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Russian Geology and Geophysics

2005 year, number 3

1.
STAGES IN DEVELOPMENT OF MOLLUSKS, PALEOBIOGEOGRAPHY OF BOREAL SEAS IN THE EARLY-MIDDLE JURASSIC AND ZONAL SCALES OF SIBERIA

S.V. Meledina, B.N. Shurygin, and O.S. Dzyuba
Institute of Petroleum Geology, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibisrk, 630090, Russia
Keywords: Early Jurassic, Middle Jurassic, paleobiogeography, stratigraphy, ammonites, belemnites, bivalves, northern Asia
Pages: 237-254

Abstract >>
We compared stages of the taxonomic restructurings that occurred in communities of ammonites, belemnites, and bivalves in Siberian sea basins in the second half of the Early Jurassic and at the beginning of the Middle Jurassic. In general, the main stages are similar in different groups of mollusks, but the restructuring borders often do not coincide. Degree of endemism and portion of immigrants in communities of different groups differ as well. Abiotic factors that have an effect on the evolution and migration are addressed here. On the basis of areal differentiation of mollusk assemblages, three paleobiogeographic areas have been recognized since the Late Pliensbachian: circumpolar Arctic, western Boreal-Atlantic and eastern Boreal-Pacific Realms. These realms are united into the Panboreal Superrealm. Ammonite, belemnite, and bivalve zonal scales devised for the Siberian Jurassic reflect the specific evolution of each of these groups in seas of the Arctic Realm as well as specific variations in its communication with adjacent realms.



2.
BENCHMARKS FOR CORRELATION OF CARBONIFEROUS AND PERMIAN MARINE DEPOSITS IN SIBERIA AND FAR EAST

A.G. Klets
Institute of Petroleum Geology, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Stratigraphy, paleogeography, brachiopods, Ammonoidea, Carboniferous, Permian, marginal seas, Angarida
Pages: 255-271

Abstract >>
Data on stratigraphy of Carboniferous and Permian deposits formed in marginal seas of Angarida, which is the ancient Siberian continent covering the territory of Siberia and Far East, have been generalized. The biostratigraphic base of interregional correlation consists of: zonal faunal complexes of the key Verkhoyansk-Okhotsk region, nine key correlation intervals, and six event borders. The most important event levels were soles of transgression-regression cycles corresponding to mass extinction of biota, innovation, immigration of new communities, change of dominants, etc. The most drastic changes in paleogeography of marginal seas of Angarida and composition of faunal communities are related to the Tournaisian-Visean Shcheglov and Middle Bashkirian Tylakh events. Geodynamically oriented paleogeographic flow charts have been compiled for three time sections (Late Tournaisian, border of Early and Middle Carboniferous, beginning of the Early Permian).



3.
NEOTECTONIC DEFORMATION IN THE SOUTHWESTERN SIBERIAN CRATON

A.V. Arzhannikova and S.G. Arzhannikov
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Cenozoic tectonics, deformation, southwestern Siberian craton
Pages: 272-277

Abstract >>
New neotectonic data from the southwestern periphery of the Siberian craton reveal post-Miocene reverse thrusting in the zone of the Alar'-Belaya fault. Traces of activity were found in Late Pleistocene-Holocene sections in the lower reaches of the Belaya River and on the left bank of the Angara as seismogenic rupture in sediments produced by local and remote shocks.



4.
DYNAMICS OF MELTING OF MANTLE-DERIVED ROCKS ABOVE A HOT SPOT BENEATH OCEANIC RIDGES

V.N. Sharapov, L.N. Perepechko*, and I.F. Rakhmenkulova
Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Institute of Thermal Physics, Siberian Branch of the RAS,
1 prosp. Akad. Lavrent'eva, Novosibirsk, 630090, Russia
Keywords: Magmatic systems, asthenosphere, hot spot, decompression melting
Pages: 278-286

Abstract >>
During the formation of intraplate volcanic ridges the hot upper mantle controls the decompression melting of ascending low-density upper-mantle material above hot spots. Depending on the degree to which the upper mantle is warmed up, the extent and dynamics of partial melting as well as the dimensions of the asthenosphere can considerably differ: (1) The time of existence of a zone of partial melting varies from 15 to 90 Myr; (2) the maximum degree of partial melting may differ by a factor of 5-10 (from 5-7 to 50-60 vol.%); (3) horizontal dimensions of the asthenosphere may change from 150-200 to 400-500 km.
Depending on given conditions, such parameters as melting depth, asthenosphere thickness, and position of the upper solidus boundary can considerably change with time. Our calculations show that the ultimate depths of the upper solidus boundary is about 100 km. It is unlikely that any standard (in composition of magmas and extent of melting) types of plume-related magmatic systems existed in the geological past. Given no initial conditions in the upper mantle, the dynamics of hot spots can be inferred from geochemical characteristics of magmas rather than from temperature and size of hot spots.



5.
GEOCHEMISTRY AND GEOCHRONOLOGY OF PERMO-TRIASSIC BASITES IN THE NORTHWESTERN ALTAI-SAYAN FOLDED AREA

G.S. Fedoseev, V.I. Sotnikov, and L.P. Rikhvanov*
Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Tomsk Polytechnical University, 30 prosp. Lenina, Tomsk, 634050, Russia
Keywords: Basites, dikes, sills, geochemistry, 40Ar/39Ar geochronology, Kolyvan'-Tomsk folded zone, Kuznetsk basin
Pages: 287-301

Abstract >>
Petrogeochemical and isotope-geochronological data are of paramount importance for refining the geologic correlation between weakly exposed magmatites of the Kolyvan'-Tomsk folded zone and Kuznetsk basin. We report data on dike and dike-sill basite complexes of shallow-depth genesis. According to the 40Ar/39Ar age estimates, basites in the Kuznetsk and Gorlovka basins are older than dikes free of xenogenous matter in the Tomsk magmatic area (248.95 ± 1.81-246.2 ± 1.4, 259.1 ± 3.4, and 241.6 ± 2.7-238.0 ± 5.2 Ma, respectively) and are close in petrogeochemistry to basites of the Tashara complex in the Novosibirsk magmatic area. These data suggest the synchronous occurrence of magmatic events related to the Permo-Triassic plume activity within the West Siberian Plate, Siberian Platform, and their southern folded framing.



6.
FACTORS DETERMINING CONCENTRATION OF TRACE ELEMENTS IN ORES OF PYRITE-POLYMETALLIC DEPOSITS IN SOUTHERN SIBERIA

I.V. Gas'kov, E.G. Distanov, and K.R. Kovalev
Institute of Geology, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Pyrite-polymetallic deposits, trace elements, ores, volcanogenic ore formation
Pages: 302-316

Abstract >>
Volcanogenic pyrite-polymetallic deposits are widespread in southern Siberia (Rudny Altai, eastern Tuva, northern Baikal region, and western Transbaikalia). They are divided into two groups according to their formation mechanism: volcanogenic hydrothermal (VHMS-type) (Kyzyl-Tashtyg deposit in Tuva; Korbalikhinskoe, Zolotushinskoe, Yubileinoe, and other deposits in Rudny Altai) and volcanosedimentary (SEDEX-type) (Kholodninskoe deposit in the northern Baikal region and Ozernoe deposit in western Transbaikalia).
Comparison of major ores and ore minerals during the volcanosedimentary and volcanogenic hydrothermal formation of pyrite-polymetallic deposits showed their significant difference in the spectrum and contents of trace elements.
Ores of the VHMS deposits have high concentrations of trace elements as they formed under drastic drop in temperature (as a result of boiling-up of solutions and their mixing with sea water), which caused mass deposition of ore matter and coprecipitation of accompanying elements, including their sorption by crystallizing sulfide minerals.
The poorer spectrum and lower concentrations of trace elements in ores of the SEDEX deposits are, on the contrary, related to the rhythmic formation of chemogenic ores from sea-floor brine pools. The reduced amount of endogenic sulfur and gradual reduction of sea water sulfur and its involvement in sulfide formation prevented rapid mass deposition of sulfides and, accordingly, coprecipitation of accompanying elements.



7.
KIMBERLITES OF ZIMBABWE: ABUNDANCE AND COMPOSITION

A.D. Khar'kiv, E.F. Roman'ko, and B.M. Zubarev
Central Research Geological Prospecting Institute of Nonferrous and Noble Metals,
129B Varshavskoe shosse, Moscow, 113545, Russia
Keywords: Kimberlites, composition, diamond association, Zimbabwe
Pages: 317-327

Abstract >>
The composition of kimberlites from the Chingwisi, QK1, and QK2 pipes is considered, and new data on a single operating pipe, River Ranch, are reported. Original calcretized kimberlites of the upper horizons of the Chingwisi pipe are described. Garnets of ultrabasic paragenesis of diamond association are extremely scarce and pyrope-almandine garnets of eclogite paragenesis of the same association are abundant in QK1 and QK2. These garnets can serve as indicators in search for diamond pipes with the use of the heavy-concentrate-mineralogical method. Kimberlite bodies of mobile belts can differ in composition from those of Archean cratons.



8.
FOCUSING TRANSFORMATION OF ARRAY SEISMIC DATA

V.A. Pozdnyakov and V.A. Cheverda*
KrasnoyarskGeofizika, 21c ul. Partizana Zheleznyaka, Krasnoyarsk, 660022, Russia
* Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Scattered waves, focusing transformation, selective image, consolidated block, faults
Pages: 328-337

Abstract >>
The focusing transformation approach is applied to array seismic data to detect scatterers and/or diffractors in wavefields in 3D imaging of complexly structured subsurface. The efficiency of the method is tested in 3D imaging of failure in a consolidated granite intrusion.



9.
A NUMERICAL SOLUTION TO A SYSTEM OF ELASTICITY EQUATIONS FOR LAYERED ANISOTROPIC MEDIA

A.L. Karchevsky
Sobolev Institute of Mathematics, Siberian Branch of the RAS,
4 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: System of elasticity, layered anisotropic medium, differential matrix Riccati equation
Pages: 338-350

Abstract >>
A system of elasticity equations for layered anisotropic media is solved numerically as the differential matrix Riccati equation, with analytical solutions for each layer. The suggested approach differs from the existing counterparts and is applicable to anisotropic media with layers of any anisotropy type.