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2005 year, number 4

1.
CHEMICAL AND MINERAL COMPOSITIONS OF METACARBONATE ROCKS UNDER REGIONAL METAMORPHISM CONDITIONS AND GUIDELINES ON ROCK CLASSIFICATION

O.M. Rosen, D. Fettes*, and J. Desmons**
Geological Institute, Russian Academy of Sciences,
7 Pyzhevsky per., Moscow, 119017, Russia
* British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Great Britain
** rue de Houdemont, 3, F-54500, Vandoeuvre-lиs-Nancy, France
Keywords: Carbonate rocks, metamorphism, systematics of metamorphic rocks
Pages: 351-360

Abstract >>
Analysis is given to data on the chemical and mineral compositions of Archean-Early Paleozoic metamorphic carbonate rocks of greenschist to granulite facies. A continuous series of pure carbonate to nearly carbonate-free rocks derived from a calcium-enriched sedimentary protolith has been revealed. Several groups of metacarbonate rocks (petrotypes) with statistically significant boundaries between them have been recognized. The boundaries are used as a basis for a systematic scheme of metacarbonate rock nomenclature proposed by the Subcomission on the Systematics of Metamorphic Rocks (SCMR), a branch of the Commission on Systematics in Petrology (CSP) of the International Union of Geological Sciences (IUGS).



2.
BLUESCHISTS FROM THE UIMON ZONE AS EVIDENCE FOR ORDOVICIAN ACCRETIONARY-COLLISIONAL EVENTS IN GORNY ALTAI

N.I. Volkova, S.I. Stupakov*, G.A. Tret'yakov*, V.A. Simonov*, A.V. Travin**, and D.S. Yudin**
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
** Analytical Centre of United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Blueschist, geochemistry, subduction, accretion, geochronology, Gorny Altai
Pages: 361-378

Abstract >>
Petrochemical study of metabasites from the Uimon Zone, Gorny Altai, shows essential differences in TiO2, MgO, Na2O, K2O, and P2O5 contents between glaucophane-bearing, winchite-actinolite, and actinolite schists. Geochemical discrimination of these rocks suggests that blueschists are comparable to alkaline basalts of oceanic islands (OIB), whereas greenschists correspond in composition to tholeiite basalts of N-MORB type and their tuffs. Mineralogical distinctions of the rocks (namely, the presence of glaucophane or actinolite) are stipulated by chemical compositions of their protoliths, instead of variations in P-T conditions of metamorphism. Under similar P-T conditions (350-400



3.
Fe-Mg CARBONATES AND VARIATIONS IN ISOTOPIC COMPOSITIONS OF CARBON AND OXYGEN AT THE OZERNOE HYDROTHERMAL-SEDIMENTARY PYRITE-POLYMETALLIC DEPOSIT (Transbaikalia)

K.R. Kovalev, G.S. Ripp*, E.G. Distanov, and M.V. Baulina
Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
Keywords: Hydrothermal-sedimentary sulfide-polymetallic ores, Fe-Mg-carbonates, carbon and oxygen isotopes
Pages: 379-394

Abstract >>
The Ozernoe hydrothermal-sedimentary pyrite-polymetallic deposit in western Transbaikalia abounds in Fe-Mg-carbonates (predominantly siderites), which make up two major genetic groups of siderite ores: hydrothermal-sedimentary ores and their varieties recrystallized during hydrothermal-metasomatic processes. These groups of ores differ in structure, morphology, mineralogy, and geochemistry. Hydrothermal-sedimentary siderite ores form sheet bodies, make up ore sulfide-siderite rhythmites, have a fine-grained texture, and are polymineral siliceous-sulfide-carbonate formations. Siderites of these ores are Zn-enriched. Recrystallized ore varieties form deposits of complex morphology along the southeastern limb of the Ozernoe syncline on the flanks of silicic-alkaline metasomatites. These ores occur at different stratigraphic levels and coexist with stratified bodies of sulfide and sulfide-siderite ores. Exo- and endocontact zones of siderite deposits abound in coarse-crystalline siderite-barite-sulfide ores.
The isotopic compositions of carbon and oxygen of siderites, ankerites, and dolomites of hydrothermal-sedimentary ores and chemogenic and reef limestones of the ore-bearing section are generally the same as the isotopic compositions of coeval marine carbonates: δ13C = -1.6...+2.0<194> and δ18O = +15...+22<194>. Carbon plays a minor role in the formation of hydrothermal-sedimentary ores. Siderite varieties produced through recrystallization of sedimentary siderites during superposed metasomatic processes inherit the isotopic composition of their predecessors. Great variations in isotopic compositions of carbon and oxygen are observed within the large Ozernoe-Vasil'evskoe ore-bearing structure enclosing the Ozernoe deposit. The isotopic compositions of carbon and oxygen in carbonates of ores and ore-hosting rocks of this structure become heavier from base to roof of its section. The isotopic composition trends of carbon and oxygen are similar in behavior to that of sulfur in ore sulfides. Carbonates of hydrothermal-sedimentary iron oxide ores associated with sulfide ores have significantly lighter isotopic compositions of carbon (-0.5...-5 ‰) and oxygen (-2...+5 ‰). It is shown that the isotopic compositions of carbon and oxygen depend on the content of Mn in Fe-Mg-carbonates.



4.
PALEOMAGNETIC DIRECTIONS FROM NERSA INTRUSIONS OF THE BIRYUSA TERRANE, SIBERIAN CRATON, AS A REFLECTION OF TECTONIC EVENTS IN THE NEOPROTEROZOIC

D.V. Metelkin, I.V. Belonosov, D.P. Gladkochub*, T.V. Donskaya*, A.M. Mazukabzov*, and A.M. Stanevich*
Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Neoproterozoic, paleomagnetic pole, Siberian craton, Rodinia
Pages: 395-410

Abstract >>
New paleomagnetic data have been obtained from the Neoproterozoic subvolcanic intrusions which are widespread in the Biryusa terrane of the southwestern margin of the Siberian craton. The mafic dike swarms called the Nersa complex are rift-related subvolcanics, which could be used as indicators of the initial stage of the Neoproterozoic breakup of the Rodinia supercontinent. Two stable components of magnetization from the Nersa intrusions and Neoproterozoic sedimentary rocks of the Karagas Group were obtained. One of them is possibly a synfolding component related to the Early Paleozoic deformation processes. The other is the characteristic component of remanent magnetization, with the site-mean direction characterized by D s = 146.4, I s = 11.0, and α95 =13.3}>. The nature of this component is disputable. On the one hand, it can locate Siberia in the Late Neoproterozoic, but on the other hand, it also can reflect the Early Paleozoic remagnetization processes. Detailed analysis of the data, including results of reversal, baked-contact, and fold tests, suggests that the paleomagnetic directions could be of primary origin. We have tested the new paleomagnetic pole for its correspondence to some alternative tectonic reconstructions of Rodinia and believe that Siberia had an equatorial position and its modern southern margin was oriented to Laurentia during Neoproterozoic time.



5.
PLIOCENE-PLEISTOCENE SEDIMENTARY STRATIGRAPHY OF TRANSBAIKALIA: NEW DATA

M.A. Erbaeva, V.V. Karasev*, and N.V. Alekseeva
Geological Institute, Siberian Branch of the RAS, 6a ul. Sakhyanovoi, Ulan Ude, 670047, Russia
*
Keywords: Stratigraphy, mammals, Pliocene, Pleistocene, Eopleistocene, Neopleistocene, Transbaikalia
Pages: 411-419

Abstract >>
A decade of integrate litho-, magneto-, and biostratigraphic studies in joint projects yielded new data on the Pliocene-Pleistocene stratigraphy of Transbaikalia. Upper Cenozoic deposits are mostly slope wash in Western Transbaikalia and alluvium in Eastern Transbaikalia. Paleontological constraints have been obtained for a number of formations in the two subregions. In Western Transbaikalia they are the Lower Pliocene Anosov, Middle Pliocene Tologoi, and Middle Neopleistocene Krivoi Yar Formations, and in Eastern Transbaikalia they are the Lower and Middle Pliocene, and Upper Pliocene-Eopleistocene Ikaral, Torei, and Tsasuchei Formations, respectively, the Lower and Middle Neopleistocene Kholui and Kholbon Formations, the Lower Eopleistocene Ust'-Obor solifluction slope wash, and the upper Middle Neopleistocene Borzhigantai alluvium. The Middle Pliocene-Holocene section of Western Transbaikalia includes eleven successive mammal assemblages and the Eastern Transbaikalian section includes four faunas of which three are equivalents of the Chikoi, Tologoi, and Upper Paleolithic assemblages from Western Transbaikalia. The correlation among the faunal assemblages was used to correlate the Neogene-Quaternary sections from the two subregions and made a basis for a single scale of the Cenozoic stratigraphy of Transbaikalia.



6.
ICE-RAFTING OF SAND-SILTY MATERIAL IN SOUTH BAIKAL

E.G. Vologina, N.G. Granin*, S.S. Vorob'eva*, P. Francus**, T.K. Lomonosova, I.A. Kalashnikova, and L.Z. Granina*
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
* Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia
** Climate System Research Center, Department of Geosciences, University of Massachusetts,
611 North Pleasant Street, Amherst, MA 01003-9297, USA
Keywords: Ice-rafting, kolobovnik, sedimentation, bottom sediments, diatoms, grain size, minerals, South Baikal
Pages: 420-427

Abstract >>
In March, 2002, two fields of globular granular ice (so-called



7.
APPROXIMATION OF ELECTROMAGNETIC FIELD FROM A MAGNETIC DIPOLE IN A LAYERED MEDIUM AT SMALL WAVE NUMBERS

A.I. Fedorov and M.I. Epov
Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Quasi-steady-state approximation, electromagnetic field, layered medium, wave number
Pages: 428-436

Abstract >>
A new analytical solution for approximate integral functions that describe the electromagnetic field from dipole sources in layered conducting media at small wave numbers is applied to a three-coil loop in a medium with a single interface.



8.
OPTIMIZATION INVERSION OF SEISMIC DATA FROM LAYERED MEDIA: AN ALGORITHM FOR GRADIENT

E. Kurpinar and A.L. Karchevsky*
Mathematics Department, Science Faculty, Ege University, 35100 Bornova, Izmir, Turkey
* Sobolev Institute of Mathematics, Siberian Branch of the RAS,
4 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: System of elasticity, inverse problem, residual functional gradient, layered medium, differential matrix Riccati equation
Pages: 437-445

Abstract >>
A new analytical algorithm for the gradient of residual functionals is applied to numerical inversion of seismic data from horizontally layered media. The algorithm works for any linear differential equation and second-order system where the coefficients are piecewise constant functions. It is advantageous due to its plain logic and easily programmable formalism.



9.
FIRST PALEOMAGNETIC RESULTS FROM THE BOTTOM SEDIMENTS OF LAKE HЦVSGЦL BASED ON DRILLING DATA

A.Yu. Kazansky, A.P. Fedotov*, G.G. Matasova, A.A. Yuldashev**, G.A. Ziborova*, T.O. Zheleznyakova*, E.G. Vologina**, Ts. Oyuntcimeg***, Ts. Narantsetseg***, D. Tomurkhoo***
Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia
** Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
*** Institute of Geology and Mineral Resources, Academy of Sciences of Mongolia,
POB 118, Ulaan Baatar, 210351, Mongolia
Keywords: Paleomagnetism, lake sediments, HЦvsgЦl Lake
Pages: 446-449

Abstract >>
First paleomagnetic results for a 53 m core obtained from the HÖvsgÖl lake deposits (Mongolia) in the framework of the Russian-Mongolian joint project are presented. On the basis of measured inclination of natural remanent magnetization (NRM), three distinct polarity zones corresponding to Brunhes and Matuyama Chrons and Jaramillo Subchron have been recognized. Intervals with shallow NRM inclination are interpreted as possible geomagnetic excursions. Using the boundaries of both polarity zones and excursions as time marks, sedimentation rate of lake deposits is calculated. Several layers with different magnetic properties have been distinguished within the core section. The periodic repetitions of layers with different characteristics, probably, reflect variations in magnetic material input to the lake, changes in water salinity, and fluctuations of the lake level.