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Russian Geology and Geophysics

2008 year, number 4

1.
The genetic relationship of the deformed peridotites and garnet megacrysts from kimberlites with asthenospheric melts

L.V. Solov'evaa, Yu.G. Lavrent'evb, K.N. Egorova, S.I. Kostrovitskiic, V.N. Korolyuk b , L.F. Suvorovac
a Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
b Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
c Institute of Geochemistry, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Kimberlite; deep-seated xenolith; deformed peridotites; megacrysts; geochemistry; asthenospheric liquids
Pages: 207-224

Abstract >>
New data on geochemistry of incompatible trace elements (Nb, Zr, Hf, Ti, Y, REE) in garnets (Ga) and clinopyroxenes (Cpx) from deformed high-temperature garnet lherzolites, harzburgites, and garnet megacrysts from the Udachnaya pipe are presented. According to textures and petrographic composition, the deformed Ga-bearing peridotites are subdivided into coarse- and fine-porphyroclastic. Rocks of the first type (Ga-lherzolites) are enriched in Ga and Cpx (up to 30% each); among them, there are xenoliths with deformed Ga, Cpx, and Ol megacrysts. Deformed fine-porphyroclastic peridotites (Ga-lherzolites and Ga-harzburgites) are depleted in Ga (≤10%) and Cpx (≤3-5%). Four Ga megacrysts are typical of a Cr-poor association (0.33-1.63% Cr2O3; 0.56-1.05% TiO2; Mg# = 0.67-0.86). Garnets from deformed high-temperature coarse-porphyroclastic Ga-lherzolites are similar in geochemistry to Ga megacrysts: Their chondrite-normalized REE curves rise smoothly from La to Yb, and their spidergrams show peaks of Nb, Zr + Hf, and Ti. The narrow marginal zones of Ga grains from Ga-lherzolites are enriched in LREE. The peaks of HFSE in the Ga curves are smoothed out. The element patterns of garnet from fine-porphyroclastic Ga-lherzolites and Ga-harzburgites show a sinusoidal distribution of REE and Zr + Hf and Ti minima on the spidergrams. The element patterns of Cpx from both types of rocks are nearly the same: The curves are convex in the field of LREE, drastically drop from Nd to Yb, and have minima of Nb, Zr + Hf, and Ti. The curves of melts (L) equilibrated with Ga (calculated from partition coefficients) show a drastic enrichment in LREE and peaks of Nb, Zr + Hf, and Ti for garnet from deformed coarse-porphyroclastic lherzolites and megacrysts. The L curves of fine-porphyroclastic lherzolites and harzburgites have distinct minima of Zr + Hf and more sharply slope from Nd to Yb. The L curves of Cpx have minima of Nb, Zr + Hf, and Ti and lie near the field of basaltoid kimberlites from the Udachnaya pipe. All these data are consistent with the model of percolative fractional crystallization. The matter of mantle plume arrived from the mantle transition zone might have been the source of asthenospheric liquids inducing specific magmatic metasomatism at the continental lithosphere-asthenosphere boundary. The source material was enriched in majorite and silicate-perovskite or in ancient oceanic crustal matter.



2.
Composition and origin of peculiar pyropes from lherzolites: evidence for the evolution of the lithospheric mantle of the Siberian Platform

N.S. Tychkov, N.P. Pokhilenko, S.S. Kuligin, E.V. Malygina
Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Siberian Platform; kimberlites; LAC pyropes; lherzolites; pyroxenites; lithospheric mantle; structure
Pages: 225-239

Abstract >>
More than 1000 pyropes from the Muza (J3) and Ivushka (D-C) (northeastern Siberian Platform, and Khorkich (Mz) (southwestern part of the platform) kimberlite pipes, alluvial deposits of the Muna-Markha area, and granular peridotites of the Udachnaya pipe have been analyzed for major and some minor elements. As a result, a group of pyropes was distinguished whose composition is not typical of the lherzolite paragenesis (LAC pyropes). They are predominant in the Muza pipe and are widespread over the world. This group is described as a separate paragenetic type. In all known cases, LAC pyropes belong to granular clinopyroxene-bearing harzburgites, and in situ conditions for this suite are typically below 50 kbar and 1000



3.
Isotope dating of ultrapotassic magmatism in the central Chukchi region: Rb-Sr age and geochemistry of monzonites from the Linkor stock

S.V. Efremov, V.D. Kozlov, G.P. Sandimirova, S.I. Dril'
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Ultrapotassic magmatism; isotopic age
Pages: 240-253

Abstract >>
Isotope dating of ultrapotassic magmatism in the central Chukchi region was carried out. Based on the Rb-Sr isochron age of monzonites from the Linkor stock (107 ± 2.2. Ma) and using earlier obtained geochronological isotope data, we have estimated the time of one of the stages of ultrapotassic magmatism, 107-113 Ma. Igneous rocks of this age occur throughout the central Chikchi region, which suggests their genetic relationship with some great geodynamic event there, likely, the accretion of the Mainit island arc to the Chukchi microcontinent.
The results obtained permit the reconstruction of the geologic history of the region and can be used to construct more detailed geodynamic models.



4.
Geochemistry of metavolcanics in the southern Hцvsgцl area (northern Mongolia): geodynamic implications

A.Ya. Medvedeva, I.V. Gordienkob, M.A. Gornovaa, A.I. Almukhamedova
aVinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 1a ul. Favorskogo, 664033, Irkutsk, Russia
b Geological Institute, Siberian Branch of the Russian Academy of Sciences, 6 a ul. Sakhyanovoi, Ulan Ude, 670047, Russia
Keywords: Metavolcanics; protolith; accretionary prism
Pages: 245-253

Abstract >>
The lava sequence of the Alag-Zaar-Gol River catchment (southern Hövsgöl area, northern Mongolia) is an accretionary prism composed of metamorphosed basalt and, less often, andesite alternating with siliceous tuff. The protolith of metabasalts may have formed in island-arc or ocean-island environments, and andesites were, possibly, derived from ocean-island varieties. Volcanics of different types are found coexisting within the same thrust sheets, which may be due to thrusting in the course of postmagmatic tectonic events.



5.
Neoproterozoic basalts of the Paleo-Asian Ocean (Kurai accretionary zone, Gorny Altai, Russia): geochemistry, petrogenesis, and geodynamics

I.Yu. Safonovaa, V.A. Simonova, M.M. Buslova, T. Otab, Sh. Maruyamac
a Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
b Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan
c Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo, 152-8550, Japan
Keywords: Paleo-Asian Ocean, Gorny Altai, basalts, melt microinclusions, major and trace-element geochemistry, crystallization temperature, MORB, OIB
Pages: 254-271

Abstract >>
Late Neoproterozoic (Late Vendian) basalts of the Paleo-Asian Ocean are found as fragments incorporated in the Kurai accretionary zone, Gorny Altai, Russia. Detailed study of their geochemistry and relationships with associated sedimentary rocks allowed us to recognize three types of basalts: (1) lavas depleted in REE, Nb and Ti, compositionally similar to N-MORB and associated with thin-bedded oceanic siliceous sediments; (2) basalts of transitional compositions, and (3) basalts enriched in the above elements; basalts of types 2 and 3 are similar to Pacific intraplate basalts and are associated with carbonate



6.
The role of skeletal and nonskeletal biogenic material in the accumulation of organic matter of the Bazhenov Formation

Yu.N. Zanin, A.G. Zamirailova, V.R. Livshits, V.G. Eder
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Upper Jurassic-Lower Cretaceous; Bazhenov Formation; black shales; organic carbon; primary nature; West Siberia
Pages: 272-279

Abstract >>
Organic matter of the Upper Jurassic-Lower Cretaceous highly carbonaceous black-shale Bazhenov Formation in the central part of the West Siberian sea basin is composed of predominantly sapropelic planktonogenic material, which was earlier assumed to have derived from siliceous-skeletal microorganisms (first of all, radiolarians) and nonskeletal ones (bacteria and algae). Comparative analysis for organic carbon in the formation rocks and in radiolarites and radiolarian oozes from some other areas showed that the siliceous-skeletal organisms make up <1% of the organic matter of the Bazhenov Formation. This is confirmed by correlation analysis of two main types of Bazhenov rocks - siliceous-argillaceous (bazhenovites) and argillaceous (mudstones), - which showed the absence of correlation between the contents of organic carbon and quartz (which is mainly authigenic) for both rocks.



7.
Current crustal deformation in the northern Tien Shan: GPS and seismological data

S.A. Tychkova, O.A. Kuchaib, N.A. Bushenkovab, V.D. Braginc, Z.A. Kalmetievad
a Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 3 prosp. Koptyuga, Novosibirsk, 630090, Russia
b Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3 prosp. Koptyuga, Novosibirsk, 630090, Russia
c United Institute of High Temperatures, Russian Academy of Sciences, Science Station, Bishkek, 720049, Kyrgyzstan
d Testing Seismological Group, Kyrgyz National Academy of Sciences, 52/1 Asanbai, Bishkek, 720060, Kyrgyzstan
Keywords: Seismotectonic strain field; geodetic strain field; seismology; GPS; travel-time tomography; velocity anomaly
Pages: 280-290

Abstract >>
We study crustal deformation in the northern Tien Shan as recorded in strain rates derived from earthquake and GPS data. Geodetic strain rates indicate general shortening along the N-S component and agree with Quaternary fault slip rates and with the strain field obtained from earthquake mechanisms, all being signature of overall north-south contraction in the region.
The GPS strain field changes quickly in time, especially in the W-E direction, and is produced by a joint effect of elastic, plastic, and quasi-plastic deformation. The seismological strain field reflects the effect of external tectonic forces applied to seismogenic crust and the stress change due to crustal heterogeneity and geometry of the study area. Seismological and GPS strain rates carry information of different kinds. The former reflect formation of local structures mainly in the brittle crust, such as a local pull-apart in the center of the Lake Issyk Kul area, whereas the latter provide clues to mechanisms that drive the geological evolution of the mountain terrain as a whole. The current evolution may involve the effect of density instability at the lithosphere-asthenosphere boundary.



8.
Monte-Carlo simulation for estimating topographic disturbance to heat flow data

D.E. Ayunov, A.D. Duchkov
Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3, prosp. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Thermal logging; topographic disturbance of geothermal field; Monte-Carlo simulation; digital elevation model of Baikal bottom; topographic correction to heat flow data; Lake Baikal
Pages: 291-296

Abstract >>
The Earth's surface topography disturbs the thermal field making measured superficial thermal gradients (heat flow) higher or lower than the deep value. Topographic disturbance can be removed from data by applying the respective correction. We estimated the effect of the rugged bottom of Lake Baikal on thermal gradients from the vicinity of boreholes BDP-93 and BDP-96 drilled in the lake sediments and deep borehole L-2 on the western shore of the South Baikal basin near Listvyanka Village. The corrections were computed using a Monte-Carlo simulation algorithm specially designed to quantify the effect of surface topography on the thermal field. The performance of the algorithm was checked by a test example. The corrections turned out to be vanishing in the area of underwater drilling but significant at L-2 where the thermal gradient in the upper 1 km of rocks was about 20% underestimated. The corrected heat flow in L-2 data increased from 50 to 60 mW/m2 and approached the values measured in the central South Baikal basin.