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Russian Geology and Geophysics

2019 year, number 4

1.
A MODEL OF NONUNIFORM EXHUMATION OF KEYBOARD-LIKE STRUCTURES AS A POSSIBLE CAUSE OF METALLOGENIC BELT ZONING IN FOLDED AREAS

I.N. Kigai
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, 119017, Moscow, Russia
Keywords: Metallogenic belts, terranes, folding, orogeny, tectonometallogenic cycles, ore-magmatic stages and levels, keyboard-like structures of folded areas, nonuniform exhumation, ore-magmatic assemblages, ore zoning

Abstract >>
A metallogenic belt zoning is observed in folded areas of all metallogenic epochs from Precambrian to Cenozoic. The changes observed across the strike of folded areas on geological mapping are usually treated as geodynamic, petrologic/metallogenic, or temporal specialization of corresponding belts (terranes) and zones. However, there are serious petrological reasons to expect a significant downward change in the basicity of igneous rocks and the composition of related mineralization in folded areas. The change may be caused both by the relationship between the densities of magmas and country rocks and by the relationship between the solidus temperatures of different magmas and the thermal gradient pattern on the path of magma ascent. More basic igneous rocks and related mineralization should be detained at deeper levels of the lithosphere because of the higher density and higher solidus temperature of magmas. During the orogeny, the vertical differential movements of several adjacent terranes located between deep-seated faults can create some keyboard-like structures composed of parallel belts variously uplifted and subsided, similarly to white and black keys of a piano. In more uplifted and then more denuded belts and blocks of such structures, the ore-magmatic complexes earlier located at deeper levels can be exposed at the surface. As a result, a metallogenic belt zoning can be generated, which can be absolutely of no (or only partial) relation to any prefolding primary sedimentary or other material or temporal specific features of terranes. The possibility of such nature of metallogenic belt zoning is shown here with the use of geophysical data for two important ore-producing regions of Russia: eastern Transbaikalia and southern Primorye. The model is proposed as one of the possible ways of formation of metallogenic belt zoning in some other folded areas. This research work will probably favor a more profound analysis of the nature of metallogenic belt zoning in the considered and other regions of the world with the application of modern geophysical methods. The use of the described regularities of the stage arrangement of igneous rocks of different basicity and oxidation state and genetically related hydrothermal ores can facilitate prospecting for ores of different metals and their 3D forecasting in keyboard-like structures of Phanerozoic orogens and ancient platforms.

DOI: 10.15372/RGG2019030



2.
HIGH-GRADE CONTACT METAMORPHISM IN THE KOCHUMDEK RIVER VALLEY (Podkamennaya Tunguska basin, East Siberia): EVIDENCE FOR MAGMA FLOW

E.V. Sokol1, O.P. Polyansky1, A.N. Semenov1, V.V. Reverdatto1, S.N. Kokh1, A.S. Devyatiyarova1, V.Yu. Kolobov1, P.V. Khvorov2, A.V. Babichev1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptuga 3, Novosibirsk, 630090, Russia
2Institute of Mineralogy, Ural Branch of the Russian Academy of Sciences, Miass, 456317, Russia
Keywords: Contact metamorphism, spurrite-merwinite facies, Kuzmovka complex traps, simulation, indicator minerals, East Siberia

Abstract >>
Spurrite-merwinite marbles on the right bank of the Kochumdek River in the Podkamennaya Tunguska basin formed along the top margin of a flood basalt intrusion (Kuzmovka complex) from a marly limestone protolith of the Rhuddanian Lower Kochumdek Subformation, at a pressure of ~200 bars. The contact aureole comprises four zones of successively decreasing temperatures marked by the respective mineral assemblages: T ≥ 900 C (merwinite, spurrite, and gehlenite ( rankinite, bredigite); T ≥ 750 C (spurrite); T ≥ 700 C (tilleyite, wollastonite, and melilite (Gehl<50)); and ~500-550 C (diopside, amphibole, and grossular). Very high temperatures at the contact ( T cont > 2/3 T melt) result from magma flow along a conduit. The temperature profiles for the Kochumdek metamorphic complex show good fit between measured and geothermometer-derived values at a magma temperature of 1200 C, an intrusion thickness of ≥40 m, a heating time of six months, and a magma flow lifespan within one month. Stagnant magma in a conduit of any thickness cools down and crystallizes rapidly and fails to heat up sediments to the temperatures required for spurrite-merwinite metamorphism (above 790 C).

DOI: 10.15372/RGG2019088



3.
RECONSTRUCTION OF MAGMA FLOW IN PERMO-TRIASSIC INTRUSIONS OF THE ANGARA-TASEEVA SYNECLISE (Siberian Platform) BASED ON MAGNETIC SUSCEPTIBILITY ANISOTROPY DATA

A.V. Latyshev1,2, P.S. Ulyakhina1,2, R.V. Veselovskii1,2
1Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119991, Russia
2Schmidt Institute of Physics of the Earth, ul. Bolshaya Gruzinskaya 10, Moscow, 123242, Russia
Keywords: Anisotropy of magnetic susceptibility, Siberian traps, large sills, rock magnetism, Permian, Triassic

Abstract >>
Based on detailed studies of the anisotropy of magnetic susceptibility (AMS), the directions of magmatic-melt propagation have been reconstructed in large dolerite sills of the Angara-Taseeva syneclise. Half the sites studied showed a magnetic fabric of normal type, i.e., the minimum K3 axis of the AMS ellipsoid is orthogonal to the contact, and the other two axes lie in the plane of the intrusive body. Interpreting the orientation of the maximum K1 axis as the direction of the melt flow at such sites, we compiled a scheme of the main magma flow directions. The obtained results are generally consistent with the hypothesis of magma-feeding zone in the central, most downwarped part of the Angara-Taseeva depression. The distribution of the maximum axes of the AMS ellipsoid is different in sills and points to the existence of local intrusion centers. Intrusions with an AMS ellipsoid of reverse type are predominant on the periphery of the depression. The sites with a normal magnetic fabric do not demonstrate any prevailing direction of magma propagation, because there is no general direction of the melt flow in the peripheral subsurface zones of large intrusions.

DOI: 10.15372/RGG2019029



4.
THE SOURCES OF PALEOPROTEROZOIC COLLISIONAL GRANITOIDS (Sharyzhalgai uplift, Southwestern Siberian Craton): FROM LITHOSPHERIC MANTLE TO UPPER CRUST

O.M. Turkina1,2, I.N. Kapitonov3
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 1, Novosibirsk, 630090, Russia
3A.P. Karpinsky Russian Research Geological Institute, Srednii pr. 74, St. Petersburg, 199106, Russia
Keywords: Collisional granitoids, zircon, Lu-Hf isotope composition, mantle and crustal sources, Paleoproterozoic, southwestern Siberian craton

Abstract >>
The paper presents the geochemical and isotope characteristics of rocks and the U-Pb age and Lu-Hf isotope composition of zircons from three plutons of Paleoproterozoic granitoids in the Sharyzhalgai uplift (southwestern Siberian craton). The age of granitoids of the Toisuk (1838 6 and 1827 9 Ma), Nizhnii Kitoi (1846 7 Ma), and Malaya Belaya (1863 16 Ma) plutons corresponds to the Late Paleoproterozoic collision stage and is correlated with the time of mafic magmatism. The studied rocks have a wide range of silica contents. The Toisuk pluton is composed of a range of rocks from monzodiorites to granodiorites (granosyenites) and granites; the Nizhnii Kitoi pluton, of granodiorites and granites; and the Malaya Belaya pluton, of leucogranites. The rocks of the three plutons are highly ferroan, enriched in LREE, Th, and HFSE, and correspond in composition to A-type granitoids. A characteristic feature of melanocratic granitoids of the Toisuk and Nizhnii Kitoi plutons is extremely high contents of Ba: 4080-1500 ppm and 1560-990 ppm, respectively. Based on analysis of experimental data on the melting of various substrates and the results of numerical computation, it is assumed that monzodiorite-granodiorites of the Toisuk pluton and granodiorites of the Nizhnii Kitoi pluton resulted from the differentiation/melting of a mafic source similar in Ba and Sr contents to intraplate continental basalts. The isotope compositions of zircon and melanocratic granitoids of the Toisuk (eHf from -6.0 to -10.7 and eNd from -5.3 to -10.2) and Nizhnii Kitoi (eHf from -5.0 to -8.1 and eNd = -4.0 and -5.1) plutons argue for the generation of their mafic sources from the enriched lithospheric mantle formed as a result of Neoarchean subduction processes. Vein granites of the Toisuk pluton and leucogranites of the Malaya Belaya pluton formed through the melting of quartz-feldspar (granodiorite) substrate. The contrasting isotope parameters of the Toisuk vein granites (eHf from -6.7 to -10.1, zircons, and eNd = -5.5, rock) and Malaya Belaya leucogranites (eHf from 2.9 to 5.9, zircons, and eNd from +0.7 to -1.9, rocks) indicate melting of the Archean and Paleoproterozoic crust, respectively. The more radiogenic Hf isotope composition of zircons from vein granites as compared with rocks of the Archean crust of the Irkut terrane is evident of the contribution of juvenile material to the granite formation.

DOI: 10.15372/RGG2019026



5.
ZIRCONOLOGY OF THE LHERZOLITE BLOCK OF THE NURALI MASSIF (South Urals)

A.A. Krasnobaev1, A.I. Rusin1, P.M. Valizer2, I.I. Likhanov3
1Zavaritskii Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, per. Pochtovyi 7, Yekaterinburg, 620075, Russia
2Ilmeny State Reserve
3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Lherzolite, dunite, zircon, geochemistry, REE, Th and U, age, Nurali massif

Abstract >>
Data on the mineralogy, geochemistry, and geochronology of zircons from plagioclase and spinel lherzolite and dunites of the Nurali massif are presented. The age of the main-type zircons from lherzolites (plagioclase ones, 446.2 2.8 Ma, and spinel ones, 433.3 3.4 Ma) and dunites (443.8 6.9 Ma) suggests their synchronous crystallization ( T av = 440 5 Ma). The metamorphic age of zircons is 410-415 Ma. In addition to grains belonging to the main subset, the older grains with an age between 527 and 2045 Ma and newly formed metasomatic grains with an age of 380-385 Ma were found. The structure and REE patterns of most zircons from ultramafic rocks point to their magmatic origin, which is probably related to crystallization of a fluidized mantle melt.

DOI: 10.15372/RGG2019042



6.
STABILITY OF ZIRCON IN THE SYSTEM MgO-SiO2-H2O AT 2.5 GPa

N.L. Dobretsov1,2, A.I. Chepurov3, V.M. Sonin3, E.I. Zhimulev3
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3, pr. Koptyuga, Novosibirsk, 630090, Russia
2Novosibirsk State University, 2, ul. Pirogova, Novosibirsk, 630090, Russia
3S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 3, pr. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Zircon, baddeleyite, peridotitic mineral assemblage, high pressure, high temperature

Abstract >>
We report preliminary experimental data on the interaction of zircon with the peridotitic assemblage olivine + serpentine at 2.5 GPa and 1400 C. In these conditions, zircon is unstable and decomposes with the formation of baddeleyite. Therefore, it is unlikely that zircon could survive in ultramafic rocks for a long time in the presence of a fluid phase at high temperatures. Thus, rigorous constraints on boundary conditions are required for the use of zircon in the dating of high-temperature geologic processes.

DOI: 10.15372/RGG2019043



7.
SILURIAN TERRIGENOUS-CARBONATE SEDIMENTATION IN GORNY ALTAI: STRUCTURE, FACIES VARIABILITY, FAUNAL ASSEMBLAGES, AND STRATIGRAPHIC POSITION

N.V. Sennikov1,2, R.A. Khabibulina1, O.T. Obut1,2, N.V. Novozhilova1,2, D.A. Tokarev1,2, V.A. Luchinina1
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia
2Novosibirsk State Universtity, ul. Pirogova 2, Novosibirsk, 630090 Russia
Keywords: Silurian, Homerian, Gorstian, Ludfordian, event stratigraphy, Gorny Altai

Abstract >>
This paper discusses the structure and facies variability of the Silurian mixed terrigenous-carbonate strata and presents new paleontological and biostratigraphic data. It attempts to refine the stratigraphic position and provide new lithologic evidence (markers) for regional manifestations of global sedimentary events (Mulde transgressive-regressive and Linde regressive events) in the Altai sections. The chronostratigraphic position of the Kuimov Horizon, a regional stratigraphic unit, has been defined based on graptolite and conodont faunas.

DOI: 10.15372/RGG2019031



8.
TECTONIC EVOLUTION OF LATE MESOZOIC-CENOZOIC BASINS IN EASTERN CHINA AND IMPLICATIONS FOR PACIFIC PLATE SUBDUCTION

Hongliang Wang1,2,3, Jintong Liang1,2,3
1School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China
2Key laboratory of marine reservoir evolution and hydrocarbon enrichment mechanism, Ministry of Education
3Beijing key laboratory of unconventional natural gas geological evaluation and development engineering
Keywords: Late Mesozoic-Cenozoic tectonic evolution, basins in eastern China, Pacific Plate subduction

Abstract >>
During the late Mesozoic and Cenozoic, the North China Craton witnessed a series of tectonic transition and deformation events, caused mainly by the subducting Pacific Plate and forming many petroliferous basins in eastern China and adjacent areas. Based on analysis related to the reconstruction of balanced sections, the comparison of subsidence rates, and the migration of depocenters in these basins, the tectonic evolution of these basins and their relationships with each other are clearly revealed. The evolution of the Bohai Bay Basin shows a migration from southwest to northeast. The North Yellow Sea Basins evolution developed northward, while the South Yellow Sea Basin evolution migrated southward. The evolution of the East China Sea Basin was activated from northwest to southeast. In combination with the subduction of the Pacific Plate, the conclusions of this study can be summarized in three aspects: (1) The structural pattern of the basin group is characterized by east-west trending belts and north-south trending blocks, (2) tectonic activities in the late Mesozoic-Cenozoic basins evolved from west to east in response to the westward subduction of the Pacific Plate, and (3) due to the collision of the Eurasian Plate and Indian Plate and the transitional pattern of the Tan-Lu Fault Zone from sinistral to dextral, the tectonic evolution of the basin group was centered in the Yellow Sea Basin, with the Bohai Bay Basin evolution migrating northward and the ECSB migrating southward.

DOI: 10.15372/RGG2019071



9.
THE EFFECT OF CURRENT PULSE WAVEFORM ON THE ELECTROMAGNETIC-FIELD TRANSIENT IN THE EARTH

E.Yu. Antonov1, V.S. Mogilatov1,2, M.I. Epov1,2
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Electrical prospecting, TEM sounding, pulse waveform, TE field, TM field

Abstract >>
Theoretical studies and methodical recommendations in pulsed electrical prospecting are based mainly on the technique of pulsed excitation with an instantaneous turn-off of direct current. In practice, the duration of current passage is determined by the depth of sounding and by economic and time expenditures. The pulse duration affects the late stage of the electromagnetic-field transient. At the early stage, it also depends on the turn-off front, which cannot be too short for technical reasons. On the solution of the direct problem, it is necessary to take into account the real parameters of the current pulse. Earlier it was proposed to use a special current pulse waveform to optimize sounding (the compensation transient electromagnetic method was implemented). We propose to look at the problem more broadly, in terms of separation of the nonstationary geoelectromagnetic field into TE and TM polarizations, which are excited by different sources. It turns out that the dependence of these polarizations on the waveform of the exciting pulse can be radically different in media with an insulating base.

DOI: 10.15372/RGG2019027