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Russian Geology and Geophysics

2012 year, number 3

1.
EVOLUTION OF THE CENTRAL ALPINE-HIMALAYAN BELT IN THE LATE CENOZOIC

V.G. Trifonova, T.P. Ivanovab, and D.M. Bachmanova
aGeological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017, Russia
bInstitute of Geosphere Dynamics, Russian Academy of Sciences, Leninskii pr. 38/6, Moscow, 117334, Russia
Keywords: Orogeny, asthenosphere, collisional shortening, volcanism
Pages: 221-233

Abstract >>
The Late Cenozoic geodynamics of the Alpine-Himalayan belt comprised the collision between continental-lithosphere plates and blocks and the effect of the Neotethyan active residual asthenosphere, which reached the northern margin of the belt after the ocean had closed. From the late Eocene to the early Pliocene, strong deformation, lateral migrations of flaked plates, metamorphism, and magmatism (they all consolidated the crust) took place in the lithosphere with the participation of mobile asthenospheric components. In the Pliocene-Quaternary, the asthenosphere beneath the consolidated crust partly replaced the dense mantle lithosphere with remaining paleoocean mafic rocks, which subducted into the mantle. Phase transformations and deformations in the subducting metamafic slabs caused mantle earthquakes. The less compact metamafic rocks experienced metamorphic softening under the effect of the asthenosphere and incorporated into the Earth's crust. The upper-mantle and lower-crust softening led to a drastic intensification of uplifting and the formation of mountain ranges. Recent volcanism is also attributed to the activity of the Neotethyan asthenosphere.



2.
THE CONSERVATION OF AN AQUEOUS FLUID IN INCLUSIONS IN MINERALS AND THEIR INTERSTICES AT HIGH PRESSURES AND TEMPERATURES DURING THE DECOMPOSITION OF ANTIGORITE

A.I. Chepurov, A.A. Tomilenko, E.I. Zhimulev, V.M. Sonin, A.A. Chepurov, S.V. Kovyazin, T.Yu. Timina, and N.V. Surkov
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Subduction, serpentine, high pressure, high temperature, fluid inclusions
Pages: 234-246

Abstract >>
An experimental study of serpentine decomposition at high pressure (4.5 GPa) was carried out to elucidate if water can be preserved in the system in the form other than structural admixtures in minerals. This problem is of interest because it is water that plays a leading role in the melting in a subducted slab and a mantle wedge. To estimate the possible content of an aqueous fluid in deep-seated rocks, a BARS pressless split-sphere apparatus was used in complex with thermobarogeochemistry and gas chromatography.
It has been established that the serpentine decomposition is accompanied by the release of water, which concentrates in inclusions in the produced minerals (olivine and orthopyroxene) and their interstices. Chromatographic analysis with a stepwise heating of samples showed that most of the released water is localized in the interstices, and the rest is conserved in fluid inclusions in the minerals. The produced solid phases conserve 0.13 to 2.43 wt.% fluids as inclusions, with water amounting to 0.1-2.06 wt.%. The content of inclusions determined by microscopic examination falls in this region. Since the mobility of the fluid conserved as inclusions in the olivine and orthopyroxene is significantly lower than that in the interstices, this fluid might be better preserved in olivine-containing rocks subsided to depth.



3.
DJERFISHERITE IN XENOLITHS OF SHEARED PERIDOTITE IN THE UDACHNAYA-EAST PIPE ( Yakutia ): ORIGIN AND RELATION TO KIMBERLITE MAGMATISM

I.S. Sharygin, A.V. Golovin, and N.P. Pokhilenko
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Djerfisherite, kimberlite, carbonatite, melts, mantle xenoliths, upper mantle
Pages: 247-261

Abstract >>
Djerfisherite, a Cl-bearing potassium sulfide (K6Na(Fe,Ni,Cu)24 S26Cl), is a widespread accessory mineral in kimberlite-hosted mantle xenoliths. Nevertheless, the origin of this sulfide in nodules remains disputable. It is usually attributed to the replacement of primary Fe-Ni-Cu sulfides when xenoliths interact with a K- and Cl-enriched hypothetical melt/fluid. The paper is devoted to a detailed study of the composition and morphology of djerfisherite from a representative collection (22 samples) of the deepest mantle xenoliths - sheared garnet peridotite, taken from the Udachnaya-East kimberlite pipe (Yakutia). Four types of djerfisherite were distinguished in the mantle rocks on the basis of morphology, spatial distribution, and relationships with the rock-forming and accessory minerals in the nodules. Type 1 was found in the rims of polysulfide inclusions in the rock-forming minerals of the xenoliths; there, it was younger than the primary sulfide assemblage pyrrhotite + pentlandite ± chalcopyrite. Type 2 formed rims around large polysulfide segregations (pyrrhotite + pentlandite) in the xenolith interstices. Type 3 formed individual grains in the xenolith interstices together with other sulfides, silicates, oxides, phosphates, and carbonates. Type 4 was present as a daughter phase in the secondary melt inclusions which occurred in healed cracks in the rock-forming minerals of the xenoliths. Along with djerfisherite, the inclusions contained silicates, oxides, phosphates, carbonates, alkaline sulfates, chlorides, and sulfides. The results indicate that djerfisherite from the xenoliths is consanguine with kimberlite. Djerfisherite both in the sheared-peridotite xenoliths from the Udachnaya-East pipe and in different xenoliths from other kimberlite pipes worldwide formed owing to the interaction between the nodules and kimberlitic melts. Djerfisherite forming individual grains in the xenolith interstices and melt inclusions crystallized directly from the infiltrating kimberlitic melt. Djerfisherite bounding the primary Fe-Ni ± Cu sulfides formed by their replacement as a result of a reaction with the kimberlitic melt.



4.
THE GEOCHEMISTRY AND ISOTOPIC AGE OF ECLOGITES FROM THE BELOMORIAN BELT ( Kola Peninsula ): EVIDENCE FOR SUBDUCTED ARCHEAN OCEANIC CRUST

A.A. Shchipanskya, L.I. Khodorevskayab, and A.I. Slabunovc
aGeological Institute of the Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 198017, Russia
bInstitute of Experimental Mineralogy, Chernogolovka, Moscow Region, 142432, Russia
cInstitute of Geology, Karelian Scientific Center of the Russian Academy of Sciences, ul. Pushkinskaya 11, Petrozavodsk, 185910, Russia
Keywords: Eclogites, TTG gneisses, oceanic crust, isotope dating, SHRIMP, oceanic zircon, subduction, megamelange belt, Archean
Pages: 262-280

Abstract >>
We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic genesis derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalite-trondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace-element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar).
Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution-reprecipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which evidences the syn-eclogitic origin of theTTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier studied Gridino eclogites.
The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ~2.86-2.87, ~2.82-2.80, and ~2.72 Ga, which is in good accordance with the known isotopic age for majer crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust.
The presence of HP/UHP elcogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamelange belts.



5.
THE HARAA GOL TERRANE IN THE WESTERN HENTIYN MOUNTAINS ( northern Mongolia ): GEOCHEMISTRY, GEOCHRONOLOGY, AND GEODYNAMICS

I.V. Gordienkoa, A.Ya. Medvedevb, M.A. Gornovab, O. Tomurtogooc, and T.A. Gonegera
aGeological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakh'yanovoi 6a, Ulan-Ude, 670047, Russia
bA.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
cInstitute of Geology and Mineral Resources, Mongolian Academy of Sciences, Peace Avenue 63, P.O. Box 118, Ulaanbaatar-51, 210351, Mongolia
Keywords: Metabasalt, gabbros, cherty sediments, isotopic age, oceanic crust, back-arc spreading basins, island-arc systems, Mongol-Okhotsk Ocean, Haraa Gol terrane, western Hentiyn Mts., northern Mongolia
Pages: 281-292

Abstract >>
According to geological, petrological, geochemical, and geochronological studies, the Haraa Gol terrane in the western Hentiyn Mts. is dominated by two rock assemblages of different ages, associated with the initiation and development of the island arcs and marginal spreading seas of the Mongol-Okhotsk Ocean. The Late Cambrian, Early Ordovician, and Middle Ordovician were marked by the effusion of basalt and basaltic andesite and the formation of gabbro and gabbro-dolerite in back-arc spreading basins. In the Late Silurian-Devonian, after a short pause, tectonomagmatic processes were activated, with the formation of differentiated island-arc volcanics, gabbro, and granitoids. Their absolute 40Ar-39Ar age is given in the paper. The model age of the T Nd (DM) protolith of the Haraa Gol igneous rocks corresponds to the composition of the Mesoproterozoic juvenile crust.



6.
CRUST STRUCTURE AND COMPOSITION IN THE SOUTHERN SIBERIAN CRATON ( influence zone of Baikal rifting ), FROM MAGNETOTELLURIC DATA

M.I. Epov, E.V. Pospeeva, and L.V. Vitte
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Magnetotelluric (MT) soundings, resistivity, rift-related faults, collision zone, igneous and metamorphic rocks
Pages: 293-306

Abstract >>
The magnetotelluric (MT) profile traverses the southeastern edge of the Siberian craton and the adjacent Paleozoic Ol'khon collision zone, both being within the influence area of the Baikal rifting. The processed MT data have been integrated with data on the crust structure and composition, as well as with magnetic, gravity, and seismic patterns. Large resistivity lows are interpreted with reference to new geothermal models of rifted crust in the Baikal region. The northwestern and southeastern flanks of the profile corresponding to the craton and the collision zone, respectively, differ markedly in the crust structure and composition and in the intensity of rifting-related processes, the difference showing up in the resistivity pattern. The high-grade metamorphic and granitic crust of the craton basement in the northwestern profile flank is highly resistive but it includes a conductor (less than 50 ohm·m) below 16-20 km and a nearly vertical conductive layer in the upper crust. The crust in the southeastern part, within the collision zone, is lithologically heterogeneous and heavily faulted. High resistivities are measured mainly in the upper crust composed of collisional plutonic and metamorphic complexes. Large and deep resistivity lows over the greatest part of the section are due to Cenozoic activity and rift-related transcrustal faults that vent mantle fluids constantly recharged from deeper mantle.



7.
THE IMPACT OF TECHNOGENIC FACTORS ON THE SEISMICITY OF THE KUZNETSK BASIN REGION AND LAKE BAIKAL

A.A. Bryksin and V.S. Seleznev
Geophysical Survey, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Seismicity, technogenic impact, Altai-Sayan region, Kuznetsk Basin, Baikal
Pages: 307-312

Abstract >>
Using the materials from the catalogue of seismic events in the Siberian region, we estimated the impact of man's activity on natural seismicity. Local man's intervention into natural processes has been studied by the examples of commercial explosions during the quarry mineral mining in the Kuznetsk Basin and the exploitation of the railroad site along the shore of Lake Baikal. Seismic emission is shown to change with time under the impact of powerful monochromatic vibrators on the environment.



8.
THE NATURE OF TEMPERATURE VARIATIONS IN BOREHOLE Kun-1 ( Kunashir Island )

D.Yu. Demezhko, A.K. Yurkov, V.I. Utkin, and A.V. Klimshin
Institute of Geophysics, Ural Branch, Russian Academy of Sciences, 100 ul. Amundsena, Yekaterinburg, 620016, Russia
Keywords: Temperature monitoring, borehole, tide, thermal convection, tectonic regime, Kunashir Island
Pages: 313-319

Abstract >>
The study presents the result of continuous temperature monitoring in a 300 m deep borehole located on the Pacific coast of Kunashir Island. Temperature variations at 20-240 m depths with periods from a few tens of minutes to a few tens of days and amplitudes of thousandths to tenths of degree are found to result from three main causes: tidal variations of groundwater level in the borehole, natural convection in the borehole, and nonperiodic and spatially heterogeneous changes of the pore pressure resulting in an anomalous subsurface fluid regime and generating crossflow between the different aquifers through the annular space. Joint analysis of temperature and seismic data revealed that shallow-focus (up to 30 km) earthquakes with M > 5 occurred southeast of the seismic station created more expressed temperature response at 240 m depth. This response has the form of a 0.05-0.3 K temperature drop, which precedes a seismic event within a period of a few hours to a few days. Tidal analysis of borehole temperature measurements can be used to calculate the strain sensitivity, and the method of borehole temperature monitoring can be used to investigate the tectonic regime of the Earth's crust.