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Russian Geology and Geophysics

2016 year, number 3

1.
ZONED MAGMATIC AREAS AND ANOROGENIC BATHOLITH FORMATION IN THE CENTRAL ASIAN OROGENIC BELT (by the example of the Late Paleozoic Khangai magmatic area)

V.V. Yarmolyuk1, A.M. Kozlovsky1, M.I. Kuzmin2
1Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017, Russia
2A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
Keywords: Батолиты, рифтовые зоны, изотопная геология, мантийные плюмы, Batholiths, rift zones, isotope geology, mantle plumes

Abstract >>
In the Late Paleozoic-Early Mesozoic, the Central Asian Rift System formed in the southern framing of the Siberian Craton. It has a specific structure owing to zoned magmatic areas comprising the largest world’s batholiths (Angara-Vitim, Khangai, and Khentei) in the core and coeval rift zones on the periphery. By the example of the Khangai batholith, which is the core of the Hangayn zoned magmatic area, we have demonstrated that the batholith granites are a specific class of anorogenic granitoids. Each batholith is composed of nearly coeval rocks of widely varying compositions, from low-alkali to subalkalic and alkaline and from mafic to ultrafelsic. The geochemical and isotopic characteristics of rocks of the Khangai batholith are intermediate between the host continental crust and the juvenile mantle source. The melts from this mantle source formed lava sequences on the rift periphery of the batholith and were also manifested in its core as synplutonic mafic intrusions. The isotope-geochemical characteristics of the batholith granitoids evidence that mantle magmas took part in the formation of granite melts. With regard to the volume of the batholith rocks (~1 mln km3), the Khangai zoned magmatic area can be considered a silicic large igneous province, whose formation was controlled by a localized source of material and heat, i.e., mantle plume. We propose a model relating the composition, structure, and geologic position of batholiths to the impact of mantle plumes on the lithosphere of a folded area. The model assumes that mantle melts intruded into the base of crust by the underplating mechanism or formed underlayers, caused crustal anatexis, and were mixed with anatectic melts. The geochemical and isotopic characteristics of the resulted magmatic products were intermediate between the parameters typical of the convergent boundaries of lithospheric plates and those characteristic for intraplate environments.



2.
DEFORMABLE PLATE TECTONICS AND REGIONAL GEODYNAMIC MODELOF THE ARCTIC REGION AND NORTHEAST ASIA

L.I. Lobkovsky
Shirshov Institute of Oceanology, Russian Academy of Sciences, Nakhimovskii pr. 36, Moscow, 117997, Russia
Keywords: Постулаты тектоники плит, теорема Эйлера, деформируемые плиты, эволюция Арктики и Северо-Восточной Азии в мелу и кайнозое, верхнемантийная конвекция, сопряженная с субдукцией, Postulates of plate tectonics, Euler’s theorem, deformable plates, Cretaceous and Cenozoic evolution of the Arctic and Northeast Asia, subduction-induced upper mantle convection

Abstract >>
The difficulties and contradictions of the classical concept of plate tectonics are discussed. It was shown the postulate of rigid plates and Euler’s theorem on the motion of rigid body with one fixed point, which is the basis of the kinematic description of plate tectonics, cannot be universally applied, e.g., to the analysis of Cretaceous reconstructions of the North Atlantic and Arctic. The second postulate of the existence of lithosphere as separate and distinct tectonic plates, which is equivalent to the assumption on the continuity of all plate boundaries, is not satisfied. This study provides a generalization of the theory of classical plate tectonics, in which plates are treated as deformable bodies, the concept of deformable plate tectonics. The study also discusses a new regional geodynamic model for the Cretaceous and Cenozoic evolution of lithosphere beneath Arctic and Northeastern Asia. The model is based on the assumption of a laterally extended convection cell in the upper mantle driven by a conveyor-belt-like process of subduction of the Pacific plate. The proposed model provides a physically sound explanation for the standard tectonomagmatic processes, such as island arc roll-back and opening of back-arc basins, formation of continental rift zones and igneous provinces at great distances (over 1000 km) from subduction zones, etc.



3.
THE STRESS STATE OF THE NORTHERN TIEN SHAN CRUST BASED ON THE KNET SEISMIC NETWORK DATA

Yu.L. Rebetsky1, N.A. Sycheva2, V.N. Sychev2, S.I. Kuzikov2, A.V. Marinin1
1Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, ul. B.Gruzinskaya 10, Moscow, 123995 Russia
2Scientific Station of the Russian Academy of Sciences, Bishkek 49, Bishkek, 720042, Kyrgyzstan
Keywords: Тектонические напряжения, механизмы очагов землетрясений, геодинамический режим, внутриконтинентальный ороген, Tectonic stresses, earthquake focal mechanisms, geodynamic regime, intracontinental orogen

Abstract >>
In this study we present a detailed analysis of natural stresses in the Northern Tien Shan crust averaged in a window of 10-15 km obtained from seismological data of the local KNET network. The transformation of focal mechanism data into the parameters of the stress tensor was based on the method of cataclastic analysis of rupture dislocations elaborated by Yu.L. Rebetsky (Institute of Physic of the Earth, Moscow). The results, including the orientation of the principal stress axes and the reduced stresses, are presented for four depth layers. It was shown that the central part of the study area is dominated by horizontal compression, while multiple domains characterized by horizontal shear and superimposed compression or pure horizontal shear are also present (uppermost layers in the eastern part of the Chuya depression, Suusamyr depression and adjoining regions, in the central part of the Kyrgyz Range). There are also several large domains of high and low effective confining pressure, which defines the corresponding deviator stress, according to the Coulomb-Mohr law. It was shown that relatively strong earthquakes are correlated with zones with low levels of effective pressure where the ruptures are characterized by lower resistance to brittle fracturing, i.e., Coulomb friction stresses. It was also shown that a distinct segment of the ~60 km E-W striking fault on the northern slope of the Kyrgyz Range generates a uniform distribution of stresses, corresponding to a dextral slip along of its edges.



4.
THE ROLE OF DYNAMOMETAMORPHISM IN THE FORMATION OF ORE DEPOSITS (by the example of the Tishinka and Ridder-Sokol’noe pyrite deposits in Rudny Altai)

S.V. Zinoviev1,2
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: Динамометаморфизм, зоны смятия, сдвиговые зоны, тектониты, рудоконцентрация, Рудный Алтай, Dynamometamorphism, crush zones, shear zones, tectonites, ore concentration, Rudny Altai

Abstract >>
Dynamometamorphic lithologic transformations of ores and ore-hosting rocks are substantiated by the example of the Tishinka deposit and the upper beds of the Ridder-Sokol’noe deposit in Rudny Altai. The lenticular and lenticular-banded morphology of orebodies in the deposits is due to the in situ redeposition of ore substance in the form of bodies oriented concordantly with the direction of the maximum tangential stresses and, correspondingly, the general strike of the deformation-metamorphic structures. It is shown that dynamometamorphism is not only the structure- and rock-forming factor but also plays a great role in the concentration and localization of ores, favoring the formation of rich ore deposits in zones of intense deformations and structural and geochemical transformations. The age of the main phases of dynamometamorphic impact and the time of formation of the deposit orebodies are estimated. For the Tishinka deposit, it is 294-301 and 279-288 Ma (Early Permian), and for the lenticular orebodies of the upper beds of the Ridder-Sokol’noe deposit, ~307 Ma (Late Carboniferous).



5.
SOURCE ROCKS OF CARBONIFEROUS-LOWER CRETACEOUS TERRIGENOUS SEDIMENTS OF THE NORTHEASTERN SIBERIAN PLATFORM: RESULTS OF Sm-Nd ISOTOPE-GEOCHEMICAL STUDIES

S.V. Malyshev1, A.K. Khudoley1, A.V. Prokopiev2, V.B. Ershova1, G.G. Kazakova3, L.B. Terent’eva4
1Institute of Earth Sciences, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia
2Diamond and Precious Metal Geology Institute, Siberian Branch of the Russian Academy of Sciences, pr. Lenina 39, Yakutsk, 677007, Russia
3A.P. Karpinsky Russian Geological Research Institute (VSEGEI), Sredny pr. 74, St. Petersburg, 199106, Russia
4Institute of Precambrian Geology and Geochronology, nab. Makarova 2, St. Petersburg, 199034, Russia
Keywords: Сибирский континент, Верхоянская пассивная окраина, Приверхоянский прогиб, карбон, пермь, мезозой, ε, источники сноса, Carboniferous, Permian, Mesozoic, ε, source rocks, Verkhoyansk foreland basin, Verkhoyansk passive margin, Siberian continent

Abstract >>
The first Sm-Nd isotope studies of the Carboniferous-Early Cretaceous clastic rocks of the northeastern Siberian Platform have been carried out. Variation in the isotope composition of sediments within this time interval has been determined and interpreted. The high εNd( t ) values for Carboniferous-Permian sediments (from -11 to -2) testify to the large contribution of the products of erosion of island-arc and juvenile complexes localized in the Taimyr-Severnaya Zemlya fold-thrust belt in the Carboniferous. The positive εNd( t ) values for Triassic sandstones suggest erosion of the coeval igneous rocks of the trap association. The Upper Jurassic and, particularly, Cretaceous sediments of the Lena-Anabar depression and Verkhoyansk foreland basin are characterized by extremely negative εNd( t ) values (from -15 to -19), which is evidence for erosion of mature continental crust. The Carboniferous-Triassic complexes of the Verkhoyansk fold-thrust belt, which show higher εNd( t ) values, cannot have been a source of detritus for Cretaceous sandstones. The sediments filling the Verkhoyansk foreland basin accumulated as a result of the decomposition of homogenized distal source rocks with the isotope characteristics of the ancient crust rather than the breakup of the mountain range rising to the east. Salients of the crystalline basement of the Siberian Platform, such as the Aldan Shield, might have been these source provinces.



6.
THERMOCHRONOLOGY OF EARLY PALEOZOIC COLLISIONAL AND SUBDUCTION-COLLISIONAL STRUCTURES OF CENTRAL ASIA

A.V. Travin1,2,3
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
3Tomsk State University, pr. Lenina 36, Tomsk, 634050, Russia
Keywords: Коллизионный тектогенез, метаморфические деформации, U/Pb, Ar/Ar изотопное датирование , термохронология, Центральная Азия, Collisional tectogenesis, metamorphic deformations, U-Pb and Ar-Ar dating, thermochronology, Caledonides, Central Asia

Abstract >>
The thermochronology of the Early Paleozoic collisional and subduction-collisional systems and blue schist complexes of the Central Asian Orogenic Belt has been reconstructed by the proposed method of total isotope dating. The evolution of these geologic structures is divided into short synchronous stages of active thermal events related to large-scale mantle-crustal magmatism, high-pressure/low-temperature and high-temperature/low-pressure metamorphism, and intense tectonic deformations. The plume activity of different intensities, both in intraoceanic and intracontinental environments, is presumed to be the deep mechanism of synchronization.



7.
GRANULITES OF THE SOUTH MUYA BLOCK (Baikal-Muya Foldbelt): AGE OF METAMORPHISM AND NATURE OF PROTOLITH

S.Yu. Skuzovatov a1, E.V. Sklyarov b c2,3, V.S. Shatsky a d e1,4,5, K.-L. Wang f6, K.V. Kulikova g7, O.V. Zarubina a1
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2Institute of Earth's Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
3Far Eastern Federal University, ul. Sukhanova 8, Vladivostok, 690950, Russia
4V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
5Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
6Institute of Earth Sciences, Academia Sinica, 128, Academia Road, Sec. 2, Nangang, Taipei, 11529, Taiwan
7Institute of Geology, Komi Science Center, Ural Branch of the Russian Academy of Sciences, ul., Pervomaiskaya 54, Syktyvkar, Russia
Keywords: Центрально-Азиатский складчатый пояс, Южно-Муйский блок, гранулиты, гранатовые пироксениты, циркон, U-Pb-возраст, континентальная субдукция, Granulites, garnet pyroxenites, zircon, U-Pb age, continental subduction, Central Asian Orogenic Belt, South Muya block

Abstract >>
High-pressure mafic granulites and garnet pyroxenites occur within the South Muya block as boudins or lenses among metamorphic rocks of the Kindikan Group. Their primary minerals crystallized at 670-750 ºC and 9.5-12.0 kbar. Granulite metamorphism peaked at 630 Ma, according to LA-ICP-MS U-Pb zircon ages. Judging by their major- and trace-element compositions and Hf isotope ratios in zircons, the South Muya granulites were derived from differentiated within-plate basalts, which, in turn, resulted from melting of juvenile mantle source and Meso- or Paleoproterozoic crust. The events of granulite and eclogite metamorphism in the South and North Muya blocks, respectively, were coeval and the two blocks were spatially close to each other at the onset of Late Baikalian subduction and collision events



8.
GEOCHEMISTRY, ISOTOPE TRIAD (Nd-Sr-O), AND 40Ar-39Ar AGE OF PALEOZOIC ALKALINE MAFIC INTRUSIONS OF THE KUZNETSK ALATAU (by the example of the Belaya Gora pluton)

V.V. Vrublevskii1, O.M. Grinev1, A.E. Izokh2,3,4, A.V. Travin2,3,4
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, ul. Pirogova 2, Novosibirsk, 630090, Russia
3Novosibirsk State University, pr. Lenina 36, Tomsk, 634050, Russia
4Tomsk State University
Keywords: Щелочной магматизм, геохимия, геохронология, мантийный плюм, Кузнецкий Алатау, Центрально-Азиатский складчатый пояс, Alkaline magmatism, geochemistry, geochronology, mantle plume, Kuznetsk Alatau, Central Asian Fold Belt

Abstract >>
We report the first data on 40Ar/39Ar dating, trace-element geochemistry, and isotope (Nd, Sr, O) composition of the Belaya Gora gabbro-foidolite-foyaite intrusive massif, one of typical representatives of the alkaline province of the northeastern Kuznetsk Alatau. The established age of rock-forming amphibole, ~401-403 Ma, is taken as the time of the pluton formation in the Early Devonian. The distribution and ratios of LILE and HFSE in the rocks suggest that the intrusion took place in the setting of “superposition” of Devonian mantle plume on the Early Paleozoic accretion-collision complexes. Therefore, the source of magmatic products had a heterogeneous composition as a result of mixing of plume material with the substances of suprasubduction and crustal substrates. The mantle component might have been a PREMA + EM combination, which is confirmed by the parameters of the Nd isotope composition (εNd ( T ) ≈ 3.2-4.6; T Nd(DM) ≈ 0.8 Ga) of alkaline rocks. The effects of crustal contamination are reflected in the high ratios of strontium and oxygen isotopes (87Sr/86Sr) T ≈ 0.7046-0.7054; δ18O ≈ 8.5-9.2‰, SMOW).



9.
INTEGRATED ARCHEOLOGICAL AND GEOPHYSICAL STUDIES IN WEST SIBERIA

M.I. Epov1,2, V.I. Molodin3, A.K. Manshtein1, E.V. Balkov1, P.G. Dyad’kov1, G.G. Matasova1, A.Yu. Kazansky3, S.B. Bortnikova1, O.A. Pozdnyakova3, Yu.G. Karin1, D.A. Kuleshov1
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
3Institute of Archeology and Ethnography, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Lavrent'eva 17, Novosibirsk, 630090, Russia
Keywords: Археология, метод сопротивлений, электромагнитное профилирование, магнитометрия, Archeology, resistivity method, electromagnetic profiling, magnetometry

Abstract >>
We present the most informative results of archeological and geophysical field studies of the Baraba forest-steppe over the last three years. The studies were carried out for archeological sites of different types belonging to a wide time interval (~6000 BC-2000 AD). Data on the presence, size, and configuration of archeologic objects were obtained by magnetometry and electrometry. We studied contrast between the magnetic properties of the upper horizon of present-day soil and underlying substratum at archeologic sites of different types and ages. Low contrast reduces the amplitudes of magnetic anomalies above buried ancient structures. It is shown that geoelectric methods are efficient in cases when magnetometry is not.



10.
FLUCTUATIONS IN GEOMAGNETIC SECULAR VARIATION RATE ACCORDING TO DATA FROM THE GLOBAL NETWORK OF MAGNETIC OBSERVATORIES

A.V. Ladynin
Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Геомагнитное поле, скорость векового хода, флуктуации, периоды флуктуаций, амплитуды флуктуаций, колебания оси диполя и изменение его магнитного момента, Geomagnetic field, secular variation rate, fluctuation, fluctuation period, fluctuation amplitude, nutation of the dipole axis, magnetic dipole moment variation

Abstract >>
The secular variation rate (SVR) of geomagnetic-field components (horizontal intensity H , vertical intensity Z , and inclination I) shows two types of fluctuations: with a short period (3 ± 0.15 years) and a long period (10-70 years). The amplitudes of the short-period fluctuations (SPFs) were estimated. The SPFs are uniform throughout the Earth, the Z and I SVR fluctuations are synchronous and have the same phases, and H SVR fluctuations are opposite in phase. Modeling of an eccentric-dipole field with a variable axial-pole latitude has shown that the SPFs in SVR are caused by the nutation of the dipole axis (and by the outer-core current systems responsible for the dipole field). Long-period fluctuations (LPFs) in SVR manifest themselves differently in different regions, and their nature is dominated by the effect of currents in the liquid core near the mantle.



11.
CURRENT TURN-OFF IN AN UNGROUNDED HORIZONTAL LOOP: EXPERIMENT AND THEORY

N.O. Kozhevnikov
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: Метод переходных процессов, горизонтальная петля, выключение тока, ранние времена, длинная линия, обратная задача, TEM surveys, horizontal loop, current turn-off, early times, transmission line, inversion

Abstract >>
An ungrounded horizontal loop, a common transmitter type in TEM surveys, makes up a system with distributed parameters with the earth under it. It can be simulated by an equivalent circuit with lumped parameters at late times and/or low frequencies, but at early times commensurate with the period of free current oscillations, the lumped circuit model fails to account for experimental data. At high frequencies and/or early times, the wire, in combination with the underlying earth, forms a transmission line in which current behaves according to the wave equation. This model allows calculating the current at any time and at any loop point with reference to the theory of long transmission lines. At early times, the loop self-responses depend on near-surface resistivity and environment and its primary magnetic field differs from that predicted by the classical theory of TEM surveys. Therefore, inversion of early-time response in terms of the conventional TEM system model is meaningless. However, as illustrated with a loop shunted by a matching resistor, the loop model as a combination of two transmission lines enables the inversion of the early-time current response in terms of the line parameters and near-surface resistivity.