Volume 30, 3, 2011 |
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A.M. Gagieva, I.L. Zhulanova.
Geochronometry of Middle Paleozoic volcanics of the Omolon massif:
comparison of K Ar, Rb Sr, and U Pb data, and their geologic
interpretation (North-East Asia) 3 V.G. Gonevchuk, G.A. Gonevchuk, V.A. Lebedev, A.A. Orekhov. Association of monzonitic rocks of the Kavalerovo ore district (Primorye): geochronology and some issues of genesis 20 K.V. Chudnenko, À.Yu. Antonov. Assessment of the conditions of granitoid systems formation using the thermodynamic modeling method (Aldan Shield, Far East) 32 A.M. Petrishchevsky. Rheological model of the South Sikhote-Alin Earth’s crust (from gravity data) 50 A.V. Konovalov, A.S. Sychov, V.N. Solovyev. Mass estimates of scalar seismic moments of slight earthquake foci on the south of Sakhalin Island 66 N.F. Vasilenko, A.S. Prytkov, S.M. Saprygin. Horizontal movements and generation of strong earthquakes in the North Sakhalin interiors 76 T.G. Ryashenko, N.N. Ukhova, S.I. Stel'makh, N.I. Belyanina, P.S. Belyanin. Hypotheses of brown loam formation in Primorye: retrospective and a new view (Russian Far East) 80 A.I. Serezhnikov. Geological and hydrogeological characteristics and paleohydrogeological reconstructions of the Baley gold ore field (Transbaikal region) 93 V.Yu. Zabrodin. Paleogeography of the north-western part of the Sikhote-Alin fold system in the Mesozoic (Far East) 106 Jubilees Nikolai P. Romanovsky (80th birth anniversary) 116 Memorial dates Lev I. Krasny (100th birth anniversary) 117 A.M. Gagieva, I.L. Zhulanova Geochronometry of Middle Paleozoic volcanics of the Omolon massif: comparison of K-Ar, Rb-Sr, and U-Pb data, and their geologic interpretation (North-East Asia) The results of K-Ar, Rb-Sr isochronous and U-Pb datings (the zircon SHRIMP method) of Middle Paleozoic volcanogenic assemblages of the Omolon massif are summarized. A conclusion is drawn that in principle they agree with both one another and geologic data. The formation of the Kedon Group, which makes up the bulk of the volcanics of Middle Paleozoic, began at the boundary between the Early and the Middle Devonian, about 400 Ma ago (U-Pb dates 400.5 ± 4.4 and 387.6 ± 6.4, Rb-Sr isochron 402 ± 6 Ma ago). The issue of the isotope age of the upper boundary of the Kedon Groupd remains open, as there remain differences in the definition of its stratigraphic position. A histogram for 111 K-Ar datings of volcanics of the Kedon Group has been constructed. It has polymodal nature testifying to violation of K-Ar isotope systems affected by thermal events, which occurred 310–290 (end of the Carboniferous –beginning of the Permian) and 240–220 (the Middle –beginning of the Late Triassic) Ma ago. Mantle (ultrabasic-basic) volcanism, which manifested itself in a much expansive territory than the area of distribution of the Kedon Group, is the geologic expression of both thermal events. Key words: Middle Paleozoic volcanic rocks, isotope dating, Omolon massif, North-East Asia. V.G. Gonevchuk, G.A. Gonevchuk, V.A. Lebedev, A.A. Orekhov Association of monzonitic rocks of the Kavalerovo ore district (Primorye): geochronology and some issues of genesis Isotope-geochronological study of rocks, forming two large intrusive bodies and an isolated group of small bodies in the western part of the Kavalerovo ore district, has been carried out. They were described in the majority of publications as monzonitic or trachyandesite - monzonitic associations. The time of formation of the study intrusive associations is defined in the range 113-98 Ma by the biotite and amphibole K-Ar method and Rb-Sr method involving the rocks and minerals segregated from them. The interval can be wider (115-95 Ma) with regard to other isotope data, including those on rocks of the effusive facies. It does not contradict the conception of belonging of the investigated association to one magmatic complex. Significant and logical differences in the age of compositionally similar rocks of different massifs have not been established. According to the pattern of geodynamic evolution of the region, the latitic melts chamber, effusive and most intrusive rocks of the complex were formed prior to the initial stage of the Sikhote-Alin subduction volcanic belt. The initial stage of the belt formation is probably registered by the datings of biotite from monzodiorites of the Uglovaya VÒS, located in the central part of the district (90 Ma), and of late amphibole from monzonitoids of its western part (91-92 Ma). The geochemical distinctions of the rocks composing different intrusive bodies could be caused by the features of melt evolution in the intermediate hearths or crystallization chambers. Key words: monzonite, isotope geochronology, genesis, ore content, the Far East, Russia. K.V. Chudnenko, À.Yu. Antonov Assessment of the conditions of granitoid systems formation using the thermodynamic modeling method (Aldan Shield, Far East) A new universal method is suggested and tested involving thermodynamic modeling of parameters of granitoid systems formation based on data on the chemical composition and relation of minerals in the study rock sample. The method is the solution of the reverse problem of convex programming, and is based on solving for the minimum of Gibbs free energy in the physical-chemical system for the multitude of defined limitations. Capabilities of the this method are demonstrated when assessing conditions of formation (P-T and fluid regime) of different types of Late Mesozoic granitoids of four large massifs along strike of the Stanovoy Range (Aldan shield southern framing). These granitoids vary in alkalinity (calc-alkaline and subalkaline), melanocratic character (diorites to leicogranites), and genesis (magmatic and palingenetic-metasomatic). The simulated conditions of formation of the study granitoids generally coincide with the estimates based on geological data and also on the mineralogical geobarometers and geothermometers, and characterize different facies of the depth of these assemblages – from hyperabyssal (<10-12 km) to abyssal (>20 km). These data essentially supplement the available calculation diagrams (performed for the first time) at the expense of amphibole-free varieties. Generally, the simulated temperature values of the formation of magmatic granitoids lie in the range of crystallization of the bulk of the rocks, and those of water-saturated metasomatic granitoids are virtually similar to the temperatures in all geothermometers given the most actual values of the oxygen potential. This result illustrates the rightfulness of the suggested methods and the possibilities of its wide use in practice. The prospects of the suggested method for any regional and global geodynamic constructions are obvious, as this makes it possible to assess and compare the depth (and also the temperature and parameters of fluid regime) of formation of different geologic assemblages: metamorphic, metasomatic, and magmatic. The contemporary derivation of such information is essential for assessment of the ore potential of the region. Accordingly, further investigations along this line are necessary, incidentally, both in the field of further automatization of calculations and in application of the methods for not only granitoids at other sites but also for gabbroids. Key words: granitoids, Aldan shield, minerals, thermodynamics, modeling, geothermobarometers, Stanovoy Range, Aldan Shield, Far East. A.M. Petrishchevsky Rheological model of the South Sikhote-Alin Earth’s crust (from gravity data) The rheological properties of the Earth’s crust in the South Sikhote-Alin area were evaluated based on the analysis of the spatial density gradient distributions of spherical sources of gravity anomalies. These distributions are characterized by the existence of two highly rigid layers and two layers of lower viscosity. By solutions of the forward linear gravity problem the directly proportional relationship between the rigidity and density of the simulated media is confirmed. The lower crystalline layer of the Amur plate, whose roof gently slopes twards the Sea of Japan coast, and wedge-like slabs of oceanic crust beneath the Taukha and Kema terranes, thrust over the lower layer of continental crust, correspond to the rigid media. The subcrustal zone of partial melting in the base of the East-Sikhote-Alin volcanic belt, and accretionary complexes of the Samarka and Zhuravlevka terranes, involved later in the process of strike-slip deformations, correspond to the viscous media. The Late Paleozoic and Early Mesozoic geodynamics of the Sikhote-Alin Earth’s crust is characterized by the collision interaction of the continental and oceanic rigid wedges, which resulted in directional accretion of terrigenous thick sequences of turbidite basins and island arcs to the Amur plate. Key words: Earth’s crust, viscosity, gravity modeling, Sikhote-Alin. A.V. Konovalov, A.S. Sychov, V.N. Solovyev Mass estimates of scalar seismic moments of slight earthquake foci on the south of Sakhalin Island Frequency dependence of the function of seismic waves damping for the southern part of Sakhalin Island has been determined for the first time based on the coda of seismic waves from the near earthquakes using the model of single scattering. The algorithm of automated definition of scalar seismic moments of slight earthquake foci has been realized. Mass estimates of seismic moments have been obtained as exemplified by the aftershocks of the Aug.17, 2006 Gornozavodsk earthquake (MW 5.6) and Kostromskoye earthquake swarm events of May-June 2004, which took place on South Sakhalin. The dynamic parameters of the foci have been determined based on the SH-wave spectra adjusted for absorption and geometrical discrepancy. The loglinear relation linking the seismic moment to the local magnitude has been established, which is in good agreement with the estimates obtained in other regions, and in a certain sense does not contradict the world average dependence. Key words: seismic waves, seismic moment, small earthquakes, Sakhalin Island. N.F. Vasilenko, A.S. Prytkov, S.M. Saprygin Horizontal movements and generation of strong earthquakes in the North Sakhalin interiors Recent geodynamics of Sakhalin Island is identified by the convergence of the Eurasian and North American (Sea of Okhotsk) lithosphere plates, which is manifested in high seismic activity of the island. In the North Sakhalin interiors the plate boundary may be identified with the North Sakhalin deep fault and the Upper-Piltun fault; the latter was breaking after the M 7.2 1995 Neftegorsk earthquake. This paper has shown for the first time that the stationary movement of the Sea of Okhotsk plate is retarded on this fault and forms with time folding of injection and anomaly in the stress field. This anomaly is removed during the next (in 400-1,000 yrs) strong earthquake by seismic sliding of the Upper-Piltun fault edges. 2003-2006 GPS observations revealed the free state of this fault zone with a relative shift rate of the walls of 5-6 mm/yr. Key words: Earth’s surface horizontal movements and deformations, viscoelastic model, mechanism of strong earthquakes generation, North Sakhalin Island. T.G. Ryashenko, N.N. Ukhova, S.I. Stel'makh, N.I. Belyanina, P.S. Belyanin Hypotheses of brown loam formation in Primorye: retrospective and a new view (Russian Far East) The hypotheses of formation of brown loams in Primorye, which are widespread within the Razdolnoye-Khanka depression, are under consideration. These hypotheses were proposed in the 70-80s of the last century. Information on the loess-type rock formation of the adjacent region of the southern part of Eastern Siberia was accumulated. New analytical data on microstructure, mineral, chemical and microelement composition of brown loams were obtained from collection samples and a comparative analysis of sediments allowed us to formulate other suggestions regarding the place of brown loams in the community of loess-type rocks. Key words: brown loams, loess, Quaternary sediments, West Primorye, Razdolnoye-Khanka depression, Russian Far East. A.I. Serezhnikov Geological and hydrogeological characteristics and paleohydrogeological reconstructions of the Baley gold ore field (Transbaikal region) Baley is an epithermal lode gold-quartz deposit hosted in sedimentary Lower Cretaceous rocks. Its geology and hydrogeology have been investigated in detail. Based on these investigations, the paleohydrogeological conditions of the deposit formation are reconstructed. It was particularly conditioned by preservation of the original geological and hydrogeological structure - a graben-like depression with a basin of groundwater. The ore field was forming as a result of activity of the Early Cretaceous high-temperature hydrothermal system. The ore-bearing solutions resulted from interaction of deep fluids with rocks and local groundwater. At a depth of ~ 600 m the solutions boiled. Gold deposited from the degassed liquid phase. Key words: epithermal gold-quartz deposit, paleohydrogeological reconstruction, boiling of solutions, Baley, Transbaikal region. V.Yu. Zabrodin Paleogeography of the north-western part of the Sikhote-Alin fold system in the Mesozoic (Far East) Paleogeographic schemes (Late Triassic, Early, Middle and Late Jurassic, and Cretaceous) have been constructed for the north-western part of the Sikhote-Alin fold system. The reconstruction of the paleogeographic setting requires specification of contemporary schemes of tectonic zoning; they should be taken into account when reconstructing Late Jurassic-Cretaceous events at the plates convergence boundary along the eastern margin of Eurasia. Key words: paleogeography, Triassic, Jurassic, Cretaceous, Sikhote-Alin fold system. |