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2010 year, number 8

1.
PALEOPROTEROZOIC BASALTOIDS IN THE NORTH BAIKAL VOLCANOPLUTONIC BELT OF THE SIBERIAN CRATON: AGE AND PETROGENESIS

M.N. Shokhonovaa, T.V. Donskayaa, D.P. Gladkochuba, A.M. Mazukabzova, and I.P. Paderinb
aInstitute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
b A.P. Karpinsky All-Russian Geological Research Institute (VSEGEI), Srednii pr. 74, St. Petersburg, 199106, Russia, Russia
Keywords: Basaltoids, dolerites, lithospheric mantle source, subduction-derived components, geochemistry, geochronology, postcollisional extension, Paleoproterozoic, Siberian craton
Pages: 815-832

Abstract >>
The oldest igneous rocks in the Paleoproterozoic (~1.88-1.85 Ga) North Baikal postcollisional volcanoplutonic belt of the Siberian craton are the basaltoids of the Malaya Kosa Formation (Akitkan Group). The youngest are the composite (dolerite-rhyolite) and doleritic dikes cutting the granitoids of the Irel' complex and the felsic volcanic rocks of the Khibelen Formation (Akitkan Group). The position of Malaya Kosa basaltoids in the Akitkan Group section and published geochronological data on the felsic volcanic rocks overlying Malaya Kosa rocks suggest that their age is ~1878 Ma. The rhyolites from the center of a composite dike were dated by the U-Pb zircon method at 1844±11 Ma, and the dolerites in the dikes are assumed to be coeval with them. Malaya Kosa basaltoids correspond to high-Mg tholeiites and calc-alkaline andesites, whereas the dolerites in the dikes correspond to high-Fe tholeiites. Geochemically, these basaltoids and dolerites are both similar and different. As compared with the dolerites, the basaltoids are poorer in TiO2 (an average of 0.89 vs. 1.94 wt.%), Fe2O3* (9.54 vs. 14.71 wt.%), and P2O5 (0.25 vs. 0.41 wt.%). However, these rocks are both poor in Nb but rich in Th and LREE, ε Nd (T) being negative. According to petrographic and geochemical data, they derived from compositionally different sources. It is assumed that the basaltoids originated from subduction-enriched lithospheric mantle, whereas the dolerites originated from refractory lithospheric mantle metasomatized by subduction fluids. The isotopic and geochemical features of mafic rocks in the North Baikal belt are well explained by their formation during crustal extension which followed subduction and collision in the region. The early stages of postcollisional extension evidenced the melting of subduction-enriched lithospheric mantle with the formation of parent melts for Malaya Kosa basaltoids. At the final stages of the formation of the North Baikal belt, during the maximum crustal extension, Fe-enriched melts rose to the surface and generated the dolerites of the dikes.



2.
THE DEVONIAN MAGMATISM IN THE KROPOTKIN RIDGE ( EAST SAYAN ) AND SOURCES OF BASITES: GEOLOGICAL, GEOCHEMICAL, AND SR-ND ISOTOPE DATA

A.A. Vorontsov and I.V. Sandimirov
A.P. Vinogradov Institute of Geochemistry, ul. Favorskogo 1a, Irkutsk, 664033, Russia
Keywords: Devonian magmatism, rifting, East Sayan
Pages: 833-845

Abstract >>
In the east of the Tuvinian trough within the Kropotkin Ridge, the formation of Devonian volcanic associations was intimately conjugate with rifting on the southwestern framing of the Siberian Platform. The associations include picrite-like basalts, trachybasalts, basaltic trachyandesites, trachyandesites, trachytes, trachyrhyodacites, trachyrhyolites, comendites, and subvolcanic dolerites. The basic and normal-basic rocks are subdivided into two groups by TiO2 contents: high-Ti (TiO2 ~ 2.2-4.2 wt.%) and medium-Ti (TiO2 ~ 1.3-2.0 wt.%). Compared with the high-Ti basites, the medium-Ti ones are depleted in K, Rb, REE, Nb, Ta, Th, and U and have features of magmatic series of active continental margins. The high-Ti rocks are similar in composition to within-plate basalts. But in the isotopic compositions of Sr and Nd the above groups of basanites are similar and correspond to mantle sources forming enriched within-plate basalts of the OIB type. This combination of within-plate and continent-marginal geochemical features in the basites localized in the same structure-geologic conditions might indicate the formation of rock associations in the rift zone at the rear of active continental paleomargin during the evolution of their common plume source. Its interaction with the suprasubductional lithospheric mantle determined the geochemistry of rocks.



3.
DISTRIBUTION OF GOLD IN IGNEOUS GRANITOID COMPLEXES IN THE CENTRAL AND SOUTHWESTERN AREAS OF EASTERN TRANSBAIKALIA

A.M. Spiridonov, V.D. Kozlov, L.D. Zorina, V.I. Men'shikov, and V.A. Bychinskii
V.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
Keywords: Granitoids, granitoid formations, igneous complexes, gold concentrations in granitoids, gold mineralization, cluster analysis of correlations between gold and trace-element concentrations
Pages: 846-856

Abstract >>
The distribution of gold in rocks from some igneous complexes of the central and southwestern areas of eastern Transbaikalia (Daurian, Aga, and Argun structure-formational zones) was studied by quantitative extraction-atomic-absorption analysis and mass spectrometry with inductively coupled plasma (Element-2 mass spectrometer). High gold concentrations (on average, 0.0043 ppm) are typical of the most widespread hornblende-biotite granodiorites and granites of the main phases of batholith intrusions in the Upper Paleozoic Unda complex in the east of the study area and in the Triassic-Middle Jurassic Kyra complex in the west. The rocks of the Early-Middle Jurassic (Sokhondo) and Middle-Upper Jurassic (Shakhtama, Kharalga, and Kukul'bei) complexes have much lower Au concentrations (mainly 0.0014-0.0030 ppm), with the minimum ones established in the Shakhtama complex. During the magmatic differentiation of granitoid intrusions, the concentrations of gold in the late leucogranite differentiates decreased.
The Au concentrations in the studied complexes do not depend on the composition of the host terrigenous rocks of different ages, which evidences the endogenous nature of the revealed differences in Au concentrations in the regional granitoids. Abnormally high concentrations of gold in some studied samples are observed mainly to the regional hydrothermal mineralization occurrences.
The classification R-type cluster analysis showed that all variables of the studied igneous rocks are subdivided into three groups by the degree of correlation. Gold shows a distinct tendency to the correlation with siderophile oxy- and sulfurophile groups of metals. The Q -type analysis generally confirmed the correctness of the known formational classification of the regional granitoids.



4.
APATITE-BEARING GREENSTONE BELTS IN RUSSIA

E.V. Belyaev
Central Research Institute for Geology of Industrial Minerals, ul. Zinina 4, Kazan, 420097, Russia
Keywords: Apatite, Archean greenstone belt, shield, metavolcanic sedimentary complex, mineral resources of Russia, prospects, guidelines
Pages: 857-862

Abstract >>
The paper discusses the important and urgent problem of enlarging the mineral resources of Russia. Volcanosedimentary complexes of greenstone belts are suggested as a new alternative source of apatite ores. The distinguishing features and minerageny of greenstone complexes in the Karelian, Aldan, and Anabar Shields have been considered. Apatite occurrences have been described in brief. The paper suggests the main lines of study and geological exploration of apatite-bearing metavolcanosedimentary complexes.



5.
LATEST EROSIONAL INCISION IN RIVER VALLEYS OF SOUTHERN EAST SIBERIA

G.F. Ufimtsev, A.A. Shchetnikov, and I.A. Filinov
Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
Keywords: Erosional incision, oscillatory tectonic movements, neotectonics, Baikal rift system, Siberian craton
Pages: 863-867

Abstract >>
Examples of the geological and geomorphic framework of river valleys in the Tunka rift basin (Baikal rift system) and in the Irkutsk amphitheater (Siberian craton) have been used to show that horizontal and vertical motions of tectonic units in southern East Siberia are superposed with periodic movements. In the latter, the waves of slow uplift are attendant with erosional incision events, whereas during the subsidence cycles, the incised valleys become filled with mostly alluvial sediments. The latest erosional incision in the area occurred in the past 70 kyr.



6.
THE HISTORY OF THE KARAGAS SUPERGROUP EVOLUTION IN THE BIRUSA REGION: SYNTHESIS OF PALEOMAGNETIC AND SEDIMENTOLOGICAL DATA

D.V. Metelkina,b, V.V. Blagovidovb, and A.Yu. Kazanskya,b
a A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, prosp. Akad. Koptyuga 3, Novosibirsk, 630090, Russia
b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Neoproterozoic, sedimentary basin, depositional environment, paleomagnetic pole, Siberian craton, Rodinia
Pages: 868-884

Abstract >>
We summarize the results of paleomagnetic and sedimentological studies of the Neoproterozoic rhythmic terrigenous-carbonate rock unit in the Karagas Supergroup, which were carried out in the middle reaches of the Biryusa and Uda Rivers. The paleomagnetic data are presented along with a detailed description of the studied sections and the specific sedmentological characteristics of the studied deposits. The new data refine the position of Late Precambrian paleomagnetic poles in Siberia and mark the paleogeographic and facies features of the rock unit formation. They also show that the rock unit was deposited much more rapidly than it was supposed earlier, in the environment of shallow-water sea basin on the margin of the Siberian continent. The origin and evolution of the Karagas sedimentary basin was governed by both global and regional tectonic factors. We associate the initiation of the basin with a global tectonic event, namely, the opening of ocean in southern Siberia during the break-up of Rodinia. Regional tectonic processes controlled the position of the Karagas sedimentary basin and its evolution. The position of the mean paleomagnetic pole (Plat = 3.9°, Plong = 292.3°, A95 = 7.1°) confirms the corresponding interval of the Neoproterozoic trend of APW in Siberia and proves the near-equatorial position of the continent in Karagas time.



7.
UPPER MANTLE ROOTS OF SIBERIAN CRATON BASEMENT STRUCTURES ALONG THE RIFT DSS PROFILE

V.D. Suvorov, Z.R. Mishen'kina, and E.A. Mel'nik
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, prosp. Akad. Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Peaceful nuclear explosions, upper mantle, Siberian craton, Rift DSS profile
Pages: 885-897

Abstract >>
We investigate the upper mantle velocity structure through processing first arrival data fr om peaceful nuclear explosions. The reported 2D model has been obtained by ray tracing for a spherical Earth, unlike the classical plane-approximation approach with subsequent spherical symmetry corrections, which is not always applicable to a laterally heterogeneous subsurface. The upper mantle velocity highs and lows imaged to 200-220 km depths show obvious correlation with major structures of the craton basement. Namely, low-velocity zones are observed beneath basins, the largest (to 8.0-8.1 km/s) under the Vendian-Cambrian Sayan-Yenisei syneclise. A discontinuous high-velocity layer (8.6-8.7 km/s) at depths between 150 and 240 km is underlain by a zone of lower velocity (8.50-8.55 km/s) down to the 410 km discontinuity, wh ere the velocity at the top of the transition zone is 9.4-9.5 km/s.