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Russian Geology and Geophysics

2016 year, number 2

1.
GRANITOID MAGMATISM AND METALLOGENY OF THE LAKE ZONE IN WESTERN MONGOLIA (by the example of the Bumbat-Hairhan area)

S.N. Rudnev1, A.E. Izokh1,2, A.S. Borisenko1,2, I.V. Gaskov1,2
1V.S. Sobolev Institute of Geology and Mineralogy, 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: Central Asian Fold Belt, Lake Zone of Western Mongolia, Late Neoproterozoic-Early Paleozoic intrusive magmatism, geochronology, geochemistry

Abstract >>
We present results of study of the geologic structure and age sequence of formation of the Late Neoproterozoic-Early Paleozoic plagiogranitoid and gabbroid associations in the Bumbat-Hairhan intrusive area of the Lake Zone in Western Mongolia. The petrogeochemical characteristics of the plagiogranitoids provide information about the conditions of formation of their parental melts at the island-arc and accretion-collision stages of the regional evolution. They also help to establish the main magma-generating sources as well as the major mechanisms of large-scale formation of granitoid melts and their relationship with ore generation processes. According to the trace-element and REE patterns and indicative ratios of these elements, the plagiogranitoids are subdivided into high- and low-alumina ones. Among the island-arc plagioranitoids (551-524 Ma) of the Bumbat-Hairhan area, high-alumina varieties are the most widespread. They resulted from the partial melting of metabasites in equilibrium with garnet-containing restite at ≥15 kbar during their subsidence into the subduction zone. In geochemical features these plagiogranitoids are similar to high-Si adakites of different world regions. Island-arc low-alumina plagiogranitoids are scarcer. Their geochemical characteristics indicate that the parental melts were generated through the partial melting of metabasites in the basement of the island-arc system in equilibrium with amphibole-containing restite at ≤8 kbar. Plagiogranitoid associations of the accretion-collision stage (511-468 Ma) are the most widespread in the Bumbat-Hairhan area. They are subdivided into high- and low-alumina ones. According to the contents of trace elements and their indicative ratios, the low-alumina plagiogranitoids resulted from the partial melting of metabasites in equilibrium with plagioclase-containing restite at ≤8 kbar in the upper part of the collisional structure, and the high-alumina ones were generated through the melting of metabasites in the basement of thick crust in equilibrium with garnet-containing restite at ≥15 kbar. Geochronological studies in the Bumbat-Hairhan area revealed two stages of ore-generating processes spatially and temporally related to the formation of low-alumina plagiogranitoids. The early stage (518 5 Ma), development of vein Cu(Au) mineralization, coincided in time with the formation of island-arc low-alumina plagiogranitoids of the Darbi massif (~524 Ma). The late stage (456 4 Ma), formation of porphyry Cu-Mo (Au) mineralization, was synchronous with the formation of the low-alumina plagiogranites at the accretion-collision stage (~468 Ma).



2.
EVOLUTION OF THE PALEOZOIC GRANITOID MAGMATISM IN THE KUZNETSK ALATAU: NEW GEOCHEMICAL AND U-Pb (SHRIMP-II) ISOTOPE DATA

V.V. Vrublevskii1, A.D. Kotelnikov1, S.N. Rudnev2, V.I. Krupchatnikov3
1Tomsk State University, pr. Lenina 36, Tomsk, 634050, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
3Gorno-Altaiskaya Ekspeditsiya Joint-Stock Company, Maloeniseiskoe, Altai Territory, 659370, Russia
Keywords: Granitoid magmatism, geochemistry and isotope geochronology, Kuznetsk Alatau, Central Asian Fold Belt

Abstract >>
A U-Pb isotope analysis has revealed zircons, dated at ~510−490 and ~430−400 Ma, in granitoid intrusions on the eastern slope of the Kuznetsk Alatau Range (Batenev Ridge). This suggests two stages of regional granitoid magmatism: Middle-Late Cambrian and Silurian-Early Devonian. The ages of two zircon grains from the rocks of the Tigertysh pluton are ~1.9 and ~2.6 Ga; they suggest the involvement of fragments of the Paleoproterozoic continental crust in magma generation. The granitoids, independently of their age, are mostly products of rocks of the calc-alkalic (K2O/Na2O ≈ 0.5−1.7) series. In the proportions of alumina, calcium, and alkalies most of them correspond to I-type granites (A/CNK ≤ 1.1) with reduced Fe/(Fe + Mg) values and contents of HFSE, including REE (ΣREE ≈ 100−300 ppm). The trace-element patterns of the rocks point to the possible participation of both IAB and OIB in magma genesis. Presumably, the influence of the within-plate component increased as a result of the superposition of plume on accretion-collisional structures, and their melting might have favored the preservation of geochemical features of subduction magmatism in the granitoids. The isotopic inhomogeneity (εNd(T) = 2.3−4.8, (87Sr/86Sr) T ≈ 0.7036−0.7051) of granitoid complexes suggests the heterogeneous composition of the sources of their material, which consisted of the matter of moderately depleted (PREMA) and enriched (EM) lithospheric mantle and crustal substrate, mixed in different proportions. Some products of the regional alkali-basic magmatism are coeval with the Cambrian granitoids of the Kuznetsk Alatau and are similar to them in isotope composition. This magmatism confirms the interaction of plume with the active continental margin at the Caledonian stage of evolution of the Central Asian Fold Belt.



3.
LATE PALEOPROTEROZOIC VOLCANIC ASSOCIATIONS IN THE SOUTHWESTERN SIBERIAN CRATON (Angara-Kan block)

A.D. Nozhkin1, O.M. Turkina1,2, I.I. Likhanov1, N.V. Dmitrieva1,2
1V.S. Sobolev Institute of Geology and Mineralogy, 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: Volcanic associations, geochemistry, U-Pb age, Paleoproterozoic, geodynamics, correlation, metamorphism, southwestern Siberian craton

Abstract >>
We present data on the geologic setting, mineral composition, and conditions of metamorphism of volcanics of andesite-dacite-trachyrhyodacite, leucobasalt-basalt, basalt-andesite-basalt-trachyandesite, and basalt associations in the Yenisei amphibolite-gneiss complex of the Angara-Kan terrane. We have determined the age of zircon (SHRIMP-II U-Pb dating) from two samples of volcanics of the andesite-dacite-trachyrhyodacite association. The volcanics formed in the Late Paleoproterozoic (~1.74 Ga) and were metamorphosed at 750 Ma. Inherited zircon and the Sm-Nd model age (2.4-2.5 Ga) of intermediate-felsic volcanics testify to their formation predominantly from the ancient crustal source. The Late Paleoproterozoic (1.78-1.74 Ga) sedimentation and volcanism in the Angara-Kan terrane followed the main collision events: high-temperature metamorphism (1.89-1.87 Ga) and formation of the first-phase granitoids of the Taraka massif (1.84 Ga). The volcanics have a predominantly bimodal character and belong to the tholeiitic and subalkalic series; this testifies to their formation in an extension setting. The formation of the volcanics of the Yenisei complex correlates with the intrusion of the intraplate granites of the Taraka massif, whereas the terrigenous sediments correlate with the sediments of the middle Subluk Group in the Urik-Iya graben. The rocks of the Yenisei complex are characterized by inhomogeneous metamorphism, which corresponds to the conditions of transition from epidote-amphibolite to amphibolite facies and to the entire PT -range of the amphibolite facies.



4.
ORDOVICIAN DEPOSITS ON THE WESTERN SLOPE OF THE SOUTHERN URALS AND THEIR CORRELATION BASED ON CONODONTS AND CHITINOZOANS

T.M. Mavrinskaya, R.R. Yakupov
Institute of Geology, Ufa Scientific Center, Russian Academy of Sciences, ul. K. Marksa 16/2, Ufa, 450077, Russia
Keywords: Ordovician, biostratigraphy, biofacies, isotopy, conodonts, chitinozoans, Southern Urals, western slope

Abstract >>
Biostratigraphic subdivision and correlation of Ordovician different-facies sections on the western slope of the Southern Urals are carried out based on conodonts and chitinozoans. Upper Ordovician conodonts are defined in the West Zilair zone, in a carbonate-terrigenous section near Nabiullino Village. They belong to important cosmopolitan taxa which permit distinguishing biostratigraphic units corresponding to the North Atlantic standard conodont scale. Successions of Middle and Upper Ordovician conodont and chitinozoan assemblages are detected in the terrigenous-siliceous sections of the East Zilair zone. Middle Ordovician chitinozoans are found in the Uraltau zone, in the metamorphic rocks of the Belekei Formation of the Suvanyak complex, previously assigned to the Precambrian. Most of the taxa in the conodont and chitinozoan assemblages have a good correlation potential owing to their wide geographic range. A positive δ13C excursion of 3.3 has been first recorded in the Nabiullino section (West Zilair zone), suggesting the initial stage of the HICE isotopic event in the Southern Urals. The development of shallow-water conodont biofacies at the top of the section suggests a sea level fall in the late Ordovician.



5.
NEW DATA ON THE STRUCTURE AND AGE OF THE TERMINAL PERMIAN STRATA IN THE SOUTH VERKHOYANSK REGION (northeastern Asia)

A.S. Biakov1,2, Yu.D. Zakharov3, M. Horacek4, S. Richoz5, R.V. Kutygin6, Yu.Yu. Ivanov1, E.V. Kolesov1, A.G. Konstantinov7,8, M.I. Tuchkova9, T.I. Mikhalitsyna1
1N.A. Shilo Northeast Interdisciplinary Scientific Research Institute, Far Eastern Branch, Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia
2Kazan Federal University, ul. Lenina 18, Kazan, 420008, Russia
3Far East Geological Institute, Far Eastern Branch, Russian Academy of Sciences, pr. Stoletiya Vladivostoka 159, Vladivostok, 690022, Russia
4BLT Wieselburg, HBLFA Francisco-Josephinum, Rottenhauserstrasse, 1, 3250 Wieselburg, Austria
5Karl-Franzens-Universitt Graz, Heinrichstrasse, 26, Graz, 8010, Austria
6Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, pr. Lenina 38, Yakutsk, 677891, Russia
7A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
8Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
9Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017, Russia
Keywords: Biostratigraphy, bivalves, carbon isotopes, upper Permian, Permian-Triassic boundary, South Verkhoyansk region, northeastern Asia

Abstract >>
We present new data on the structure and age of the upper Permian strata in the basin of the Setorym River (tributary of the Vostochnaya Khandyga River) in the South Verkhoyansk region, represented by the Imtachan Formation and the lower part of the Nekuchan Formation. Based on the new findings of bivalve fossils from the Intomodesma costatum Zone, as well as on the study of carbon isotopes in the shells and carbon isotopes of the organic matter of the host mudstones, we have for the first time proved the completeness of the Permian section. The detailed description of the contact between the Imtachan and Nekuchan Formations bears evidence for a certain continuity of the lithologic characteristics of the sandstones. The sharp lithologic contact between the formations reflects not a regional sedimentation hiatus but a shift of depositional settings from the upper parts of a delta to the deep shelf, caused by forced marine transgression. It is concluded that there was no long-term hiatus between the accumulation of the Imtachan and Nekuchan Formations. Images of the most characteristic species of bivalves and crinoids from the Imtachan Formation are presented.



6.
FORAMINIFERAL BIOSTRATIGRAPHY OF THE CALLOVIANVOLGIAN SEDIMENTS AND BIOFACIES OF THE SOUTHWESTERN WEST SIBERIAN LOWLAND

E.V. Zaichikova, L.K. Levchuk
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Foraminifera, biostratigraphy, Jurassic, Callovian, Kimmeridgian, Volgian, biofacies, West Siberia

Abstract >>
The study explored the taxonomic composition of foraminiferal assemblages from cores sampled in the Silga and Yamal-Tyumen facies regions-the southern and southwestern West Siberian sedimentary basin. A complete sequence of biostratons was established for the Callovian-Kimmeridgian sediments of the southwestern West Siberian Lowland and was correlated with the units of the General Stratigraphic Scale. A number of biostratons have been revealed for the south of West Siberia. The biofacies analysis of the foraminiferal assemblages revealed that the Yamal-Tyumen facies region belonged to a relatively deep marine environment, far away from the Jurassic Sea littoral zone, in the Callovian through the Kimmeridgian and that the Silga facies region was a relatively shallow marine environment in the Oxfordian through the Middle Volgian.



7.
THE PALEOGENE/NEOGENE BOUNDARY IN CONTINENTAL DEPOSITS OF THE WEST SIBERIAN PLAIN

V.S. Volkova1, O.B. Kuzmina1, Z.N. Gnibidenko1, A.G. Golovina2
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Siberian Research Institute of Geology, Geophysics, and Mineral Resources, Krasnyi pr., 67, Novosibirsk, 630091, Russia
Keywords: Continental lacustrine deposits, Turtas, Zhuravka, and Abrosimovka Formations, palynology, magnetostratigraphy, upper boundary of the Oligocene, West Siberia

Abstract >>
The paper presents palynologic and paleomagnetic characteristics of three West Siberian horizons are presented: Zhuravka (Upper Oligocene, Chattian), Abrosimovka, and Beshcheul (Lower-Middle Miocene, Aquitanian, Burdigalian, Langhian, and Serravallian). We consider the distribution of dinocysts of the genus Pseudokomewuia in the stratotype section of the Zhuravka Horizon (Zashchitino Village) and in the section of borehole 13 (Chelyuskintsev Village), which exposes the Turtas, Abrosimovka, and Beshcheul Formations. Similar compositions and structures of dinocyst assemblages and similar palynofloras in the Turtas and Abrosimovka Horizons, as well as the sedimentation environment and cyclic structure of lacustrine deposits, suggest that the Paleogene/Neogene boundary should be localized at the base of the Beshcheul Horizon. We propose to include the Abrosimovka Horizon into the Upper Oligocene (upper Chattian). The data are correlated with the regional paleomagnetic scale. These views should be reflected in a new regional stratigraphic chart for the Paleogene-Neogene deposits of the West Siberian Plain.



8.
THE EFFECT OF NATURAL AND ANTHROPOGENIC FACTORS ON THE EVOLUTION OF REMOTE LAKES IN EAST SIBERIA FOR THE LAST 200 YEARS

A.P. Fedotov1, S.S. Vorobyeva1, N.A. Bondarenko1, I.V. Tomberg1, N.A. Zhuchenko1, N.P. Sezko1, O.G. Stepanova1, M.S. Melgunov2,3, V.G. Ivanov1, T.O. Zheleznyakova1, N.I. Shaburova4, L.G. Chechetkina5
1Limnological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Ulan-Batorskaya 3, Irkutsk, 664033, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
3Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
4Zapovednoe Pribaikalie Federal State Institution, ul. Baikal'skaya 291B, Irkutsk, 664050, Russia
5Vitimsky State Nature Reserve, ul. Irkutskaya 4a, Bodaibo, 666902, Russia
Keywords: Phytoplankton, lake bottom sediments, diatoms, hydrochemistry, acidification, glaciers, climate, Lake Oron, East Siberia

Abstract >>
We performed a regional analysis of the effect of anthropogenic (acid precipitation) and natural (climatic changes, endogenous methane) factors on the hydrochemical composition and phytoplankton of mountain lakes in East Siberia for the last 210 years. The lacustrine diatom community responded to the intense acid precipitation that led to the acidification of lakes in Europe and North America in 1950-1985: Cyclotella-complex was partly replaced by more pH-tolerant species of diatoms, such as Aulacoseira lirata, A. italica, and Tabellaria flocculosa. This anthropogenic impact, however, was not dramatic for the ecosystem of mountain Lake Oron. The diatom records distinctly show a tendency for the reduction of lacustrine-alga population since the end of the Little Ice Age. We assume that the decrease in the Oron bioproductivity was mainly due to a deficit of nutrients caused by the inflow of ultrafresh waters from the thawing glaciers, snow patches, and seasonal snow cover of the Kodar Ridge during the global warming in the Northern Hemisphere. In addition, the changes in the lake ecosystem might have been accelerated by emissions of endogenous methane.



9.
THREE-DIMENSIONAL MODELING OF PULSED SOUNDINGS USING A FAST FOURIER TRANSFORM

M.I. Epov1,2, E.P. Shurina1,3, E.P. Shtabel1, N.V. Shtabel1
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
3Novosibirsk State Technical University, pr. Karla Marksa 20, Novosibirsk, 630073, Russia
Keywords: Vector finite element method, three-dimensional modeling, Fourier transform, pulsed electromagnetic soundings

Abstract >>
A method for three-dimensional modeling of pulsed soundings using a fast Fourier transform is developed: instead of the time-domain problem, it is proposed to solve a set of frequency-domain problems obtained by processing the original excitation pulse by a fast Fourier transform. The error of the proposed method compared to the method of solving the time-domain problem is numerically estimated for a model problem with a sinusoidal signal.



10.
PALEOSEISMOLOGICAL STUDIES IN THE EPICENTRAL AREA OF THE 1911 KEMIN EARTHQUAKE (northern Tien Shan)

E.V. Deev1,2, A.M. Korzhenkov3
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
3O.Y. Schmidt Institute of Earth Physics, Russian Academy of Sciences, ul. Bol'shaya Gruzinskaya 10, Moscow, 123995, Russia
Keywords: 1911 Kemin earthquake, paleoseismicity, fault scarp, Aksu fault, Tien Sha

Abstract >>
The area of the Chon-Aksu and Kichi-Aksu grabens abounds in seismic deformation produced by historic and prehistoric events, among which the great Kemin (Kebin) earthquake of 1911, with a magnitude of Ms ≈ 8 and a shaking intensity of I 0 = 10-11, generated by the Aksu fault. Trenching across a fault scarp of the 1911 event has revealed signatures of reverse slip resulted from another earthquake that occurred about 3000 years ago. Traces of a large event at ~12,700 yr BP appear in sediments of a tectonically dammed lake. The trenching results, along with radiocarbon dating and published evidence, show the Late Pleistocene-Holocene history of the Aksu fault to comprise prolonged quiescence periods separated by large earthquakes or earthquake clusters at 19,500-20,000, 12,700, 4000-3000, and <850 yr BP (including the 1911 Kemin shock).



11.
JERKS AND CONDUCTIVITY ANISOTROPY OF LOWER MANTLE

V.V. Plotkin
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Magnesiowstite, conductivity anisotropy, global electromagnetic induction, geomagnetic variations, jerks, lower mantle, outer fluid core, core-mantle boundary

Abstract >>
Conductivity anisotropy of the lower mantle presumably caused by phase change of dielectric magnesiowstite at depths of 1500-2000 km is detectable from jerks. Jerks are induced by currents in the fluid outer core, propagate upward from the CMB through anisotropic conducting mantle, and appear on the Earths surface. The surface jerk patterns are studied theoretically from the potential of the geomagnetic field presented as a sum of magnetic and electric modes. Equations for the fields of both modes and their relationship in a weakly anisotropic earth are obtained by the perturbation method. The field potential is expanded into a series of spherical harmonics, and the equations are solved in the frequency and time domains. The surface jerk responses can be inverted to retrieve anisotropy parameters; the goal function in the inversion may correspond to misfit between the model and experimental values either along the horizontal or vertical components.