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Russian Geology and Geophysics

2012 year, number 12

1.
MIGRATION PATHS OF MAGMA AND FLUIDS AND LAVA COMPOSITIONS IN KAMCHATKA

N.L. Dobretsov, I.Yu. Koulakov, Yu.D. Litasov
Keywords: Seismic tomography, subduction, volcanic arcs, lava compositions, melting zone, migration of melts and fluids, Kamchatka

Abstract >>
Geophysical and geochemical data have been analyzed jointly in order to gain better understanding of subduction-related active volcanism in Kamchatka. The velocity structure of lithosphere beneath volcanic arcs has been imaged on three scales. Regional tomography to distances of thousands of kilometers has allowed constraints on slab geometry, which changes markedly in dip angle and thickness beneath the Kuriles-Kamchatka arc, possibly, because of a change in the interplay of the subduction driving forces. Intermediate-scale regional tomography (hundreds of kilometers) has been applied to the cases of Toba caldera in Sumatra, Mount Merapi in Java, and volcanoes in the Central Andes and provided evidence of magma conduits marked by low-velocity zones that link the suprasubduction volcanic arcs with clusters of earthquake hypocenters on the slab top. Local tomography resolves the shallow structure immediately under volcanoes and the geometry of respective melting zones. An example time-lapse (4D) seismic model of the crust beneath the Klyuchevskoy group of volcanoes has imaged a decade-long history of anomalous velocity zones and their relation with the activity cycles of Bezymyanny and Klyuchevskoy Volcanoes. As modeling shows, andesitic Bezymyanny and basaltic Klyuchevskoy Volcanoes have different feeding patterns during their eruption cycles: The former feeds directly from the mantle, while the material coming to the latter passes through a complicated system of intermediate chambers. The local tomography model has been applied as reference to interpret the available major- and trace-element data from the Klyuchevskoy and Bezymyanny Volcanoes. The lava compositions of the two volcanoes have becoming ever more proximal since 1945 in many major and trace elements, while some parameters remain different. Paroxysmal eruptions of Bezymyanny for several recent decades correlate with the time when Klyuchevskoy erupted lavas with high percentages of high-Mg basalts. The difference in the evolution trends of the Kamchatka volcanic rocks may be due either to fractional crystallization or to the presence of concentrator minerals in the source, titanomagnetite, orthopyroxene, rutile, garnet, and plagioclase being especially active as to uptake of some elements. The natural compositions of rocks have been compared in this context with published experimental data. According to the seismic-velocity structure and lava compositions analyzed jointly, there are five levels of crystallization beneath the studied volcanoes, while the number and spatial patterns of magma sources are different for two types of andesitic volcanoes.



2.
THE LOMONOSOV RIDGE AS A NATURAL EXTENSION OF THE EURASIAN CONTINENTAL MARGIN INTO THE ARCTIC BASIN

V.A. Poselov, G.P. Avetisov, V.V. Butsenko, S.M. Zholondz, V.D. Kaminsky, S.P. Pavlov
Keywords: Junction zone, geological and geophysical studies, Lomonosov Ridge, Arctic Ocean

Abstract >>
The integrated geological and geophysical studies carried out in recent years in the Lomonosov Ridge and at its junction with the Eurasian shelf revealed evidence for thinned (reduced) crust in the ridge (20–25 km) and its genetic relationship with shelf structures. We compared the parameters of deep seismic cross-sections of the shelf and Lomonosov Ridge, thus proving the existence of continental crust in the latter. Also, we analyzed the deep structure of the junction between the Lomonosov Ridge and the shelf and established a genetic geologic relationship, with no evidence that the Lomonosov Ridge moved as a terrane with respect to the shelf. In addition, seismological studies independently confirm the genetic relationship between the Lomonosov Ridge and the adjacent shelf. The Lomonosov Ridge is a continental-crust block of a craton. The craton was reworked during the Caledonian tectonomagmatic activity with the formation of a PrecambrianCaledonian seismically unsegmented basement (upper crust) and an epi-Caledonian platform cover. Afterward, the block subsided to bathyal depths in the Late Alpine. This block and the adjacent areas of the Eastern Arctic shelf developed in the platform regime till the Late Mesozoic.



3.
Protoliths of Paleoproterozoic calciphyres from the Irkut block (Sharyzhalgai uplift of the Siberian craton): composition and origin

L.N. Urmantseva, O.M. Turkina, I.N. Kapitonov
Keywords: Calciphyres, geochemistry, isotopic composition, zircon, UPb age, Sharyzhalgai uplift, Siberian craton

Abstract >>
The paper presents data on high-grade silicate-carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and alumina-rich (cordierite- and sillimanite-containing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous component ranges in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary unit. The geochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.42.7 Ga) of the calciphyres and the value THf(DM-2st) = 2.53.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ~1.87 Ga. This is close to the previous age estimates for the terrigenous-rock metamorphism (1.851.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).



4.
METASOMATIC PARAGENESES IN DEEP-SEATED XENOLITHS FROM THE UDACHNAYA AND KOMSOMOLSKAYA-MAGNITNAYA PIPES AS INDICATORS OF FLUID TRANSFER THROUGH THE MANTLE LITHOSPHERE OF THE SIBERIAN CRATON

L.V. Soloveva, T.A. Yasnygina, K.N. Egorov
Keywords: Mantle lithosphere, metasomatism, trace elements, fluids, diamonds, Siberian craton

Abstract >>
The petrography, major element, and trace element (TE) compositions of minerals from two types of modal metasomatites (metasomatized peridotites and pyroxenites) from the Udachnaya and Komsomol’skaya-Magnitnaya kimberlite pipes, Yakutia, have been studied. It is shown that texturally and chemically equilibrated metasomatites A consist of a set of superimposed minerals: phlogopite + diopside ± ilmenite apatite sulfides graphite. Their major and trace element compositions have specific features. The contents of TEs in garnet and clinopyroxene from these metasomatites are close to those in garnet and clinopyroxene from low-temperature coarse-grained peridotites richest in TEs. The distribution of a significant portion of TEs between garnet and clinopyroxene from A -type metasomatites and from coarse-grained lherzolites rich in TEs is close to experimental values reported for minerals coexisting with carbonatitic and basaltic fluids. We assume that this metasomatic process was nearly synchronous with the global metamorphism and cratonization of the mantle lithosphere and that high-density silicate–carbonate fluid melts were metasomatizing agents.
Another large mantle metasomatism process in the lithosphere of the Siberian craton was associated with the Middle Paleozoic kimberlite magmatic event, induced by the Yakutian thermochemical plume. Metasomatic minerals (Mg phlogopite + Cr diopside + chromite sulfides graphite) intensely replaced the minerals of the primary paragenesis, particularly, garnet. These reaction metasomatites show a sine-shaped REE pattern in garnet and disequilibrium between garnet and clinopyroxene. It is supposed that the reaction metasomatism in the mantle lithosphere of the Siberian craton was associated with ingress of reduced asthenospheric fluids at the early stages of the kimberlite formation cycle. Metasomatic graphite formed in metasomatites of both types, and this fact evidences for two diamond formation epochs in the history of the mantle lithosphere of the Siberian craton.



5.
CONCENTRATION OF TRACE ELEMENTS IN SMALL LAKES OF THE INGODA BASIN (Chita Region, Russia)

O.A. Sklyarova, E.V. Sklyarov, Yu.V. Menshagin
Keywords: Saline lakes, trace elements, evaporation, waterrock interaction

Abstract >>
By the example of the Ingoda basin (Transbaikalia) including a system of small lakes, the role of two leading processes of concentration of trace elements in small reservoirs (water–rock interaction and evaporation) has been estimated. It is shown that the accumulation of U, As, Se, Sb, and Ge does not depend on the kind of concentration. Their contents increase linearly with total dissolved solids in both lake and feeding waters. Accumulation of Sc, V, Y, Th, REE, and W is mainly due to evaporation, with their maximum concentration by three to four orders of magnitude. Only U, W, and REE, whose contents in the lakes reach n·100 μg/l, are of practical interest.



6.
A RELATIONSHIP BETWEEN THE Source OF THE 25 MAY 1887 EARTHQUAKE AND THE FAULT ALONG THE BILIKTUIKA RIVER VALLEY (southern Siberian Platform)

A.S. Gladkov, Ya.B. Radziminovich, O.V. Lunina
Keywords: Tectonic deformation, earthquakes, southern Siberian Platform

Abstract >>
Several active fault zones were studied in the southern Siberian Platform along the Kovykta–SayanskIrkutsk gas pipeline. Late Cenozoic seismogenic faults are observed here. The fault zone in the Biliktuika River valley shows the strongest displacements and deformations. The radiocarbon dating of buried soil horizons for this fault was compared with seismological data on the earthquakes observed in this area. The comparison permitted attributing the latest slip along the fault to the 25 May 1887 earthquake.



7.
THE 1912 EARTHQUAKE IN SOUTH BAIKAL: TRACES IN BOTTOM SEDIMENTS AND GAS RELEASE INTO THE WATER COLUMN

E.G. Vologina, M. Sturm, Ya.B. Radziminovich, S.S. Vorobeva, A.A. Shchetnikov
Keywords: Earthquake, methane, bottom sediments, magnetic susceptibility, diatoms, rate of recent sedimentation, Lake Baikal

Abstract >>
Large earthquakes took place in southern Cisbaikalia in the first half of 1912. They might have caused a mass release of large amounts of gas (methane?) into the water of Lake Baikal and the atmosphere near Sharyzhalgai station of the Circum-Baikal Railroad. This phenomenon was observed in August 1912 by the residents as rising water columns several meters high and reported in the regional press.
To find traces of this event, core was recovered from bottom sediments at a depth of 1300 m in winter 2010. The depth interval 1–8.7 cm is a homogeneous layer, no more than 100 years old (210Pb dating). The sediments here are poor in SiO2biog but richer in Corg than the underlying sediments. Also, they are marked by a considerable content of terrestrial plant remains, a lower content of planktonic diatoms, and higher contents of benthic and ancient diatoms. These data indicate that the layer under study formed as a result of the 1912 earthquake, with a considerable contribution from the littoral and shallow-water zones of Lake Baikal.



8.
RECONSTRUCTION OF THE HOLOCENE CLIMATE BASED ON A CARBONATE SEDIMENTARY RECORD FOR SALINE LAKE VERKHNEE BELOE (western Transbaikalia)

E.P. Solotchina, E.V. Sklyarov, P.A. Solotchin, E.G. Vologina, V.N. Stolpovskaya, O.A. Sklyarova, N.N. Ukhova
Keywords: Carbonates, XRD analysis, IR spectroscopy, modeling, stable isotopes, geochemistry, saline lake, paleoclimate, Holocene, western Transbaikalia

Abstract >>
We present results of mineralogical and crystallochemical studies of the Holocene carbonate sediments of a small saline lake localized in the Borgoi dry-steppe region, western Transbaikalia. Mg-calcites with a varying Mg content are predominant in the assemblage of endogenic carbonate minerals from bottom sediments. Mathematical modeling of the XRD spectra of carbonates permitted us to identify excess-Ca dolomites, which are an indicator of a shallow (playa) lake. The studies showed that the lacustrine Mg-calcites do not form a continuous series from low- to high-Mg varieties. We discuss the cause of this phenomenon and also consider the existing viewpoints of the structure of low-temperature Mg-calcites and excess-Ca dolomites and their formation conditions in lacustrine sediments. Juxtaposing the carbonate record with the data of lithological analysis, determined stable isotopes (δ18O and δ13C), and distribution of some geochemical indicators of climatic changes, we reconstructed the intricate evolution of Lake Verkhnee Beloe, which was controlled by the regional climate since the postglacial period till the present day.



9.
CLIMATE RECONSTRUCTION IN THE URALS FROM GEOTHERMAL DATA

I.V. Golovanova, R.Yu. Salmanova, D.Yu. Demezhko
Keywords: Geothermy, temperature, inversion, paleoclimate, climate reconstruction, Urals

Abstract >>
Results of paleoclimatic analysis of geothermal data in the Middle and Southern Urals for different time intervals are presented. Climate reconstruction for the past millennium was made using data from 44 boreholes, and the magnitude of the Wurm–Holocene warming event was estimated based on data from two deep boreholes. The method of functional space inversion was used. The resolution of the method for reconstruction of various climatic events in the past was investigated. Parameters specified a priori and the required duration of the period to be reconstructed were chosen from the results of numerical modeling. According to the inversion results, the ground surface temperature at the maximum of the Medieval Warm Period in 11001200 was approximately the same as the present temperature, and at the minimum of the Little Ice Age around 1720, it was 1.23 ºC lower than at present. The subsequent temperature rise was more pronounced in the past century. The magnitude of the WurmHolocene warming event, reconstructed using data from two deep boreholes, is 1011 ºC.



10.
HIGH-MOLECULAR HETEROATOMIC COMPONENTS OF CRUDE OILS OF THE TIMANPECHORA PETROLIFEROUS BASIN

A.K. Golovko, V.F. Kamyanov, V.D. Ogorodnikov
Keywords: Crude, oil, petroleum resins, petroleum asphaltenes, molecular structure

Abstract >>
Based on the experimentally measured parameters of elemental composition, average molecular masses, and PRM spectrometry data, we calculated the average structural parameters of resin and asphaltene molecules in Paleozoic oils of some oilfields in the Timan–Pechora petroliferous basin. The values of the structural parameters of cyclic and aliphatic fragments in the molecules of resinasphaltene components of the crudes studied here are within the ranges established earlier for high-molecular compounds of crudes from other petroliferous basins.
The average structural-group characteristics of resins in crudes from carbonate reservoirs vary over narrow ranges of values independently of the age and depth of occurrence of the reservoir.
The resinous fractions of crudes from terrigenous horizons show symbatic enrichment with paraffin fragments and depletion with naphthene fragments of molecules with depth. This points to the presence of a paragenesis of petroleum hydrocarbons and heteroatomic compounds in the Earth’s interior.