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Russian Geology and Geophysics

2016 year, number 6

1.
THE MECHANISM OF MAGMA ASCENT THROUGH THE SOLID LITHOSPHERE AND RELATION BETWEEN MANTLE AND CRUSTAL DIAPIRISM: NUMERICAL MODELING AND NATURAL EXAMPLES

O.P. Polyansky1,2, V.V. Reverdatto1, A.V. Babichev1, V.G. Sverdlova1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
pol@igm.nsc.ru
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Diapirs, magma, lithosphere, viscosity, rheology, numerical modeling, continental crust, melting, intrusion

Abstract >>
Diapirism can be regarded as the main mechanism of transport through the lithosphere for both felsic and mafic/ultramafic magmas. However, the lack of field observations makes it difficult to identify the key mechanism responsible for the formation of dome-shaped structures. In this study, emplacement of natural diapirs is reconstructed by numerical experiments handling realistic rheological and petrological models for the crust and mantle lithosphere. Three different regimes of diapiric ascent were established depending on the chosen model rheology: (1) single-stage diapir ascent; (2) pulsating ascent of successive batches of mantle-derived magma to the base of the crust with a periodicity of 2-3 Myr; (3) emplacement of extensive magma bodies in the form of sills either beneath the base of the crust (underplating) or to deeper mantle levels. The timescale of 30 Myr for a heat source at the base of the lithosphere is sufficient to initiate the ascent of a diapir through the mantle and crust. The study provides the estimates of rheological properties of the lithosphere and partially molten material at which diapiric ascent through the mantle and crust can occur.



2.
GEODYNAMIC REGIMES OF THERMOCHEMICAL MANTLE PLUMES

A.A. Kirdyashkin1,2, A.G. Kirdyashkin1, V.E. Distanov1, I.N. Gladkov1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
aak@igm.nsc.ru
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Thermochemical plume, thermal power, plume conduit, plume head, melt, intrusive bodies, batholiths, diamondiferous plumes

Abstract >>
Laboratory and numerical experiments simulating the heat transfer and flow structure of thermochemical mantle plumes provide insights into the mechanisms of plume eruption onto the surface depending on the relative thermal power of plumes, Ka = N / N 1, where N and N 1 are the heat transferred fr om the plume base to the plume conduit and the heat transferred from the plume conduit to the surrounding mantle, respectively, under steady thermal conduction. There are three main types of plumes according to the Ka criterion: (i) plumes with low thermal power (Ka < 1.15), which fail to reach the surface, (ii) plumes with intermediate thermal power (1.15 < Ka< 1.9), which occur beneath cratons and transport melts from depths below 150 km, wh ere diamond is stable (diamondiferous plumes), and (iii) plumes with a mushroom-shaped head (1.9 < Ka < 10), which are responsible for large intrusive bodies, including batholiths. The volume of erupted melt and the depth from which the melt is transported to the surface are estimated for plumes of types (ii) and (iii). The relationship between the plume head area (along with the plume head diameter) and the relative thermal power is obtained. The relationship between the thickness of the block above the plume head and the relative thermal power is derived. On the basis of the results obtained, the geodynamic-regime diagram of thermochemical mantle plumes, including the plumes with Ka > 10, has been constructed.



3.
PHYSICOCHEMICAL CONDITIONS OF CRYSTALLIZATION OF DUNITES OF THE NIZHNII TAGIL PLATINUM-BEARING MASSIF (Middle Urals)

V.A. Simonov1,2, V.N. Puchkov3, V.S. Prikhodko4, S.I. Stupakov1, A.V. Kotlyarov1, N.S. Karmanov1, A.S. Stepanov5
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
3Institute of Geology, Ufa Science Center, Russian Academy of Sciences, ul. Karla Marksa 16/2, Ufa, 450000, Russia
4Yu.A. Kosygin Institute of Tectonics and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, ul. Kim-Yu-Chena 65, Khabarovsk, 680000, Russia
5ARC Centre of Excellence in Ore Deposits (CODES COE), University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
Keywords: Physicochemical conditions of crystallization, silicate melt inclusions, Cr-spinels, dunites, Pt-bearing ultramafic massifs

Abstract >>
Studies of primary multiphase silicate inclusions in accessory Cr-spinels from the fine-grained dunites of the Nizhnii Tagil Pt-bearing massif reveal their similarity to melt inclusions trapped by chromite during its growth. The analyzed Cr-spinels with multiphase silicate inclusions differ in composition from ore chromites of the same massif and from chromites (with melt inclusions) from ultramafic oceanic complexes but are similar to Cr-spinels in dunites from Pt-bearing alkaline ultramafic massifs (Konder and Inagli). According to petro- and geochemical data on heated multiphase silicate inclusions, the studied Cr-spinels crystallized with the participation of subalkalic picrobasaltic melts similar to the magmas of the Konder Pt-bearing massif and having almost the same chemical composition as tylaites. The differences between the compositions of olivines formed within the multiphase silicate inclusions and of the rock-forming minerals show that the studied Cr-spinels formed from an intercumulus liquid melt in the olivine crystal interstices during the cumulate crystallization of most of the Nizhnii Tagil massif dunites in the intrusive chamber. Numerical modeling based on the compositions of heated multiphase silicate inclusions in accessory Cr-spinels demonstrates that olivines and Cr-spinels from the studied dunites crystallized at 1430 to 1310 ºC and then olivine formation continued to 1280 ºC during the evolution of melts.



4.
TRACE ELEMENTS AND Hf ISOTOPE COMPOSITION AS INDICATORS OF ZIRCON GENESIS DUE TO THE EVOLUTION OF ALKALINE-CARBONATITE MAGMATIC SYSTEM (Ilmeny-Vishnevogorsky complex, Urals, Russia)

I.L. Nedosekova1, B.V. Belyatsky2, E.A. Belousova3
1Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620075, Russia
2A.P. Karpinsky Russian Geological Research Institute, Srednii pr. 74, St. Petersburg, 199106, Russia
3GEMOC ARC National Key Centre, Macquarie University, Sydney, Australia
Keywords: Zircon, rare elements, Hf isotope composition, carbonatites, alkaline rocks

Abstract >>
We present results of investigation of the trace-element (REE, HFSE) and Hf isotope compositions and U-Pb age of single zircons crystallized from alkaline-carbonatite magmas of the Ilmeny-Vishnevogorsky complex (IVC) (Urals, Russia). It has been established that the geochemical characteristics of the early zircon (U-Pb age is 430-410 Ma) from alkaline rocks and carbonatites of this complex are determined mainly by the magmatic evolution of parental fluid-saturated alkaline-carbonatite melts and, to a large extent, are associated with the cocrystallization of zircon and uranium rare-metal minerals (gatchettolite and pyrochlore) at the final stages of the magmatic-system activity. Early IVC zircons have a moderately depleted Hf isotope composition (εHf from +11.3 to +4.7), confirming the mantle nature of the magma source and indicating the participation of DM-like and enriched-source (probably, lower-crust component) substances in the magma generation. The considerable variations in the initial Hf isotope composition of the early zircons testify to the multistage zircon crystallization involving new portions of melts with different isotope compositions related to mixing of substances at their source. Late IVC zircons (250-350 Ma) have strongly disturbed «rejuvenated isotope systems and a geochemical composition different from that of the magmatic zircons. They formed apparently at the metamorphic stage without a substantial input of additional material.



5.
PALEOPROTEROZOIC HIGH-Mg LOW-Ti GABBRO-GRANITE SERIES IN EASTERN SARMATIA: GEOCHEMISTRY AND FORMATION CONDITIONS

R.A. Terentiev, K.A. Savko
Voronezh State University, Universitetskaya pl. 1, Voronezh, 394006, Russia
Keywords: High-Mg granitoids, boninite-like complexes, differentiation, petrogenesis, Elan complex, Voronezh Crystalline Massif

Abstract >>
We present nomenclature and geochemical classification of Paleoproterozoic LILE-enriched high-Mg low-Ti mafic-granitoid rocks of the eastern margin of the Sarmatia paleocontinent and substantiate their tectonic position. Two differentiated rock series are recognized: 1) biotite-orthopyroxene melanorite-quartz-meladiorite-melagranodiorite and 2) hornblende-biotite quartz-diorite-tonalite-granodiorite. Both series correspond in chemical composition to calc-alkalic gabbro-diorites, diorites, tonalites, and granodiorites. As follows fr om their mineralogical and geochemical compositions, these are norite-diorite rocks (intrusive analogs of boninites) (SiO2 = 52-65 wt.%, MgO = 5-20 wt.%, TiO2 = 0.2-0.8 wt.%) and high-Mg granitoids (SiO2 = 60-70 wt.%, Na2O/K2O = 0.65-1.33, MgO = 3.23-7.4 wt.%, K2O = 1.9-4.0 wt.%), respectively. Their high Mg# values (67-87) and Cr contents (>100 ppm), on the one hand, and their isotope-geochemical characteristics similar to those of the host metaterrigenous rocks, the magma enrichment in LILE, and the presence of Ni sulfide ores with a predominance of light sulfur isotopes, on the other, testify to crustal contamination of mantle magmas. The rock series are nearly of the same age and belong to the same magmatic system, wh ere high-Mg granitoids are differentiates of parental high-Mg (boninite-like) norite-dioritic magma. This is confirmed by a gradual increase in SiO2 and K2O contents and a decrease in Mg# and Ni, Co, V, and Cr contents in the sequence from norites to granodiorites and by the facies and phase relationships between the series. Intrusion of rocks took place at shallow depths after low-temperature metamorphism and folding under postcollisional collapse of the East Sarmatian orogen.



6.
NEW RESULTS OF OSL DATING OF QUATERNARY SEDIMENTS IN THE UPPER KATUN VALLEY (Gorny Altai) AND ADJACENT AREA

I.D. Zolnikov1,2, E.V. Deev3,4, S.A. Kotler1,4, G.G. Rusanov5, D.V. Nazarov6,7
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Institute of Archeology and Ethnography, Siberian Branch of the Russian Academy of Sciences, pr. Lavrentieva 17, Novosibirsk, 630090, Russia
3A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
4Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
5Gorno-Altaisk Expedition JSC, ul. Sovetskaya 15, Maloeniseiskoe Village, Altai Territory, 659370, Russia
6A.P. Karpinsky Russian Geological Research Institute, Srednii pr. 74, St. Petersburg, 199106, Russia
7St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia
Keywords: Quaternary sediments, OSL dating, fluvial and lacustrine deposits, Gorny Altai

Abstract >>
Several sections have been studied to understand the distribution and interrelation of basic genetic types of Quaternary deposits in the Uimon basin and adjacent area. The OSL date of 101 10 ka from the glaciolacustrine terrace on the northeastern rim of the basin corresponds to the cool substage of MIS 5. The glaciolacustrine sediments of the northern rim are covered with widespread diamictic flows of the outburst draining of the ice-dammed lake. The diamicts are overlain by a subaerial complex of three loesses and three paleosoils. From the OSL loess dates in the range of 43 to 49 ka we infer that the complex formed from the early MIS 3 through the Holocene. This is also supported by radiocarbon dates from fluvial deposits incised into the glaciolacustrine terrace and into megaflood sediments of the final draining of the last paleolake. The OSL dates in the range of 78 to 89 ka from alluvial sediments indicate that postglacial downcutting of the present-day Katun valley probably started just after MIS 5. The discrepancy between the beryllium dates from dropstones and Holocene TL-dates of the Katun valley floods ranging from 23 to 6 ka can be explained if the younger floods are related to the draining of moraine- and landslide-dammed rather than ice-dammed lakes. The younger floods, though being less voluminous than the glacial megafloods, were capable to produce giant ripple marks.



7.
SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE PALEOGENE LOWER SECOND MEMBER OF THE SHAHEJIE FORMATION, W79 BLOCK, WENLIU OILFIELD, Bohai Bay Basin, China

Li Jingzhe1, Zhang Jinliang1,2, Liu Shasha1, Fan Zhongli2, Xue Huanhuan2, Sun Zhongqiang2, Yu Tao2
1College of Resources Science & Technology, Beijing Normal University, Beijing 100875, China
2College of Geological Science and Engineering, Shandong University of Science and Technology, Qingdao 266500, Shandong, China
Keywords: Distributive fluvial complex, Wenliu Oilfield, W79 Block, Shahejie Formation, Sequence stratigraphy, Sedimentology

Abstract >>
High-resolution stratigraphic analysis of the lower second member of the Shahejie Formation of the W79 Block of Bohai Bay Basin, China, has revealed that the study area, previously interpreted as a shallow water delta system, actually originated mainly in a subaerial setting with a distributive pattern. Depositional systems of the distributive fluvial complex (DFC) have been recognized. The regional correlation of the lithofacies within the different depositional systems has led to a high-resolution-stratigraphic framework of two long-term base level cycles, six middle-term base level cycles, and more than 58 short-term base level cycles. Sequence boundary SB1 marks the top of the target interval and is characterized by thick and continuous mudstone overlying sand-rich channel sediments. Sequence boundary SB2 marks the bottom of the target interval and is characterized by stably distributed shales between sand-rich sediments. The base-level fluctuation has a strong tectonic component consistent with the regional tectonic setting; during the active subsidence stage, base-level rising semi-cycles were recorded in the strata, and during the relatively stable stage, base-level falling semi-cycles were recorded.



8.
ADSORBED OIL OF GAS CONDENSATE FIELDS

N.N. Mikhailov1, O.M. Ermilov2, L.S. Sechina3
1Gubkin Russian State Oil and Gas University, Leninskii pr. 65, build. 1, Moscow, 119991, Russia
2Yamal-Nenets Branch of A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, ul. Komsomol'skaya 15a, Nadym, 629730, Russia
3Institute of Oil and Gas Problems, Russian Academy of Sciences, ul. Gubkina 3, Moscow, 119333, Russia
Keywords: Residual oil, gas condensate field, adsorbed layer

Abstract >>
The pore space of reservoir rocks has a lot of active centers capable to form different types of bonds (from hydrogen to chemical). Therefore, almost any hydrocarbon can be adsorbed to a particular extent on the internal pore surface and form an adsorbed layer, which has a definite spatial configuration and makes contact with a certain part of the rock surface. Study of gas condensate reservoirs brings up the question of the composition of adsorbed hydrocarbons. The properties of such hydrocarbons in many giant gas condensate fields are of special interest as these compounds can serve as an additional source of resources after production of mobile-gas reserves. The performed study of gas condensate reservoirs of the Karachaganak, Astrakhan, and Yamburg fields has shown that their adsorbed hydrocarbons (adsorbed oil) have polar components with sulfur and oxygen atoms.



9.
ACTIVE FAULTS OF THE NORTHERN TIEN SHAN: TECTONOPHYSICAL ZONING OF SEISMIC RISK

Yu.L. Rebetsky1, S.I. Kuzikov2
1United Schmidt Institute of the Physics of the Earth, Russian Academy of Sciences, ul. Bol'shaya Gruzinskaya 10, Moscow, 123995, Russia
2Science Station of the Russian Academy of Sciences, Bishkek, 720049, Kyrgyzstan
Keywords: Current stress, seismic risk, tectonophysical zoning, active fault, hazardous fault, Coulomb stress

Abstract >>
This study continues the work by Mikhail Gzovsky on geological (tectonophysical) criteria for seismic risk. It is suggested to perform seismic-risk zoning according to parameters of normal and shear stresses on fault planes converted from results of tectonophysical stress reconstructions. The approach requires the knowledge of both dip and strike of the respective fault segments. Slip geometry is estimated from stress tensor, assuming that it is directed along shear stress. The suggested approach is applied to faults in the northern Tien Shan, and the current stress parameters are reconstructed using source mechanisms of catalogued earthquakes recorded by the KNET seismological network of the RAS Science Station in Bishkek. Stress modeling is performed by the method of cataclastic analysis providing constraints on stress ellipsoids, as well as on relations between the spherical and deviatoric components of the stress tensor. Plotted on the Mohr diagram, the fault stress points allow estimating whether the respective fault segments are close to the critical state (brittle failure). The suggested seismic-risk zoning of faults in the northern Tien Shan reveals up to 25 km long hazardous fault segments.



10.
THERMOPHYSICAL METHOD FOR QUANTITATIVE ESTIMATION OF HYDRATE CONTENT IN SAMPLES IMITATING BOTTOM SEDIMENTS

I.I. Fadeeva1,2, A.A. Duchkov2,3, M.E. Permyakov2
1N.A. Chinakal Institute of Mining, Siberian Branch of the Russian Academy of Sciences, Krasnyi pr. 54, Novosibirsk, 630091, Russia
2A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
3Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Methane hydrate, measurement of thermal conductivity and thermal diffusivity, quantitative evaluation

Abstract >>
Gas hydrates in sediments can dissociate on heating, which causes a characteristic change in temperature over time. This can be used to detect hydrates and estimate their content. The geothermal method for gas hydrate prospecting consists in obtaining thermograms and defining regularities related to the presence of gas hydrates in the medium. The aim of this study was a quantitative estimation of hydrate content in sediments, based on data from repeated measurements of the temperature of a linear heat source (needle probe). The power of the source is chosen so that hydrate does not decompose in the first measurement but decomposes in the second. Afterward, we solve the optimization inverse problem of determination of the model parameters, one of which is a hydrate content. Experimental data on heating of the laboratory samples are processed, and hydrate contents are obtained with an accuracy of 30%. These values agree with independent estimates. The analytical solution of the axially symmetric problem of gas hydrate dissociation, based on the solution of the Stefan problem, is applied as a mathematical model for hydrate heating and decomposition in the sample.