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Russian Geology and Geophysics

2014 year, number 1

1.
CHEMICAL COMPOSITION OF SEDIMENTS IN BAIKAL DEEP-WATER BOREHOLES AS A BASIS FOR RECONSTRUCTIONS OF CLIMATIC AND ENVIRONMENTAL CHANGES

M.I. Kuzmin, V.A. Bychinskii, E.V. Kerber, A.V. Oshchepkova, A.V. Goreglyad, E.V. Ivanov
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1A, Irkutsk, 664033 Russia
Keywords: X-ray diffraction, illite-smectite, Selektor, paleoclimatic reconstructions, Baikal

Abstract >>
Earlier reconstructions of climatic and environmental changes from data of deep-water drilling on the Akademichesky Ridge in Lake Baikal were based both on the content of biogenic silica or the abundance of diatom valves and on the results of X-ray diffraction (XRD) analysis of the sediment mineralogy. It was established that clay minerals are the main carrier of information about climatic variations in a drainage basin. The content of biogenic silica strictly correlates with two chrystallochemical parameters: the abundance of smectite beds in illite-smectite and the abundance of illite. However, detailed analysis of clay minerals calls for exclusive XRD techniques without mass determination of minerals in the sediments of long Baikal cores. We propose a new approach to determine the mineral composition of bottom sediments, based on their chemical composition. We compared the average chemical compositions of Pleistocene, Pliocene, and Miocene core sediments from the boreholes BDP-96 and BDP-98 and sediments of the Paleo-Barguzin River avandelta and recognized groups of chemical elements marking warm and cold climatic periods. However, the difference in the chemical compositions of sediments in warm and cold periods is insignificant. Since an XRD analysis of mineral composition is usually performed for short time intervals, it was necessary to identify cold and warm intervals by the mineral composition calculated from the chemical composition of sediments. The mineral contents were estimated using the Selektor software. Based on the average contents of chemical elements, we computed the mineral composition of the bottom sediments throughout the BDP-98 section and studied its warm and cold periods. We have established that feldspars weakly respond to climatic changes, their contents show minor variations in warm and cold epochs, whereas the contents of mica minerals change seriously. Thus, clay minerals, together with biogenic silica, are a good indicator of paleoclimatic environmental changes.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.001



2.
THE RHYOLITE–GRANITE ASSOCIATION IN THE CENTRAL TAIMYR ZONE: EVIDENCE OF ACCRETIONARYCOLLISIONAL EVENTS IN THE NEOPROTEROZOIC

V.F. Proskurnin1, V.A. Vernikovsky2,3, D.V. Metelkin2,3, B.S. Petrushkov1, A.E. Vernikovskaya4, A.V. Gavrish1, A.A. Bagaeva1, N.Yu. Matushkin2,3, N.P. Vinogradova1, A.N. Larionov1
1, V.O., Srednii pr. 74, St. Petersburg, 199106, Russia
2A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, ul. Pirogova 2, 630090, Russia
3Novosibirsk State University
4A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, 630090, Russia
Keywords: Granites, rhyolites, geochemistry, geochronology, Neoproterozoic, accretion and collision, paleomagnetic reconstructions, Arctic framing of the Siberian craton

Abstract >>
The Central Taimyr accretionary belt includes two granite-metamorphic terranes: Faddei and Mamont-Shrenk, which include the oldest igneous formations of the Taimyr folded area in the Arctic framing of the Siberian craton—granitoids and granite-gneisses with U–Pb zircon ages of 900830 Ma. The [FeO*/(FeO* + MgO)] enriched granitoids of these terranes are products of highly fractionated I -type magmas. This paper presents results of new petrographic, geochemical, geochronological, and paleomagnetic investigations of acid rocks fr om a volcanic-plutonic association (in the region of the Leningradskaya River) in the Faddei terrane in the northeastern Taimyr area. These rocks formed during the final stage of continentisland arc accretion and collision that occurred at approximately 870820 Ma. We established that the studied rocks belong to a long granitoid belt extending from Mamont-Shrenk to Faddei terrane, wh ere all the igneous bodies are deformed and oriented uniformly. The paleomagnetic pole we calculated differs significantly from the apparent polar-wander path interval of corresponding age for Siberia. The 33.8°± 5.4 angular distance between the poles indicates that the formation of this volcanic-plutonic association took place at a significant distance from the Taimyr margin of the Siberian paleocontinent.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.002



3.
CRYSTALLIZATION HISTORY OF PALEOZOIC GRANITOIDS IN THE OL’KHON REGION, LAKE BAIKAL (SHRIMP–II ZIRCON DATING)

V.A. Makrygina1, E.V. Tolmacheva2, E.N. Lepekhina3
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2Institute of the Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034, Russia
3A.P. Karpinsky Russian Geological Research Institute, V.O., Srednii pr. 74, St. Petersburg, 199106, Russia
Keywords: Granitoids, metamorphism, tectonic processes, zircons, inclusions, age

Abstract >>
At the Center of Isotope Studies of the A.P. Karpinsky Russian Geological Research Institute, the structure and isotope composition of zircons from two granitoid complexes, the age of their sequential growth zones, and the hosted inclusions have been studied using a SHRIMP-II ion mass spectrometer. The zircons consist of deformed cores with crystalline melt inclusions and of shells: inner, with glassy, partly devitrified inclusions, and outer metamorphogene, with fluid inclusions. Judging from the zircon zoning, crystallization of melts of both complexes proceeded in several stages: (1) The generation of melts and the beginning of zircon core growth (505 and 493 Ma) were synchronous with the overthrusting in the Olkhon region; (2) The rapid ascent of melts (the inner shell, 479 and 475 Ma) together with the host rocks was caused by upthrust faulting and shear dislocations; (3) The metamorphogene shell (456 Ma) reflects the second stage of metamorphism. At the same time, the Shara-Nur migmatitegranite complex corresponds in composition, structures, and textures to syncollisional K-granites, whereas the differentiated Khaidai gabbro-dioritedioritegranodioritegranite complex is close in geochemical features (similar to those of the Anga sequence metavolcanics) and the mantle (juvenile) source of substance to the recent island-arc magmatism. It is suggested that the Caledonian island-arc magmatism was close in time to the accretion of the sediments of back-arc basin (Olkhon Group) to the continental margin, on the one hand, and to the island-arc block, on the other.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.010



4.
SEISMIC CONVOLUTIONS IN THE QUATERNARY DEPOSITS OF LAKE SEVAN (Armenia)

A.M. Korzhenkov1, M.A. Avanesian2, E. Virgino3, A.S. Karakhanyan2
1O.Yu. Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, ul. Bolshaya Gruzinskaya 10, Moscow, 123995, Russia
2Institute of Geological Sciences, National Academy of Sciences, 24a Baghramian Ave., Yerevan, 0019, Republic of Armenia
3Livermore National Laboratory, Ministry of Energy, Livermore, United States
Keywords: Seismic convolutions, seismites, lacustrine deposits, ancient earthquakes, Caucasus, Armenia, Lake Sevan

Abstract >>
The aim of the study is to detect deformations in the soft sediments in a tectonically active area (the Armenian Highland) and to examine the significance of the deformations as paleoseismicity indicators. Deformations in the form of pillows, pockets, sharp waves, and ovoids are exposed in the Sevan basin, within interbedded shallow lacustrine, beach, and fluvial sediments. Also, broken beds and low-amplitude thrusts are observed. Eight field criteria for assigning soft-sediment deformations to paleoseismic triggering provide strong evidence for the seismic origin of the deformations. According to the local relative stratigraphic scale, the Sevan seismites are of PleistoceneHolocene age.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.003



5.
UNIQUE LOCATION OF HYDROTHERMAL BIOTA IN THE LOWER CAMBRIAN SEDIMENTARY–VOLCANOGENIC COMPLEX OF THE KYZYLTASHTYG ORE FIELD (Eastern Tuva)

A.A. Terleev1, V.A. Simonov2, A.V. Kanygin1, D.A. Tokarev1, S.I. Stupakov2, A.V. Kotlyarov2
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Koptyuga 3, Novosibirsk, 630090, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Hydrothermal biota, Lower Cambrian sedimentary-volcanogenic sequences, KyzylTashtyg pyrite deposit, Eastern Tuva

Abstract >>
Study of Early Cambrian sedimentary-volcanogenic complexes hosting the KyzylTashtyg pyrite deposit in Eastern Tuva has revealed cyanobacteria and algae that existed in zones of ore-forming hydrothermal systems similar in characteristics to present-day black smokers at ocean bottoms. Along with archaeocyaths and Cyanophyta from the host sedimentary rocks and microfossils from basalt amygdules, various cyanobacteria, monocyatheans, and sponge spicules have been found in ferrosiliceous deposits and metasedimentary rocks. Scanning electron microscopic, mineralogical, and thermobarogeochemical studies helped to reconstruct their hydrothermal environment.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.004



6.
GEOCHEMISTRY AND PETROGENESIS OF SUPRASUBDUCTION VOLCANIC COMPLEXES OF THE CHAR SHEAR ZONE (Eastern Kazakhstan)

E.V. Kurganskaya, I.Yu. Safonova, V.A. Simonov
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Central Asian Orogenic Belt, Paleo-Asian Ocean, VendianCambrian subduction, melting conditions, fractional crystallization, mantle sources

Abstract >>
The paper presents new petrographic, geochemical, and petrological data from volcanic rocks of suprasubduction origin of the Char shear zone in eastern Kazakhstan. We discuss bulk rock composition (concentrations of major and trace elements), types of mantle sources and parameters of their melting, conditions of crystallization of mafic magma, and geodynamic settings of basalt eruption. According to the major element composition, the volcanic rocks are basalt, andesibasalt, and andesite of tholeiitic and transitional, from tholeiitic to calc-alkaline, series. They are characterized by low TiO 2 (0.85 wt.% on average) and crystallization trends in MgOmajor elements plots. In term of trace element composition, the volcanic rocks show moderately LREE-enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/La pm = 0.140.47; Nb/Th pm = 0.71.6). The distribution of rare-earth elements (La/Sm N = 0.82.3, Gd/Yb N = 0.71.9) and the results of geochemical modeling in the NbYb system suggest high degrees of melting of a depleted mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase, and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry calculations suggest that the melts crystallized within the range of 10201180 . We think that this volcanic complex formed on the western active margin of the Paleo-Asian Ocean.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.005



7.
ASSEMBLAGES AND STRUCTURE OF ORE MINERALS IN INTRUSIVE TRAPS OF THE WESTERN PART OF THE SIBERIAN PLATFORM

M.P. Mazurov1,2, Yu.R. Vasilev3, A.V. Shikhova3, A.T. Titov1,2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, ul. Pirogova 2, Novosibirsk, 630090, Russia
2Novosibirsk State University
3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Dolerites, iron and titanium oxides, copper and nickel sulfides, exsolution, assemblages of ore minerals

Abstract >>
Phase compositions and microstructures of ore minerals in intrusive traps of the western part of the Siberian Platform have been studied using scanning electron microscopy. At the magmatic stage, oxide and sulfide solid solutions crystallize; their grain and aggregate shapes are determined by the cooling rate of magmatic bodies. We have revealed a gradual transition of oxides from fine-grained texture in the quenching zone, through skeleton, case, and frame forms, to isometric aggregates of mixed crystals in the holocrystalline silicate matrix. Sulfide spheroids (either conjugate with oxides or separated from them) are changed by dissemination and nests. The chemical compositions of both oxides and sulfides are correlated with the petrochemical types of rocks. Chrome-spinels or chrome-enriched ulvospinels crystallize first in the most magnesian dolerites. Iron and titanium oxides with Mn, V, Mg, and Al impurities prevail in the rest rock varieties. As temperature decreases, ilmenite, ulvospinel, and titanomagnetite crystallize after spinels. Exsolution structures are very intricate for titanium and iron oxides and depend on the oxidation regime and on the assemblage of impurities and their quantities. The first exsolution particles of ilmenite are more magnesian, while the following ones are more manganese. Subsolvus decomposition is accompanied by the release of impurities, grain stripping, and rearrangement and natural enrichment of ore material. Conjugate transformation of silicates and ore minerals results in aggregate pseudomorphs and minerals such as titanite, zircon, and baddeleyite. Nickel-containing sulfides formed at the magmatic stage prevail in more magnesian rocks. Copper minerals are more diverse. These are polymorphic modifications of chalcopyrite and cubanite in ore solid solutions formed at the magmatic stage, chalcopyrite in paragenesis with monoclinic pyrrhotite in zones of hydrothermal metasomatites, and chalcopyrite in solid solutions with bornite and chalcosine and in assemblage with low-temperature sulfides. The obtained data on mineral structures and assemblages can be used as indicators to classify the genesis and formation types of ores.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.006



8.
Approximation of a near-vertical boundary in the problems of pulsed electromagnetic soundings

N.V. Shtabel1, M.I. Epov1,2, E.Yu. Antonov1, M.A. Korsakov1
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Vector finite-elements method, 3D modeling, pulsed electromagnetic soundings

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.007



9.
DEEP STRUCTURE AND MARGINS OF THE KURAI BASIN (Gorny Altai), FROM CONTROLLED-SOURCE RESISTIVITY DATA N.N. Nevedrova, E.V. Deev, and A.M. Sanchaa

N.N. Nevedrova1,2, E.V. Deev1,2, A.M. Sanchaa1
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Neotectonics, resistivity surveys, TEM, VES, Kurai basin, Gorny Altai

Abstract >>
According to the data processing results, the basin is the deepest along its northern, southern, and eastern margins. The sedimentary fill comprises two resistivity units corresponding to two sequences deposited at different stages of the basin history. The lower, less resistive unit consists of PaleogeneNeogene lacustrine clay and the higher-resistivity upper unit represents coarser Quaternary deposits. In PaleogeneNeogene time, the basin formed by the left-lateral pull-apart mechanism. The earliest Quaternary strike-slip faulting in the setting of overall compression produced the Central Kurai basin within the northern Kurai basin, while the flanking ranges and fault blocks thrust upon the basin transforming it into a ramp. Thus, piedmont steps rose along the basin margins, and the marginal grabens became ramps and half-ramps.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.008



10.
LATE PALEOZOIC GEOMAGNETIC–FIELD ESTIMATES FROM PALEOMAGNETIC STUDIES OF PERMIAN LAVAS IN NORTHEASTERN KAZAKHSTAN

M.L. Bazhenov1, R. Van der Voo2, J.J. Meert3, N.M. Levashova1, I.S. Ipateva1
1Geological Institute of the Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017, Russia
2Earth and Environmental Sciences, University of Michigan, 2534C.C. Little Building, Ann Arbor, Michigan, United States
3Department of Geological Sciences, University of Florida, Gainesville, Florida, United States
Keywords: Geomagnetism, paleomagnetism, Paleozoic, thick lava series, geomagnetic-field characteristics

Abstract >>
Paleomagnetic studies of thick lava series are one of the most reliable sources of data on the ancient geomagnetic field. However, most of such data are younger than 5 Ma, with much fewer results on the rest of the Cenozoic and the Mesozoic. Two wholesome results are available for the Precambrian but none for the Paleozoic. Late Permian basalts and rhyolites from northeastern Kazakhstan were studied to obtain first estimates of the geomagnetic-field characteristics during that period. We present preliminary results on part of the collection (66 flows (sites)) from a section ~1600 m thick. The characteristic component of reversed polarity was isolated by stepwise demagnetization at all the sites with a slight error. This component is of prefolding age and, most likely, primary. No abnormal magnetization direction is observed in the data, and the average directions of the characteristic component at the sites are tightly clustered ( D = 243.3º; I = 57.0º, k = 79.1; α 95 = 2.0º; 65 sites). As compared with the published data on Cenozoic and Mesozoic thick lava series, secular variation was much weaker in the Late Permian than in the Mesozoic or Cenozoic, and the geomagnetic field was less disturbed. Secular-variation models based on the Late Cenozoic data show even more dramatic differences.

DOI: http://dx.doi.org/10.1016/j.rgg.2013.12.009