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Russian Geology and Geophysics

2013 year, number 4

1.
GEODYNAMICS AND STRESS-STRAIN PATTERNS IN DIFFERENT TECTONIC SETTINGS

N.L. Dobretsov, I.Yu. Koulakov, O.P. Polyansky
Keywords: Strain, stress, subduction, sedimentary basin, geodynamics, seismic tomography

Abstract >>
Deformation patterns in subduction zones, feeder systems of volcanoes, and rifts are compared and investigated in terms of relations among elastoplastic strain, rheology, pore fluids, and temperature. Regional-scale subduction processes have been explored in segments of the KurilesKamchatka, Izu-Bonin, and Mariana zones. Slab geometry constraints from the 3D velocity structure are used to model the balance of forces in the three subduction zones and to distinguish the regions of predominant push or pull. Stress and strain variations in suprasubduction crust are considered for the case of magma sources beneath the Klyuchevskoy group of volcanoes. Time-lapse (4D) seismic tomography shows crustal magma reservoirs to appear and disappear rapidly as the volcanoes become active or dormant, respectively. This behavior is due to rapid strain changes, which cause fast flow of fluids and an ensuing decrease or increase in melting temperature in the magma reservoirs. In addition to subduction zones, stress-strain patterns are modeled for collisional (compressive) settings, with the example of the AltaiSayan area and the Caucasus, and for the conditions of rifting (extension), in the case of the Vilyui basin. As the modeling shows, formation of a superdeep basin does not necessarily require the crust to stretch twice or more: only 20% stretching in the necking region is enough to produce a 1015 km deep basin.



2.
KINEMATIC RECONSTRUCTION OF THE EARLY CALEDONIAN ACCRETION IN THE SOUTHWEST OF THE SIBERIAN PALEOCONTINENT BASED ON PALEOMAGNETIC RESULTS

D.V. Metelkin
Keywords: Early Caledonian tectonic stage, kinematics, accretion, subduction, strike-slips, island arc, Siberian paleocontinent, paleomagnetic pole, paleotectonic reconstructions

Abstract >>
The kinematics of the Early Caledonian accretion process in the southwest (in modern coordinates) of the Siberian paleocontinent and the structure of its active continental margin are debatable subjects. This paper contains a generalization of paleomagnetic data on island-arc terranes of the AltaiSayan and BaikalVitim folded areas for the Late Vendian/CambrianEarly Ordovician interval, obtained mostly with the authors participation during the last two decades. The large accumulated database finally allows one to find unambiguously interpretable patterns in the distribution of paleomagnetic poles for the analyzed terrane system and to justify numerically the kinematics of the Early Caledonian accretion. In particular, the analysis of paleomagnetic data proves our idea stating that the transformation of the active continental margin in the Cambrian consisted in its breakup and segmentation as well as the detachment of fragments of the initially whole island arc along a system of sinistral strike-slips during the clockwise rotation of the craton and conform drift of the continental and oceanic lithospheric plates. This also validates the mostly oblique conditions of the subduction and subsequent accretion, which means a subduction-transform mode on the oceancontinent margin. We propose complemented version of the paleotectonic reconstruction for the Cambrian evolution of the western margin of the Siberian continent, based on the kinematic scheme constructed from paleomagnetic data.



3.
FORMATION OF EPIGENETIC GRAPHITE INCLUSIONS IN DIAMOND CRYSTALS: EXPERIMENTAL DATA

D.V. Nechaev, A.F. Khokhryakov
Keywords: Diamond, epigenetic inclusions, graphitization, experiment

Abstract >>
To elucidate the conditions of formation of epigenetic graphite inclusions in natural diamond, we carried out experiments on high-temperature treatment of natural and synthetic diamond crystals containing microinclusions. The crystal annealing was performed in the COCO2 atmosphere at 7001100ºC and ambient pressure for 15 min to 4 h. The starting and annealed diamond crystals were examined by optical microscopy and Raman spectroscopy. It has been established that the microinclusions begin to change at 900ºC. A temperature increase to 1000ºC induces microcracks around the microinclusions and strong stress in the diamond matrix. The microinclusions turn black and opaque as a result of the formation of amorphous carbon at the diamondinclusion interface. At 1100ºC, ordered graphite in the form of hexagonal and rounded plates is produced in the microcracks. A hypothesis is put forward that the graphitization in natural diamond proceeds by the catalytic mechanism, whereas in synthetic diamond it is the result of pyrolysis of microinclusion hydrocarbons. The obtained data on the genesis of graphite microinclusions in diamond are used to evaluate the temperature of kimberlitic melt at the final stage of formation of diamond deposits.



4.
THE STABILITY OF ORTHOAND CLINOPYROXENES, OLIVINE, AND GARNET IN KIMBERLITIC MAGMA

A.I. Chepurov, E.I. Zhimulev, L.V. Agafonov, V.M. Sonin, A.A. Chepurov, A.A. Tomilenko
Keywords: Experiment, kimberlite melt, high pressure and high temperature, dissolution of minerals

Abstract >>
It is generally accepted that the composition of ultrabasic nodules and their quantitative proportions do not significantly change during their transportation with kimberlitic magma to the Earths surface. We performed an experimental study of the relative stability of olivine, garnet, and pyroxenes in kimberlite melt at high pressure and temperatures (4 GPa, 13001500 ºC). The study has shown that the loss in weight of minerals and, correspondingly, the rate of their dissolution in kimberlite melt differ considerably. The following sequence of the dissolution rates of minerals has been established: Cpx ≥ Opx > Gar > Ol. Pyroxenes are characterized by the most rapid dissolution, and olivine is the most stable mineral. The assumption is made that clinopyroxenites and websterites disintegrate more rapidly than dunites and lherzolites in kimberlitic magma.



5.
Petrology and composition of rare-metal alkaline ROCKS in the South Gobi Desert ( Mongolia)

N.V. Vladykin
Keywords: Petrology, alkali granites, carbonatites, chemical composition, rare-metal rocks, Mongolia

Abstract >>
Earlier, a belt of alkali-granite plutons and a carbonatite province were discovered in the South Gobi Desert, Mongolia. The Lugingol pluton of pseudoleucitic syenites with carbonatites was assigned to the alkali-granite belt. However, new dating showed that it is 40 Myr younger than the Khan-Bogdo pluton and a large fault separates it fr om the alkali-granite belt. In the same part of the South Gobi Desert, a dike series of alkaline K-shonkinites with a rare-metal carbonatite vein was found by V.I. Kovalenko west of the Lugingol pluton, near Mt. Baruun Hasar Uula, and a dike series of alkali and nepheline syenites was found by us northeast of the Lugingol pluton. These data give grounds to distinguish an intrusive complex of K-alkaline shonkinites and leucitic syenites with Late Paleozoic REE-bearing carbonatites. Thus, three alkaline-rock complexes of different ages are distinguished in the South Gobi Desert. We present refined geological maps of these complexes. The plutons of all three complexes are deposits of trace elements (REE, Nb, Zr, Y, P). The chemical composition of the silicate rocks of the complex, rare-metal agpaitic pegmatites, and carbonatite and apatite rare-metal ores was considered in detail. Shonkinites from Mt. Baruun Hasar Uula and the Mountain Pass mine (United States) and their carbonatites, along with the Lugingol carbonatites, belong to a single association of K-alkaline rocks and carbonatites, as evidenced by their identical chemical, mineral, and geochemical rare-metal compositions. Rare-earth element patterns and spidergrams show similarities and differences between the rare-metal rocks of three complexes as well as paragenetic differences between their rare-metal minerals. A rare process is described the amorphization of rare-metal minerals, related to their high-temperature crystallization in a medium with abnormal silica contents of the Khan-Bogdo pegmatites. The parental magmas of the alkali-carbonatite complexes were generated from the EM-2 contaminated mantle that had undergone recycling, wh ereas the parental magmas of the Khan-Bogdo agpaitic alkali granites were produced from depleted mantle.



6.
CRYSTALLIZATION OF LOPARITE IN ALKALINE FLUID-MAGMATIC SYSTEMS (from experimental and mineralogical data)

N.I. Suk, A.R. Kotelnikov, A.A. Viryus
Keywords: Loparite, experiment, fluid, crystallization, Lovozero alkaline massif

Abstract >>
We studied loparite-containing rocks (lujaurites, juvites, foyaite-juvites, etc.) sampled from a complex of differentiated rocks and, partly, from a complex of eudialytic lujaurites of the Lovozero alkaline massif. Zoned crystals of loparite (the zoning is due to variations in Ti, Nb, REE, Sr, and Th contents) were examined by microprobing. We also carried out experimental studies of loparite formation in complex silicatesalt systems including sodium carbonate, chloride, fluoride, or sulfate at 4001200ºC and 12 kbar. They show that the composition of loparites depends on the physicochemical conditions of their formation (fluid composition) and that natural loparite can crystallize in a broad range of temperatures. The produced loparite crystals are zonin as a result of variations in Ti, Nb, La, Ce, Y, Ca, and Sr contents, which is probably related to the kinetic specifics of crystallization. Their zoning is similar to that of loparites of the Lovozero massif.



7.
EFFECT OF THE DEPOSITIONAL ENVIRONMENT ON THE COMPOSITIONAL VARIATIONS AMONG THE PHOSPHORITE DEPOSITS IN EGYPT

H.M. Baioumy
Keywords: Phosphorites, lithology, composition, depositional environment, Egypt

Abstract >>
Late Cretaceous economic phosphorites from the Red Sea, Nile Valley, and Abu Tartur areas, Egypt, show distinct variations in the lithology of associated sediments, mineralogy of nonphosphatic constituents, and distributions of major and trace elements. In the Red Sea area, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and calcite, ankerite, and pyrite cements. In the Nile Valley, the phosphorite beds are intercalated with chert, marl, and sandstone, and the nonphosphatic constituents are detrital quartz and calcite and chalcedony cements. In the Abu Tartur Plateau, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and ankerite and pyrite cements. The phosphorites studied also show distinct variations in major- and trace-element concentrations. The Abu Tartur phosphorites have higher contents of TiO2, Al2O3, Fe2O3, K2O, Co, Nb, Pb, Sr, Th, Y, and Zr and lower contents of SiO2, Ba, and U as compared to those in the Red Sea and Nile Valley areas. The positive correlations between Al2O3 and TiO2, K2O, Nb, Y, and Zr suggest the detrital origin of these constituents. Similarity in the phosphatic constituents, which were derived from outside the depositional sites, and variations in the lithology of associated sediments and the mineralogy and geochemistry of the nonphosphatic constituents, which reflect the conditions at the depositional sites, suggest that the variations in the depositional environment of the phosphorites are the potential controlling factor of the compositional variations among these phosphorites. The abundance of black shales in the Red Sea and Abu Tartur areas, as well as the occurrence of ankerite and pyrite as cementing materials for the phosphatic constituents, might reflect reducing conditions in these areas, while the abundance of siliciclastic sediments and calcite and chalcedony cements suggests oxidizing conditions in the Nile Valley. The reducing conditions in the Red Sea and Abu Tartur areas were probably developed within the pre-existing depressions in a shelf environment. These depressions might have formed as a result of a change in the movements of the North Atlantic, Eurasian, and African Plates during the late Santonian, which led to transgressive inversion of rifts along northern Egypt and consequent folding in the continental interior. The higher contents of detrital components in the Abu Tartur phosphorites, as compared to the Red Sea and Nile Valley areas, suggest more detrital inputs during the deposition of the phosphorites in Abu Tartur. The products of the diagenesis and weathering of these deposits also reflect the variations in the depositional conditions.



8.
THERMAL MAGNETIC SUSCEPTIBILITY DATA ON NATURAL IRON SULFIDES OF NORTHEASTERN RUSSIA

P.S. Minyuk, E.E. Tyukova, T.V. Subbotnikova, A.Yu. Kazansky, A.P. Fedotov
Keywords: Magnetic susceptibility, temperature dependence of magnetic susceptibility, pyrite, marcasite, greigite, arsenopyrite, chalcopyrite, pyrrhotite

Abstract >>
Dependences of magnetic susceptibility (MS) on the temperature of natural iron sulfide samples (pyrite, marcasite, greigite, chalcopyrite, arsenopyrite, pyrrhotite) fr om the deposits of northeastern Russia were studied. The thermal MS curves for pyrite and marcasite are the same: On heating, MS increases at 420450ºC, and unstable magnetite (maghemite) and monoclinic pyrrhotite with a well-defined Hopkinson peak are produced. In oxygen-free media with carbon or nitrogen, magnetite formation is weak, whereas pyrrhotite generation is more significant. The heating curves for chalcopyrite are similar to those for pyrite. They show an increase in MS at the same temperatures (420450ºC). However, stable magnetite is produced, wh ereas monoclinic pyrrhotite is absent. In contrast to that in pyrite, marcasite, and chalcopyrite, magnetite formation in arsenopyrite begins at > 500ºC. Arsenopyrite cooling is accompanied by the formation of magnetite (S-rich arsenopyrite) or maghemite (As-rich arsenopyrite) with a dramatic increase in MS. Arsenopyrite with an increased S content is characterized by insignificant pyrrhotite formation. Greigite is marked by a decrease in MS on the heating curves at 360420ºC with the formation of unstable cation-deficient magnetite. Monoclinic pyrrhotite is characterized by a decrease in MS at ~320ºC, and hexagonal pyrrhotite, by a transition to a ferrimagnetic state at 210260ºC. The addition of organic matter to monoclinic pyrrhotite stimulates the formation of hexagonal pyrrhotite, which transforms back into monoclinic pyrrhotite on repeated heating. The oxidation products of sulfides (greigite, chalcopyrite) show an increase in MS at 240250ºC owing to lepidoroite.



9.
A PHYSICOCHEMICAL MODEL FOR THE FORMATION OF GAS HYDRATES OF DIFFERENT STRUCTURAL TYPES IN K-2 MUD VOLCANO ( Kukui Canyon, Lake Baikal)

A.Yu. Manakov, O.M. Khlystov, A. Hachikubo, A.G. Ogienko
Keywords: Gas hydrate, bottom sediments, Lake Baikal

Abstract >>
Earlier, the coexistence of spatially separated layers of gas hydrates of cubic structures I and II in the bottom sediment cores fr om K-2 mud volcano (Kukui Canyon, Lake Baikal) was described. The layers of gas hydrates of different structural types were situated at different depths and overlain by the lacustrine sediments. Hydrate of CS-II contained 1315 mol.% ethane, wh ereas CS-I hydrate contained only 34 mol.% ethane. We present a physicochemical model explaining the formation of such an unusual natural object. The model suggests that only CS-I hydrate was originally present at the sampling site. Some geologic event (tectonic shifts, landslide, etc.) stopped natural-gas emanation from the mud volcano or increased the heat flow in the hydrate pool. As a result, CS-I hydrate began to dissolve in interstitial water. We assume that the ethane-enriched CS-II hydrate is an intermediate product of the dissociation (dissolution) of CS-I hydrate.