Geologiya I Geofizika, 2002, V 43, N 4, April.

STRATIGRAPHY 

BAIKALIAN IN SIBERIA
V. V. Khomentovskii
313

      The main stratigraphic datums of the Late Precambrian in Siberia are the 
Uchur-Yudoma, Baikal-Lena, and Yenisei-Angara structure-facies areas (SFA) of 
the periphery of the old Siberian Platform. Correlation of local stratigraphic 
subdivisions in structure-facies areas and zones of each of them has been 
refined. This allowed safe comparison of stratigraphic schemes of the above-
mentioned SFA, reasonable support to their common event framework, and following 
the Baikalid belt along the whole margin of the southern Siberian Platform. The 
boundaries of the main stratigraphic divisions of the Siberian Neoproterozoic 
such as Mayaian (Kerpylian + Lakhandinian), Baikalian, Yudomian, or Vendian 
define principal tectonic restructurings. These units are separated from the 
underlying formations by the event boundary having something in common with the 
Grenwill orogeny. The information obtained in Siberia (microfossils, 
geochronology) suggests the Upper Riphean age of the Mayaian.
The Baikalian unit (850-650 Ma) is separated from the enclosing stratigraphic 
subdivisions by the Lower and Upper Baikalian intense tectonic events. The 
Baikalian geotectonic stage and related Baikalids are meaningful only within its 
volume. All the mentioned recurrent events result from the interaction of the 
margin of the Siberian Craton with its surrounding microcontinents and oceanic 
crust or are related to motion of lithosphere plates. The boundaries dividing 
the Baikalian and Vendian units into two parts have also an event base.
Though the Baikalian unit, like other subdivisions of the General Scale of the 
Late Precambrian (Riphean and Vendian), was initially substantiated 
tectonically, now it is characterized by quite a representative set of 
geochronological and paleontological signatures. Of great importance is that it 
contains glacial deposits, which is in agreement with the general cooling 
reflected in the name Cryogenian, the Baikalian-coeval subdivision of the 
International Precambrian Scale. The Baikalian is to be introduced into the 
General Stratigraphic Scale of the Russian Late Precambrian and into the 
chronostratigraphic version of the International Scale.
     Neoproterozoic, Baikalian, craton, oceanic crust, microcontinent, 
collision, taphrogeny, correlation, microfossils, geochronology, glaciation, 
International Precambrian Scale 

PETROLOGY, GEOCHEMISTRY, AND MINERALOGY 

BIOSTRATIGRAPHY OF THE UPPER VENDIAN, LOWER AND MIDDLE CAMBRIAN STRATA IN A 
SECTION ON THE YENISEI RIVER NEAR PLAKHINSKY ISLAND (NORTHWESTERN SIBERIAN 
PLATFORM) 
I. V. Korovnikov, S. M. Rovland, V. A. Luchinina, Yu. Ya. Shabanov,
and A. V. Fedoseev 
334
     A section is described, which includes the upper part of the Vendian, Lower 
Cambrian, and lower Middle Cambrian strata. The section is situated on the right 
bank of the Yenisei River about 5 km downstream from Plakhinsky Island. It has 
been established that the rocks in the section under study  differ 
signifficantly in lithology from the formations (Izluchina, Sukharikha, Krasny 
Porog, and Shumny) of the Igarka district typical of this age interval. It is 
confirmed that a break exists between the Early and Middle Cambrian, which 
covers the greatest part of the Botomian Age (except the lower part of the 
Calodiscus-Erbiella Zone) and extends to the Anopolenus henrici-Corynexochus 
perforatus Zone of the Middle Cambrian Mayaian Age. Abundant remains of the 
Cambrian fauna have been found and identified: small-shell organisms, 
trilobites, algae, brachiopods, and gastropods. For the first time this section 
has been characterized to a sufficient degree. The boundary between the Vendian 
and Cambrian strata has been drawn, and the entire range of deposits to the 
Upper Cambrian strata has been stratified.
     Stratigraphy, Vendian, Lower and Middle Cambrian, algae, trilobites, small-
shell fauna, northwestern Siberian Platform 

VEIN DIFFERENTIATION OF MANTLE MELTS: EXAMPLE OF PYROXENITE XENOLITHS 
IN VITIM PICRITE BASALTS 
I. V. Aschepkov and L. Andre
343

     Pyroxenite xenoliths in Miocene Vitim picrite basalts were produced by 
polybaric fractional crystallization of melts in the mantle, as determined from 
their morphology and major- and trace-element chemistry. Black coarse-grained 
pyroxenites and megacrysts make a long trend (F# = 0.12-0.23) as a result of 
polybaric cotectic Gar-Cpx fractionation, possibly, during initiation of the 
feeding system for the basaltic eruption. Low-temperature black pyroxenites are 
products of interaction of residual melts with the lower crust into which they 
intruded. The highest-temperature hybrid websterites may result from mixing of 
basaltic melts with 5-10% partial melts from metasomatic mantle lherzolites. 
Anatectic Cr-pyroxenites were formed mostly by segregation of partial melts from 
lherzolite material and remelting of former veins. Low-temperature hybrid garnet 
pyroxenites were produced by interaction of metasomatic lherzolites (or earlier 
Cr-diopside veins) and differentiated basaltic melt enriched with volatiles. 
Remelting of earlier mineral generations, which occurs in all rising melt 
systems, is reflected by mirror peaks on spidergrams, and LREE and HFSE (and ore 
element) enrichment.
     Megacryst, basalt, clinopyroxene, garnet, ilmenite, mantle, melt, 
fractional crystallization, partition coefficients, melting, assimilation, 
inductively coupled plasma

GEOCHEMISTRY AND METALLOGENY OF CARBONACEOUS DEPOSITS OF DIFFERENT GEODYNAMIC 
ENVIRONMENTS IN THE SAYAN-BAIKAL MOUNTAIN PROVINCE 
A. G. Mironov, S. M. Zhmodik, Yu. Ch. Ochirov, L. I. Gurskaya, V. D. Popov, 
and D. Yu. Sapozhnikov
364

     Carbonaceous shales from the North Baikal (Koter depression) and 
southeastern East Sayan (Ilchir and Oka depressions) regions of the Sayan-Baikal 
mountainous province are considered in terms of their geochemistry and 
metallogeny. The rocks formed in the Late Riphean-Early Paleozoic in different 
geodynamic environments of active continental margin (turbidite terrane), back-
arc spreading zone (oceanic terrane), or shelf (carbonate deposition). Several 
intervals of the section show noble-metal mineralization. The black shales 
associated with ophiolites typically demonstrate a high auriferous potential of 
sulfides, which are equivalents of sulfides from back-arc spreading zones. The 
carbonaceous shales of continental margins show platinum and palladium 
enrichment, their elevated concentrations being attributed to zones of high Ba, 
Mo, and U abundances. The isotope composition of sulfide sulfur varies from -10-
-17 - outside these zones to +3-+6.8 - within them.
     Carbonaceous shales, geodynamic environments, gold, platinum, palladium, 
geochemistry, metallogeny

GEOPHYSICS 

DETERMINATION OF ELASTIC PARAMETERS FROM MULTIWAVE AVO DATA 
T. V. Nefedkina and V. V. Buzlukov
382

     It is suggested to determine three independent elastic parameters of the 
medium (VP, VS, and ) by combined inversion of AVO data on compressional, 
converted, and shear waves recorded by three-component land seismic profiling. A 
linear solution to the problem is considered on the assumption of nearly 
constant elastic properties on a rigid plane boundary between two isotropic 
elastic half-spaces. The solution is obtained through an iterative algorithm 
that yields stable estimates of the model parameters. The stability and 
exactness of the solution has been tested by fitting and comparison of various 
wave combinations: (PP + PS), (PP + SH), and (PP + SV). The combination of (PP + 
SV) waves provides the optimum inversion on P (and S) waves, in the presence of 
up to 50% (100%) noise. The parameters determined from (PP + PS) and (PP + SH) 
waves are less accurate, as stable estimates can be obtained at lower noise (25 
or 40% of the signal) on P, S, and converted waves.
     AVO, inversion, multiwave seismic surveys, shear and converted waves, 
parameter estimates, iterative algorithm 

BRIEF COMMUNICATIONS 

STAGES OF CONTINENTAL CRUST FORMATION IN THE CENTRAL DZHUGDZHUR- STANOVOY 
FOLDED AREA (SM-ND ISOTOPE DATA ON GRANITOIDS) 
A. M. Larin, A. B. Kotov, V. P. Kovach, V. A. Glebovitskii, E. B. Sal'nikova, 
E. M. Zablotskii, and N. Yu. Zagornaya
395
     The paper presents preliminary data of Sm-Nd systematics in granitoids from 
the central Dzhugdzhur-Stanovoy folded area. The Nd ages range from 3.1 to 1.0 
Ga, <$E epsilon sub { roman Nd } ( T ) ><|>=<|>-21.0-+ 4.3. The obtained results 
reveal at least four stages of continental crust formation in the history of the 
region: a Late Archean stage (3.1-2.8 Ga), two Early Proterozoic stages (2.5-2.4 
and 2.0-1.9 Ga), and a Mesozoic stage (0.23-0.10 Ga).
     Isotope geology, crust formation, granitoids, Dzhugdzhur-Stanovoy folded area 

TRACES OF COPROPEL-LOVING ORGANISMS IN PHOSPHATIZED COPROLITE 
Yu. N. Zanin, V. A. Luchinina, and E. A. Zhegallo 
400

     Organic matter of the fossilized excrement is subject to biological 
decomposition and transformation. But the nature of the organisms engaged in 
this process in the past is usually unknown and can be established only by 
examination of microfossils in ancient coprolites. The data obtained during the 
study of Paleogene phosphate coprolites from the Zaisan Depression, Kazakhstan, 
suggest that fungi were involved in these processes.
     Phosphatic coprolite, copropel-loving organism, fungi, Paleogene, 
Kazakhstan