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Russian Geology and Geophysics

2012 year, number 1

1.
ECLOGITES FROM THE BELOMORIAN MOBILE BELT ( Kola Peninsula ): GEOLOGY AND PETROLOGY

A.A. Shchipanskya, L.I. Khodorevskayab, A.N. Konilova, and A.I. Slabunovc
aGeological Institute of the Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017, Russia
bInstitute of Experimental Mineralogy, ul. Akademika Osip'yana 4, Chernogolovka, Moscow Region, 142432, Russia
cInstitute of Geology of Karelian Research Center of the Russian Academy of Sciences, ul. Pushkinskaya 11, Petrozavodsk, Karelia, 185910, Russia
Keywords: Eclogites, partial melting, TTG gneisses, high-pressure metamorphism, subduction, exhumation, Archean oceanic crust
Pages: 1-21

Abstract >>
The paper focuses on the metamorphic geology of the oldest crustal eclogites discovered in the Late Archean tonalite-trondhjemite-granodiorite (TTG) complex of the Belomorian Mobile Belt on the Kola Peninsula. Eclogite bodies are, most likely, widespread. We studied one of the key objects, the Kuru-Vaara quarry, where several tens of retrogressed eclogite blocks randomly embedded in the TTG gneisses were stripped at the benches. Based on the field observations, two visually different types of eclogites have been recognized: "southern", strongly retrogressed coarse-grained, and "northern", well-preserved fine-grained. The southern eclogite blocks bear evidence for their partial melting with the formation of veins and melt percolation channels. The northern eclogite blocks show no evidence for melting. Despite the significant mineralogic difference, both types of eclogites can be assigned to amphibole eclogite facies. The applied jadeite solubility geobarometers yielded the minimum pressures of ~12 kbar for the northern eclogites and ~14-15 kbar for the southern ones. The used geothermometers yielded ~700 and ~750?C, respectively. But the presence of quartz lamellae in Na-clinopyroxenes in both types of eclogites and their bulk compositions corresponding to high-Mg basalts indicate that the Kuru-Vaara eclogites might have reached ultrahigh-pressure metamorphism. Analysis of the tentative P-T paths of metamorphic evolution for both types of eclogites showed that their burial-exhumation cycle might have taken as short as a few million years.
The set of presented data suggests that the formation of the Kuru-Vaara eclogites was related to the subduction of the Archean oceanic crust, which should have differed in composition and structure from the modern oceanic crust.
																								



2.
THE LATE TRIASSIC KATAEV VOLCANOPLUTONIC ASSOCIATION IN WESTERN TRANSBAIKALIA, A FRAGMENT OF THE ACTIVE CONTINENTAL MARGIN OF THE MONGOL-OKHOTSK OCEAN

T.V. Donskayaa, D.P. Gladkochuba, A.M. Mazukabzova, B. De Waeleb, and S.L. Presnyakovc
aInstitute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
bSPK Consulting, 10 Richardson Street, West Perth WA 6005, Australia
cA.P. Karpinsky All-Russian Research Geological Institute, Srednii pr. 74, St. Petersburg, 199106, Russia
Keywords: Volcanics, granites, U-Pb geochronology, geochemistry, active continental margin, Triassic, Mongolo-Okhotsk ocean, Transbaikalia
Pages: 22-36

Abstract >>
The Kataev volcanoplutonic association has been recognized in western Transbaikalia. It unites the volcanosedimentary rocks of the Kataev Formation and associated granites localized within the lower plates of the Buteel-Nuur and Zagan metamorphic-core complexes. The rocks of the Kataev association are dynamometamorphosed to different degrees, which is due to the tectonic exposure of metamorphic-core complexes in the Early Cretaceous. The U-Pb zircon dating of the Kataev Formation rhyolites yielded their age of 226 ± 3 Ma. The U-Pb zircon age of the granites intruding the Kataev Formation rocks is 223.4 ± 5.0 Ma. The volcanics of the Kataev Formation belong to the subalkalic basalt-andesite-dacite-rhyolite series. The trachybasalts and trachyandesite-basalts of the Kataev Formation have geochemical characteristics of igneous rocks formed as a result of subduction, e.g., they show distinct negative Nb and Ti and positive Ba and Sr anomalies on multielemental patterns. The specific composition of mafic volcanics points to their formation through the melting of a mantle source resulted from the mixing of depleted mantle and subduction components. Trachyandesites have higher Th and U contents than basaltoids. They can result from the contamination of a mantle source, similar in composition to the Kataev Formation basaltoids, with crustal material. The felsic volcanics of the Kataev Formation and granites inbruding them show nearly identical geochemical characteristics corresponding to both A - and I -type granites. These rocks might have formed through the melting of a moderately water-saturated magmatic source of diorite-tonalite composition at 742-833?C.
We have established that the rocks of the Kataev volcanoplutonic association in western Transbaikalia and northern Mongolia formed in the Late Triassic synchronously with the calc-alkalic granitoids of the Henteyn-Daurian batholith and the alkali granites and bimodal volcanic associations of the Kharitonovo and Tsagaan-Hurtey volcanoplutonic associations. The synchronous formation of volcanoplutonic associations of normal and high alkalinity agrees with the geodynamic setting of the Andean-type active continental margin existing in the area of present-day western Transbaikalia and northern Mongolia in the Early Mesozoic. This setting was the result of the subduction of the Mongolo-Okhotsk oceanic plate beneath the Siberian continent.
																								



3.
CAMBRIAN PALEOMAGNETISM IN THE BATENI RIDGE: EVOLUTION OF THE KUZNETSK ALATAU ISLAND ARC ( southern Siberia )

D.V. Metelkina,b and D.G. Koz'minc
aA.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
bNovosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
cPolimetall Geologiya LLC, ul. Kalinina 82, Krasnoyarsk, 660061, Russia
Keywords: Cambrian, paleomagnetic pole, strike-slip activity, Kuznetsk Alatau paleoisland arc, Siberia
Pages: 37-50

Abstract >>
We present paleomagnetic data on the Cambrian volcanosedimentary complexes of the Bateni Ridge, which correspond to the evolution of the Kuznetsk Alatau island arc. The paleomagnetic poles which served as the basis for the apparent polar-wander path (APWP) were determined. The paleomagnetic data were compared with those on the Cambrian poles for other fragments of the Kuznetsk Alatau island arc. For example, the APWP the Bateni fragment corresponds to that for the Kiya (Martaiga) fragment of the eastern slope of the Kuznetsk Alatau. The distribution of the paleomagnetic poles suggests that the relative drift of the Kuznetsk Alatau arc fragments and the entire island-arc system resulted from large-amplitude strike-slips. In the Cambrian, this paleoarc moved from ~10? N to ~10? S and rotated more than 50? clockwise. In the paleomagnetic record, its accretion is reflected in the coincidence of the Late Cambrian-Early Ordovician poles on the eastern slope of the Kuznetsk Alatau with one another and the coeval poles for the Siberian craton. This coincidence also suggests that the general structure of the present Kuznetsk Alatаu formed as early as the Early Caledonian. Nevertheless, the differences in the position of the poles on the western and eastern slopes of the Kuznetsk Alatau suggest that strike-slip activity along the Kuznetsk-Altai deep fault (the major one in the region) continued in the Hercynian and, probably, in the Mesozoic.
																								



4.
MODELING OF DEEP-SEATED HIGH-ALUMINA PARAGENESES ON THE BASIS OF THE STABILITY FIELDS OF CORUNDUM- AND SPINEL-NORMATIVE ASSEMBLAGES OF THE SYSTEM CaO-MgO-Al2O3-SiO2

N.V. Surkov and Yu.G. Gartvich
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Phase diagram, alkali-earth series of rocks, monovariant reactions, nonvariant equilibrium, "eclogitization" reaction, eutectic, magma genesis
Pages: 51-61

Abstract >>
To elaborate physicochemical models for the origin of crystalline rocks, experimental studies of the field of high-alumina assemblages of the system CaO-MgO-Al2O3-SiO2 were carried out at 10-30 kbar and 1250-1535 ?C. We have determined the phase relations between the melt (L) and An, Sp, Cpx, Cor, and Ga, the slope of the rays of the monovariant reactions An + Sp = Cpx + Cor (+ Ga) and L = Cpx + Ga + Cor + Sp, the position of the nonvariant point (An, Sp, Cpx, Cor, Ga, L), and the compositions of phases participating in these reactions.
Based on a topological analysis of the studied site of the system CaO-MgO-Al2O3-SiO2, we have substantiated that "eclogitization" must follow the reaction Opx + An + Sp = Cpx + Ga. A fundamental continuous series of eutectic monovariant equilibria was observed: L = Cpx + Opx + Fo + An, L = Cpx + Opx + An + Sp, L = Cpx (+ Ga) + An + Sp, and L = Cpx + Cor (+ Ga) + An. A change in the melt composition in this series of eutectic reactions depending on pressure must reflect the most likely magma genesis trend in nature. Composition fields in which the above series of reactions is observed with the composition fields of the rocks of magmatic formations showed that this series is most similar to the alkali-earth series of rocks. The mineralogical compositions of cumulates and phenocrysts found in the effusive and dike varieties of these rocks correspond to unique sets of subsolidus phase associations and individual subsolidus phases crystallizing in this fundamental eutectic series.
																								



5.
BEHAVIOR OF MAJOR AND RARE-EARTH ELEMENTS DURING THE POSTMAGMATIC ALTERATION OF KIMBERLITES

V.B. Vasilenkoa, L.G. Kuznetsovaa, V.A. Minina, and A.V. Tolstovb
aV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
bALROSA CJSC, Botuobiya Geological Survey, Mirnyi, Yakutia, Russia
Keywords: Kimberlite, postmagmatic metasomatism, rock-forming oxides, REE, normative quartz, supply and loss
Pages: 62-76

Abstract >>
During serpentinization and subsequent alteration in the absence of brucite, kimberlites accumulate uncompensated silica. Its amount can be calculated from the average compositions of the rock-forming minerals (olivine, calcite, phlogopite) and the chemical compositions of the rocks. The contents of rock-forming oxides and REE were determined in 12 kimberlite pipes of the Yakutian kimberlite province, in 413 samples from secondary-alteration zones and of unaltered kimberlites. Columns of successive kimberlite alterations were constructed for each pipe on the basis of secondary-quartz data; here, the behavior of rock-forming oxides and REE was assessed. All the studied rocks had experienced different degrees of postmagmatic hydrothermal metasomatism at different depths in all the pipes. The changes were reflected in the supply/loss of rock-forming oxides and REE. During the hydrothermal metasomatism, the supply of REE enriched the kimberlites with TiO2, P2O5, and CaO. During the removal of REE, most of the rock-forming oxides were partially lost. The maximum REE supply was 67% in the Udachnaya-Vostochnaya pipe and 59% in the Nyurbinskaya pipe as compared with the unaltered kimberlites. The maximum REE loss was 87% in the Aikhal pipe and 81% in the Internatsional'naya pipe as compared with the unaltered kimberlites. The initial REE contents of postmagmatically altered kimberlites changed considerably in all the studied cases. This conclusion has been drawn owing to the use of normative-quartz content as a criterion for secondary alteration.
																								



6.
THE KARCHIGA COOPPER MASSIVE SULFIDE DEPOSIT IN THE HIGH-GRADE METAMORPHOSED ROCKS OF THE KURCHUM BLOCK: GEOLOGIC STRUCTURE, FORMATION, AND METAMORPHISM ( Rudny Altai )

K.V. Lobanova and I.V. Gas'kovb
aOrsu Metals Corporation, 1 Red Place, W1K 6PL London, United Kingdom
bV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Chalcopyrite deposit, Irtysh crumpled zone, Kurchum block, Rudny Altai
Pages: 77-91

Abstract >>
The Karchiga copper massive sulfide deposit is located in the Kurchum block of high-grade metamorphosed rocks. This block is part of the Irtysh shear zone, which belongs to the largest transregional fault in Central Asia. The deposit is associated with the gneiss-amphibolite middle unit of the metamorphic complex, which is distinct in the geochemical fields. The mineralization is spatially and paragenetically related to the amphibolite beds, which are ore-bearing together with terrigenous rocks.
The deposit contains two spatially isolated ore lodes, in which all the discovered commercial reserves concentrate. They conformably overlie the host rocks and are tabular and ribbon-like. The mineralization has a close spatial relationship with Mg-rich anthophyllite-containing rocks. The sulfide ores are disseminated or massive and comprise pyrite, chalcopyrite, pyrrhotite, sphalerite, and magnetite. The ore is of Zn-Cu composition, in which Cu dominates considerably over Zn (average contents 2 and 0.4%, respectively; Cu/(Cu + Zn) = 0.83). The ores are rich in Co (up to 0.16%, averaging 0.02%), poor in Au and Ag (0.3 and 7.2 ppm, respectively), and almost free of Pb and Ba.
All the rocks and ores experienced epidote-amphibolite metamorphism. Meanwhile, the ores experienced recrystallization and partial regeneration, but the initial shape of the accumulations remained unchanged.
The essentially chalcopyritic ores, the volcanosedimentary ore-bearing rocks, and the spatial and genetic relationship of the mineralization with undifferentiated basaltoids suggest that this deposit belongs to the Besshi type, formed in a back-arc environment, near large uplifts.
The studies show that Besshi-type chalcopyrite deposits differ from most of the complex (Kuroko-type) deposits in Rudny Altai in the composition of volcanic rocks and geodynamic settings but belong to the same evolutionary series in this pyrite-bearing province. Both types of deposits might have formed in the Middle-Late Paleozoic, during the longest peak of pyritization in the Earth's history.
																								



7.
GEOELECTRICAL STUDIES OF THE CHUYA BASIN SEDIMENTARY FILL ( Gorny Altai )

E.V. Deeva, N.N. Nevedrovaa, I.D. Zol'nikovb, G.G. Rusanovc, and P.V. Ponomareva
aA.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
bV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
cGorno-Altaisk Expedition, ul. Sovetskaya 15, Maloyeniseiskoe Village, Altai Territory, 659370, Russia
Keywords: Cenozoic, Neopleistocene, intermontane basins, vertical electrical sounding (VES), Gorny Altai
Pages: 92-107

Abstract >>
The VES method was used to determine the geoelectric parameters of Cenozoic rocks in the Chuya intermontane basin. It was shown that the polyfacies and polychromous sediments filling the basin are well differentiated by their electrical parameters. A combination of methods used in data acquisition and processing, due to their high resolution capacity at shallow depths, provided information on the structure and physical properties of the Chuya rocks from surface to depths of 250-300 m. Despite a reasonable consistence among these methods to the above depths, the VES method is a more effective tool for detecting shallower layers. Interpretation of the VES data proved the existence of a large outburst channel as new geological evidence for an ice-dammed origin of the Middle Neopleistocene lake. A more detailed layering of the topmost part of the geoelectrical section across the Irbistu-Kokozek interfluve using the VES data revealed a number of fine-grained lenticular structures of the ancient lakes in the Chuya depression, which are hidden beneath a cover of fluvioglacial and lacustrine bouldery pebbles and moraine diamictons.
																								



8.
MEDIUM AND FIELD INHOMOGENEITY: ZONE OF INFLUENCE DURING MAGNETOTELLURIC SOUNDING

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: Magnetotelluric sounding, impedance, admittance, zone of synchronous observations, nonlocal response functions, lateral inhomogeneity, conductivity
Pages: 108-115

Abstract >>
The study is devoted to the possibilities of MTS in the case of arbitrary medium and electromagnetic-field inhomogeneity. It has been shown that the local tensor impedance and admittance ratios between the field components are differential. Useful information about the study region, with complex behavior of sounding curves, can be obtained by unconventional processing techniques, with the help of nonlocal medium response functions (component matching). Experiments can be considerably more cost-effective if we divide the study area into several small zones of synchronous observations and perform independent experiments in each of them at different time.
																								



9.
INFLUENCE OF THE OFF-AXIS POSITION OF THE TRANSMITTER AND RECEIVER CIRCUITS ON THE RESULTS OF DIFFERENTIALLY NORMALIZED ELECTROMAGNETIC SOUNDING

E.V. Ageenkov, Yu.A. Davydenko, and V.A. Fomitskii
Siberian Geophysical Research Production Company Ltd., ul. Baikal'skaya 291, Irkutsk, 664050, Russia
Keywords: Electromagnetic sounding, grounded circuit, differentially normalized electromagnetic method, sounding of polarizable conducting media, Cole-Cole model, 3D forward modeling for polarizable conducting medium
Pages: 116-121

Abstract >>
In the differentially normalized method of electromagnetic sounding (DNME), the transmitter and receiver are grounded electrical circuits. The conduction and polarization properties of a section are studied by measuring the electrical potential difference (Δ U ( t )) and the second potential difference (Δ2 U ( t )); the latter characterizes the spatial inhomogeneity of the electromagnetic field. Measurements of Δ2 U ( t ) are strongly influenced by three-dimensional inhomogeneities within the receiver spread. To reduce this effect, measurements are made in two positions (left and right) of the transmitter circuit relative to receiver with subsequent averaging of the measured data. Often in field studies, the transmitter and receiver circuits are at an angle to each other, and the use of two transmitters in measurements leads to the need to determine a generalized transmitter for one-dimensional forward numerical modeling of field data.
The effect of the off-axis (diagonal) position of the transmitter and receiver circuits on the data of electromagnetic pulse sounding and their inversion for a one-dimensional polarizable conducting medium have been studied in real and numerical experiments. In modeling, the effect of induced polarization (IP) is taken into account by introducing the resistivity frequency dispersion (Cole-Cole equation). Validity of the calculation of the generalized transmitter is estimated for the solution of the one-dimensional forward problem with the inversion of field diagonal measurements. The effect of three-dimensional objects on the results of measurements using the above observation system is estimated by solving the 3D forward problem for a polarizable conducting medium.