microplates


RECENSIONS

Recension of the book “Physics of the Earth” by K. F. Tyapkin
Alekseev A. S. and Aksenov V. V.
304

Recension of the book “Tectonics, geodynamics, and metallogeny of the 
Sakha Republic (Yakutia)” 
304
 Belichenko V. G., Bukharov A. A., Mel'nikov A. I., and Ufimtsev G. F