Home – Home – Jornals – Russian Geology and Geophysics 2012 number 10

We present a new three-dimensional model of P-velocity anomalies in the upper mantle beneath the Circum-Arctic region based on tomographic inversion of global data from the catalogues of the International Seismological Centre (ISC, 2007). We used travel times of seismic waves from events located in the study area which were recorded by the worldwide network, as well as data from remote events registered by stations in the study region. The obtained mantle seismic anomalies clearly correlate with the main lithosphere structures in the Circum-Arctic region. High-velocity anomalies down to 250-300 km depth correspond to Precambrian thick lithosphere plates, such as the East European Platform with the adjacent shelf areas, Siberian Plate, Canadian Shield, and Greenland. It should be noted that lithosphere beneath the central part of Greenland appears to be strongly thinned, which can be explained by the effect of the Iceland plume which passed under Greenland 50-60 million years ago. Beneath Chukotka, Yakutia, and Alaska we observe low-velocity anomalies which represent weak and relatively thin actively deformed lithosphere. Some of these low-velocity areas coincide with manifestations of Cenozoic volcanism. A high-velocity anomaly at a depth of 500-700 km beneath Chukotka might be a relic of the zone of subduction which occurred here about 100 million years ago. In the oceanic areas, the tomography results are strongly inhomogeneous. Beneath the North Atlantic, we observe very strong low-velocity anomalies, which indicate an important role of the Iceland plume and active rifting in the opening of the ocean basin. On the contrary, beneath the central part of the Arctic Ocean, no significant anomalies are observed, which implies a passive character of rifting.

The study presents a description of a new type of Cambrian section penetrated in Vostok 4 well in the southeastern part of the West Siberian geosyneclise (Krasnoyarsk Territory). The drilled succession is well characterized by the fossil evidence and can be considered a key section for the east of the WSG. The section comprises all three divisions of the Cambrian and is subdivided into the Oksym, Tyya, Averin, Kolchum, and Evenki Formations, based on the composite log data for an interval of 2262-5105 m. The Oksym Formation (the age equivalent of the Usol'e Formation) and Kolchum Formation are first identified and described in this study. The section is mostly carbonate, argillaceous-carbonate, sulfate-carbonate, and terrigenous sediments deposited in a back-reef setting of the Yenisei salt-bearing basin.

A volcanosedimentary complex of Middle-Late Devonian deposits in northwestern Rudny Altai is described. Analysis of sedimentologic processes, the kind of manifestation and specific composition of volcanism, and petrographic and chemical compositions of sedimentary rocks permitted reconstruction of paleogeographic and geodynamic settings, which confirmed the hypothesis of the island-arc nature of the Rudny Altai zone in the Middle-Late Devonian. Also, an alternative variant of its rift nature is considered.
The rift buildings are referred to as local isolated complexes confined to positive paleovolcanic structures rising above the bottom in the fore-arc basin. The denudated part of the rift system is formed mainly by slope facies. Clastics of rift buildings, together with fragments of silicite and felsic-volcanics layers, compose widespread olistostrome breccias. We have established the humid type of lithogenesis, which, together with minor sea level fluctuations, favors karst processes and the formation of red-colored and bauxite-like rocks. The coeval volcanics composing lava nappes and tuff layers are mainly rhyolites and are often of ultrafelsic composition.

Despite the long history of research, the presence of Precambrian complexes in the West Siberian basement has not been proven. The Tyn'yarskaya 100 and Tyn'yarskaya 101 wells were drilled in the Vakh-Elogui interfluve, in the eastern West Siberian Plate (eastern Khanty-Mansi Autonomous District). At a depth of 1790 m, they stripped a rhyolite extrusion, which graded into A-type alkali granitoids with rare-metal and REE mineralization (thorite, thorogummite, pitchblende, REE-carbonates, chevkinite, and others) downsection. This volcanoplutonic complex is Early Permian (K-Ar age, ~270 Ma; Rb-Sr age, 275.7 Ma; Sm-Nd age, 276 Ma; U-Pb age, 277 Ma). Some zircons from granites are much older (2049 ± 23 Ma, SHRIMP II), suggesting a relationship between the Early Permian granitic magma and the ancient matter. This might have been a granite-metamorphic basement, the partial melting of which produced the Tyn'yar rhyolite-granite body. The Sm-Nd model ages also suggest the participation of a Precambrian substratum in the formation of the rocks under study. Thus, it is quite possible that the Tyn'yar area is underlain by a Proterozoic (~2 Ga) sialic basement, which is an edge of the Siberian Platform thinned by Late Proterozoic-Early Paleozoic rifting and extension.

Diamond from metaultramafic rocks of the Mesoarchean (2.96-3.0 Ga) Olondo greenstone belt, located in the western Aldan-Stanovoy shield, has been studied. Diamonds occur in lenses of olivine-serpentine-talc rocks within metaultramafic rocks of intrusive habit, whose composition corresponds to peridotite komatiites. All diamonds from the metaultramafic rocks are crystal fragments 0.3 to 0.5 mm in size. Morphological examination has revealed laminar octahedra, their transitional forms to dodecahedroids, crystals with polycentric faces, and spinel twins. The crystals vary in photoluminescence color: dark blue, green, yellow, red, or albescent. Characteristic absorption bands in crystals point to nitrogen impurities in the form of A and B1 defects and tabular B2 defects. The crystals studied belong to the IaA/B type, common among natural diamonds. The overall nitrogen content varies from <<100 to 3800 ppm. The relative content of nitrogen in B1 centers varies from 0 to 94 %, pointing to long stay in the mantle. The carbon isotope ratio in the diamonds, δ¹³ C = -26 ‰, is indicative of involvement of subducted crust matter in diamond formation in the Archean.

Within the Belomorian eclogite province, near Gridino Village, rocks of different compositions (tonalite-trondhjemite-granodioritic gneisses, granites, mafic and ultramafic rocks) were metamorphosed. The metamorphism included subsidence with increasing pressure and temperature, an eclogite stage, decompression in the granulitic facies, and a retrograde stage in the amphibolitic facies. We attempted to characterize the succession and to date igneous and metamorphic events in the evolution of the Gridino eclogite association. For this purpose, we conducted the following studies: U/Pb isotope dating of zircon (conventional and SHRIMP II methods) from gneisses, a mafic dike, and a high-pressure granite leucosome; U/Pb dating of rutile from mafic dikes; ⁴⁰Ar/³⁹Ar dating of amphiboles and micas; and Sm-Nd studies of rocks and minerals. The Sm-Nd model ages of felsic (2.9-3.1 Ga) and mafic (3.0-3.4 Ga) rocks from the Gridino eclogite association and individual magmatic zircons with an age of ~3.0 Ga indicate the Mesoarchean age of metamorphic-rock protoliths. The most reliable result is the upper age bound of eclogitic metamorphism (2.71 Ga), which reflects the time of the posteclogitic decompression melting of eclogitized rocks under high-pressure retrograde granulitic metamorphism. The mafic dikes formed from 2.82 Ga to 2.72 Ga, most probably, at 2.82 Ga, in accordance with the age of crystallization of magmatic zircons from metagabbro. Amphibolitic-facies superimposed metamorphism and the "final" exhumation of metamorphic complexes at 2.0-1.9 Ga are associated with the later Svecofennian tectonometamorphic stage. Successive cooling of metamorphic associations to 300?C at 1.9-1.7 Ga is shown by U-Pb rutile dating and ⁴⁰Ar/³⁹Ar mica dating.

To study the behavior of macrocomponents and admixtures during the fractional crystallization of sulfide melts and the influence of As on noble metals in this process, we performed a quasi-equilibrium directional crystallization of melt of composition (at.%): Fe - 35.5, Ni - 4.9, Cu - 10.4, and S - 48.3, with admixtures of Pt, Pd, Rh, Ru, Ir, Au, Ag, As, and Co (each 0.1 at.%), which imitates the average (by Cu contents) compositions of massive ores at the Noril'sk Cu-Ni deposits. The following sequence of phase formation from melt has been established: mss (zone I) / mss + iss (zone II) / iss (zone III) (mss is (Fe₂Ni_{1- z} )S_1+δ, iss is (Fe_xCu_y Ni_1-x-y ) _z S_1-z ); it corresponds to the distribution of main elements along the sample (primary zoning). Distribution curves for macrocomponents in zones I and II of the sample were constructed, as well as the dependencies of their partition coefficients ( k ) between solid solutions and sulfide melt on the fraction of crystallized melt.
The secondary (mineral) zoning resulted from subsolidus phase transformations has been revealed. Five subzones have been recognized: mss + cp (Ia) / mss + cp + pn (Ib) / mss + pc + pn (IIa) / mss + pc + pn + bn (IIb) / pc + bn + pn + unidentified microphases (III).
Admixture species in the sample were studied: (1) admixtures dissolved in primary solid solutions and in main minerals resulted from solid-phase transformations and (2) admixtures forming their own mineral phases. The partition coefficients of Co, Rh, and Ru (mss/L), Ru, Ir, and Rh (mss/cp), and Co, Rh, and Pd (mss/pn) were determined.
Minerals of noble metals have been recognized: Pt₃Fe, PtFe, Au, (Ag,Pd), (Au,Pt), Ag, Ag₃Cu, Au₃ (Cu,Ag,Pd,Pt), etc., and the regularities of their distribution in the sample have been established.
It is shown that some noble-metal admixtures are prone to interact with As. Mineral arsenides and sulfoarsenides of noble metals produced during fractional crystallization have been recognized: PtAs₂, Pd₃As, (RhAsS), (IrAsS), and (Ir,Rh)AsS. The discovered drop-like inclusions of noble-metal arsenides suggest the separation of the initial sulfide-arsenide melt into two immiscible liquids.
By indirect features, the micromineral inclusions are divided into primary, crystallized from melt, and secondary, produced in solid-phase reactions.
The results of study are compared with literature experimental data obtained by the isothermal-annealing method and with the behavior of noble metals and As during the formation of zonal massive orebodies at the Noril'sk- and Sudbury-type deposits.

We studied the geologic position, geodynamic setting, petrology, and geochemistry of veined lepidolitic granitoids from the Mungutiyn Tsagaan Durulj (MTsD) occurrence (central Mongolia), found within the area of Mesozoic intraplate rare-metal magmatism. It has been established that their trace-element enrichment resulted from the intense effect of fluids rich in F, K, Li, Rb, Cs, Sn, Be, and W, which arrived from a deep magma chamber of rare-metal granitic melts, on leucogranites with originally weak rare-metal mineralization. Very high contents of F, rare alkaline metals, Sn, Be, and W, characteristic of MTD granitoids, are close only to those in greisens of rare-metal granites and topaz-lepidolite-albitic pegmatites. The difference from the greisens in each case might be due to the features of the original rocks. The difference between the greisenized MTD leucogranites and the topaz-lepidolite-albitic pegmatites is more radical: Along with evident petrographic distinctions, it includes an evolution trend toward the albite norm decrease, not typical of igneous Li-F rocks; rock shearing and gneissosity, which must have contributed to their chemical transformation according to this trend; and stably lower contents of Nb and Ta (trace elements which usually accumulate during crystallization fractionation of F-Li granitic melts and are poorly soluble in magmatic fluids). The greisenized MTD granitoids are not only high-grade rare-metal ores of Li, Rb, F, and Sn but are also regarded as an indicator of a deep concealed pluton of rare-metal granites.

The aftershock process induced by the Ms = 7.0 Uureg Nuur earthquake, one of the largest events in the Altai, has been studied comprehensively. As an additional experiment, a temporary local network of seismic stations was deployed in 2006 in the epicentral area of the earthquake to gain more insights into the current tectonic activity. The aftershocks of the Uureg Nuur event were restricted to small faults in the interior of fault blocks rather than those being localized along border faults. Seismic activity across the directions of large faults has apparently been generated by a fault (in the Tsagaan Shuvuut Range) reactivated during the Uureg Nuur earthquake. The aftershock process, at its final phase, involved an adjacent crust block.

The Balei earthquake of 6 January 2006 ( M _W = 4.5) was felt over a large part of Transbaikalia. Judging by its updated source parameters (earthquake mechanism, seismic moment, and moment magnitude), the event was generated by the Balei-Darasun fault reactivated in the Cenozoic. Exhaustive macroseismic evidence has been collected for the first time from the study area. The reported results fill up the gap in the seismological knowledge of eastern Transbaikalia and can be used for seismic risk mapping and earthquake prediction.

In an effort to develop and improve potential geophysical field interpretation, we suggest that the singularity method should be extended toward areal data. This procedure enables mass and fast calculation of singularities in area and stable solutions. Studying behavior of model fields and localization of singularities in the phase planes of vector fields permits the use of findings in interpreting geopotential fields. Model and real-life cases of this procedure as applied to tectonic dislocations and local geologic features are exemplified. We show that the proposed procedure significantly enhances the application of the singularity method to interprete geopotential fields.