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