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Russian Geology and Geophysics

2009 year, number 9

1.
Early Proterozoic diamond-bearing kimberlites of Karelia and their formation peculiarities

V.N. Ustinova, A.K. Zagainyia, C.B. Smithb, V.V. Ushkovb, E.E. Laz'koa, L.I. Luk'yanovaa, L.P. Lobkovaa
a Research Geological-Prospecting Enterprise of ALROSA Co. Ltd., 128-A Nevsky pr., St. Petersburg, 193036 Russia
b Rio Tinto, 2 Eastbourne Terrace, London, W2 6LG, United Kingdom
Keywords: kimberlite; diamond; diatreme; autolithic breccia; pyroclastic; crater phase; subaerial volcanism; Karelia; Kimozero
Pages: 739-750

Abstract >>
Early Proterozoic kimberlites of Karelia are among the most ancient diamond-bearing primary source rocks in the world. They compose the large (2.0×0.8 km) Kimozero body localized in the predicted Zaonezhskoe kimberlite field. The established and assumed occurrences of kimberlite magmatism are located within the Karelian craton, which was stabilized during the Early Archean. They are confined to the central part of a large geophysical anomaly detected by gravity, magnetic, seismic, and heat-flow studies and mark a deep-seated magma chamber. Kimberlite bodies occur within structural blocks bounded by zones of plicative-rupture dislocations.
The Kimozero kimberlites form an extensive but thin saucer-like body cut by narrow quasi-cylindrical feeders and dikes. It consists of metamorphosed kimberlites, their breccias and tuffs with widely varying amounts of mica. The body includes fragmentary fine-layered crater formations. The rocks contain olivine and phlogopite phenocrysts in an extremely altered groundmass of serpentine, chlorite, calcite, mica, and ore minerals as well as indicator minerals of kimberlites, such as Cr-spinel, manganiferous ilmenite, Cr-diopside, and rare pyrope. About 100 diamonds were extracted from 12 samples (total weight 815 kg). The crystals are colorless resorbed octahedra and, more seldom, combined octahedra-dodecahedra and spinel twins with abundant green spots caused by natural irradiation, which often make the whole crystal surface green. The diamonds contain inclusions of Mg-rich orthopyroxene and pentlandite suggestive of peridotitic lithospheric mantle derivation and dating of the sulfide inclusion implies a late Archean mantle source. By petrochemistry, the rocks are classified as kimberlites.
The Kimozero kimberlites differ from classical Phanerozoic ones in having higher Fe contents, low contents of alkalies and P2O5, and intense superimposed carbonate, magnetite, and amphibole mineralization. The saucer-like bodies with narrow feeders without developed diatremes have no analogs in Russia but are similar to the saucer-like kimberlite bodies in Canada (Fort a la Corne), India (Tokapal), and Central Africa (Bakwanga) and the West Kimberley lamproites in Australia. By analogy with these bodies and on the basis of some common petrographic features (presence of pyroclastics and specific amoeba-like autoliths, scarcity of fragments of the enclosing rocks, local reworking of the deposited matter), the Kimozero kimberlites are considered to be the products of subaerial volcanic central-type eruptions.



2.
Geochemistry and SHRIMP geochronology of alkaline rocks of the Zijinshan massif in the eastern Ordos basin, China

Yang Xingkea, Chao Huixiaa, N.I. Volkovab, Zheng Menglin c , Yao Weihua d
a Key Laboratory of Western China's Mineral Resources and Geological Engineering, Ministry of Education, Chang'an University, 126 Yanta Road, Xi'an, 710054, China
b Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
c Research Institute of Prospecting and Development, SINOPEC, 20 Xueyuan, Beijing, 100083, China
dResearch Institute of Exploration and Development, Changqing Oilfield Company of CNPC, 151 Weiyang Road, Xi'an, 710021, China
Keywords: Ordos basin; magmatic activity; tectonic thermal event; Early Cretaceous; Zijinshan; SHRIMP chronology
Pages: 751-762

Abstract >>
Characteristics of geology, petrology, and geochemistry of the Zijinshan massif were studied in the eastern part of the Ordos basin. Geochemical analysis shows that the massif is characterized by high alkali, relatively high Fe, and low Mg and Ca contents. The rocks are undersaturated in SiO2, rich in REE (with no Eu anomaly) and belong to the alkaline-peralkaline series. The geologic history of the Zijinshan massif consists of several stages of magmatism. The obtained isotope-geochronological (U-Pb SHRIMP) data show that the magmatic activity climaxed in the interval 150-110 Ma, while the age of 16 zircon grains fitted a narrower interval, 132-125 Ma, i.e., the Early Cretaceous. The younger age corresponds to the middle and late stages of the evolution of the Ordos basin and agrees with a large Early Cretaceous tectonothermal event in North China. This event led to the large-scale uplift of the eastern flank of the Ordos basin, rise of the Lüliang asthenosphere, and to the formation of a large west-sloping monocline. The U-Pb SHRIMP studies have also revealed magmatic zircons of Carboniferous-Permian age, which evidences the multistage character of the thermal process.



3.
Melt migration in the mantle beneath spreading zones and formation of replacive dunites: a review

V.G. Batanovaa, G.N. Savelievab
a V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 ul. Kosygina, Moscow, 119991, Russia
b Geological Institute, Russian Academy of Sciences, 7 Pyzhevskii per., Moscow, 119017, Russia
Keywords: mantle peridotites; melt migration; reactive porous flow; replacive dunites; ophiolites
Pages: 763-778

Abstract >>
The state of the art of the problem of mantle melt transport through the upper mantle beneath spreading zones is reviewed. The geochemical data and physical properties of the mantle constraining the melt migration processes are considered. The review concentrates on the ways of mantle magma transport and mechanisms of the localization of diffuse intergranular porous melt percolation into the channel flow. The hypothesis of the reaction of migrating mantle magmas with wall peridotites and the formation of replacive dunites as a result of this process are discussed. We examine the publications on the field, theoretical, and geochemical studies of the reactive melt migration and its role in the dunite formation in the mantle sections of the ophiolites. Alternative models of the dunite origin in the mantle are also tested. The results of experimental studies of the melt/peridotite interaction are presented.



4.
Geochemistry, petrogenesis, and geodynamic typification of metavolcanics of the Tunka terrane(Baikal-Hцvsgцl region)

S.I. Shkol'nik*, L.Z. Reznitskii, V.G. Belichenko, I.G. Barash
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033 Russia
Keywords: geochemistry of metavolcanics; back-arc basin basalts; Tunka terrane
Pages: 779-788

Abstract >>
The geochemical composition of metavolcanics (metabasalts and metaandesites) of the Tunka terrane is considered. They are differently enriched in incompatible elements relative to N-MORB. The geochemical features of the basalts (LILE and LREE enrichment, Zr and Hf negative anomalies) point to the suprasubductional nature of synsedimentary volcanism; at the same time, the rocks lack negative Nb, Ta, and Ti anomalies. Comparison of the Tunka metavolcanics with those of modern geodynamic settings shows their analogy to back-arc basin basalts.



5.
Structural features of the northern West Siberian geosyneclise and new exploration targets

I.A. Plesovskikh, I.I. Nesterov(Jr.), L.A. Nechiporuk, V.S. Bochkarev
Siberian Scientific-Analytical Center, 46 ul. Permyakova, Tyumen', 625016, Russia
Keywords: West Siberian geosyneclise; basement; cover; Paleozoic; Triassic; Jurassic; oil; gas; resources; Yamal-Nenets Autonomous Okrug
Pages: 789-796

Abstract >>
The northern part of the West Siberian geosyneclise is characterized by a thick sedimentary cover and widespread Triassic sedimentary and volcanosedimentary rocks and Paleozoic platform structures. New targets have been recognized in the basement and deeply buried horizons of the geosyneclise cover. Reservoirs might be found in the following formations: Paleozoic cover deposits, weathering crusts, zones of Paleozoic rock deconsolidation, Triassic sedimentary and volcanosedimentary deposits, buried structures in the lower part of the cover, Lower and Middle Jurassic basal layers, pinch-outs of Jurassic horizons, Upper Jurassic bituminous shales and cavernous carbonates. Exploration of these potential structures will change the structure of the existing resource base toward the long-term replenishment of hydrocarbon resources and a stable rate of production replacement.



6.
Computer simulation of Neocomian clinoform reservoirs in northern and arctic West Siberia

S.V. Ershov, G.F. Bukreeva, V.O. Krasavchikov
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Neocomian; clinoform sequence; computer simulation; regional map; West Siberia
Pages: 797-807

Abstract >>
Neocomian reservoirs in the Mesozoic sedimentary cover of West Siberia have a complex geological structure. Their wedge-like (clinoform) geometry, with abrupt thickness changes and facies diversity (continental, shoreface, and pelagic deposits), causes difficulty in correlation between drilling- and seismics-based sand formations and clay caps. East-wedging (west-dipping) clinoforms consisting of interbedded clay and sand have the greatest reservoir potential in the West Siberian basin. Prediction of new oil and gas plays and their reservoir potential assessment require regional reconstructions in addition to local contour maps of individual zones and fields. However, the simulation technology which has been applied for years to Jurassic regional reservoirs is not fully applicable to the Neocomian clinoform sequences. Therefore we have adapted it correspondingly and suggest new tools to make due regard for the clinoform reservoir structure. The new approach has been tested through computing several structure and thickness contour maps of clinoform sequences for three largest regional Neocomian reservoirs (sub-Sarman, sub-Pim, and sub-Alymka) in northern and arctic West Siberia.



7.
Nonlocal response functions: application to processing BEAR data

V.V. Plotkin, A.Yu. Belinskaya, P.A. Gavrysh
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: regional magnetotelluric soundings; apparent resistivity; lateral heterogeneity of conductivity; synchronous array data; nonlocal response function; Fennoscandian shield
Pages: 808-814

Abstract >>
It has been suggested that regional array MTS data be processed with an algorithm based on the nonlocal response of laterally heterogeneous subsurface to an arbitrary electromagnetic excitation. The algorithm is tested with synthetic models which show that the inversion quality can be improved by accumulation of information through repeated measurements, at either changing or fixed array configurations. Besides the array configuration, the inversion quality depends on the selected size of the survey area. The choice of the area size defines the inversion conditions in terms of choice between the magnetic and electric mode for correlation of electromagnetic potentials. The algorithm has been applied to data acquired during the BEAR Project in the Fennoscandian shield. Preliminary results indicate a spatial correlation between conductivity minimums and Moho depth maximums. The apparent resistivity maps obtained with the suggested algorithm highlight the boundaries between the Lapland-Kola, Karelia, and Svecofennia tectonic provinces.



8.
Thermal history, geodynamics, and current thermal activity of lithosphere in China

S.V. Lysak
Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: heat flow; thermal history; active fault; rift; crustal thickness; lithospheric thickness; thermal activity
Pages: 815-826

Abstract >>
The tectonic framework of China includes major and smaller-scale units that differ in age and in style of tectonomagmatic activity, the latter being related to the thermal history of the lithosphere. Heat flow in the area varies from 25 to 150 mW/m2 or higher, with an average of 58±11 mW/m2. It is high in active faults, rifts, and other structures of extension (or sometimes compression) subject to heating from rising lithospheric and mantle plumes. The current thermal activity in the region is controlled by the Pacific subduction beneath Eurasia in eastern China and mainly by the lateral strain and rotation of the Ordos block associated with the India-Eurasia interaction in central and western China.