Home – Home – Jornals – Russian Geology and Geophysics 2015 number 12

Three levels of the periodicity of volcanic eruptions with different causes and effects are recognized. The first, small-scale, level lasts tens to thousands of years and is due to the periodic accumulation of melt in the intermediate chambers and its periodical effusion both for internal reasons and under the trigger effect of external factors (solar activity, tidal changes in the Earth’s rotation rate). The second, medium-scale, level lasts hundreds of thousands to millions of years and is due to the periodicity of restructuring of spreading and subduction zones. The third, largest-scale and most prolonged, level of periodicity (30-120 Ma) is related to periodic accumulations of melt at the core-mantle boundary, ascent of mantle plumes, and subsequent acceleration of convection. These reasons are probably the major regulator of the average periodicity. Acceleration of convection in the asthenosphere, caused by periodic pulses of mantle jets, determines the changes in the group of moving plates through the changes of spreading zones. The intermediate periodicity level (hundreds of thousands of years to the first millions of years) is demonstrated by the example of Kamchatka and Japan, where the periodic changes in the intensity of subduction magmatism are due to the restructuring of back-arc basins (Shikoku, Sea of Japan, and South Kuril basin).

We present a comprehensive description of the blastomylonites of the Ikat thrust localized within the Ikat terrane in the East Transbaikalian branch of the Central Asian Fold Belt. The metamorphic rocks of the Garga block are thrust over the almost unmetamorphosed Cambrian deposits of the Ikat Formation. The geologic position of the thrust and the structure and mineral and petrochemical compositions of the blastomylonites in the suture zone are considered, and U-Pb dating of detrital zircon is carried out. The blastomylonites develop after the gneisses of the Garga block; the block protolith, admixed with felsic volcanic material, was no older than the Neoproterozoic (750-800 Ma). The ³⁹Ar/ ⁴⁰Ar age of the thrust is estimated at 282 ± 0.4 Ma based on metamorphic biotite from the blastomylonites. The thrust marks the transpression period during the Late Paleozoic processes of tectonometamorphic activity related to the subsidence of the Mongol-Okhotsk oceanic crust beneath the Siberian continent.

We studied the geologic position, mineralogy, petrography, chemical composition, and age of granitoids of the Gamov batholith located in the Laoelin-Grodekovo terrane, southern Primorye (Russia). The studies have revealed four intrusive rock phases, from gabbro to leucogranites, in the massif. U-Pb zircon dating of tonalites of phase II and leucogranites of phase IV (254 ± 4 and 259 ± 2 Ma, respectively) shows that the intrusion of granitoids took place in the Late Permian without a significant gap in time. Structural investigations gave evidence for the intrusion in the sublatitudinal-compression setting. Geochemical studies have revealed granitoids of strongly different compositional types in the intrusive massif: gabbroids, quartz diorites and tonalites with characteristics of low-alumina TTG, calc-alkalic granodiorites and I-type granites, and medium-alkali leucogranites. The obtained data, together with the results of previous research into the Permo-Triassic granitoids of southern Primorye, reject the relationship of their formation with the evolution of the Solonker oceanic structures and suggest a more intricate tectonic scenario, which calls for additional study.

New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO₂, Cr₂O₃, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high concentrations of REE and their pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns in the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.

The paper is concerned with study of melt inclusions in minerals of ijolite xenoliths at Oldoinyo Lengai Volcano. Melt inclusions with different phase compositions occur in forsterite macrocrysts and in diopside, nepheline, fluorapatite, Ti-andradite, and Ti-magnetite crystals. Nepheline contains primary melt inclusions (silicate glass + gas-carbonate globule ± submicron globules ± sulfide globule ± daughter/trapped phases, represented by diopside, fluorapatite, Ti-andradite, and alumoakermanite). The gas-carbonate globule consists of a gas bubble surrounded by a fine-grained aggregate of Na-Ca-carbonates (nyerereite and gregoryite). Fluorapatite contains primary carbonate-rich melt inclusions in the core, which consist of nyerereite, gregoryite, thenardite, witherite, fluorite, villiaumite, and other phases. In mineral composition they are similar to natrocarbonatite. Primary melt inclusions (glass + gas bubble ± daughter phases) are rare in diopside and Ti-andradite. Diopside and forsterite have trails of secondary carbonate-rich inclusions. Besides the above minerals, these inclusions contain halite, sylvite, neighborite, Na-Ca-phosphate, alkali sulfates, and other rare phases. In addition, diopside contains sulfide inclusions (pyrrhotite ± chalcopyrite ± djerfisherite ± galena ± pentlandite). The chemical compositions of silicate glasses in the melt inclusions vary widely. The glasses are characterized by high Na, K, and Fe contents and low Al contents. They have high total alkali contents (16-23 wt.% Na ₂O + K ₂O) and peralkalinity index [molar (Na + K)/Al] ranging from 1.1 to 7.6. The carbonate-rich inclusions in the ijolite minerals are enriched in Na, P, S, and Cl. The data obtained evidence that the parental melt in the intermediate chamber was heterogeneous and contained silicate, natrocarbonate, and sulfide during the ijolite crystallization. According to heating experiments with melt inclusions, silicate-carbonate liquid immiscibility occurred at >580 °C.

The relationships and chemical compositions of silver sulfoselenides in the ores of the Rogovik gold-silver deposit (northeastern Russia) were studied to refine the low-temperature field of the Ag ₂S-Ag ₂Se phase diagram and identify contradictions between natural and experimental data. Two types of relationships between the phases of the system Ag ₂S-Ag ₂Se have been recognized using optical and scanning electron microscopy: (1) Se-acanthite and S-naumannite occur as monomineral microinclusions or fill cracks in the grains or the interstices of other minerals, and acanthite (free of impurities) forms rims on Fe-sphalerite; (2) Se-acanthite forms rims on S-naumannite. Electron probe microanalysis of silver sulfoselenides from the Rogovik ores revealed 0-7.9 wt.% Se in acanthite and 0-3.2 wt.% S in naumannite, which corresponds to the acanthite series Ag ₂S-Ag ₂S _0.76Se _0.24 and naumannite series Ag ₂S _0.28Se _0.72-Ag ₂Se. The composition ranges of the studied acanthite and naumannite series are wider than those of natural silver sulfoselenides from the Guanajuato (Mexico), Silver City (USA), Salida (Indonesia), and other deposits (Ag ₂S- Ag ₂S _0.85Se _0.15 and Ag ₂S _0.12Se _0.88-Ag ₂Se, respectively) but are significantly narrower than the composition ranges of the synthetic samples: Ag ₂S-Ag ₂S _0.4Se _0.6 and Ag ₂S _0.3Se _0.7-Ag ₂Se. The presence of intergrowths of two phases of the Ag ₂S-Ag ₂Se series in the form of Se-acanthite rims on S-naumannite in the Rogovik ores and the absence of three-phase intergrowths of silver sulfoselenides Ag ₂S _1-x Se _x from this and other deposits do not confirm the assumption on the existence of the third solid solution. The results of earlier studies of natural solid solutions Ag ₂(S,Se) show the existence of two solid solutions (of the acanthite and naumannite series) in the system Ag ₂S-Ag ₂Se and confirm the experimental data. It is necessary to carry out a detailed examination of natural silver sulfoselenides falling in the interval from Ag ₂S _0.4Se _0.6 to Ag ₂S _0.3Se _0.7 in order to identify the limits of two-phase immiscibility.

The paper is concerned with geologic processes in river valleys, such as the movement of alluvial placers and mudflows, regarded as viscous fluids. The dynamics of distribution of the placer mineral and mud-and-gravel material in a river valley was studied by modeling. Moving material was examined as an independent flow of solid particles (solid flow), free of the enclosing rocks. The flow-environment interaction was specified by mass forces, friction forces, and substance sources (drains). For the mathematical description of a solid flow, a set of viscous-liquid equations was used. The placer and mudflow parameters obtained during the numerical experiments agree satisfactorily with the full-scale data for real objects.

We demonstrate application of the time-domain characteristics of induced polarization (IP) to the solution of prospecting problems on the flanks of the Julietta gold-silver deposit. The IP time-domain characteristics were analyzed by the Debye decomposition, which permits reconstruction of distribution of relaxation times. The experimentally determined dependences of the IP parameters on the main petrophysical properties of disseminated ores were used to interpret field data. Application of the Debye decomposition helped to distinguish zones with finely disseminated sulfide minerals in hydrothermally altered rocks, which were missed on the application of the standard technique for analysis of IP data. The estimated content of sulfide minerals in the altered rocks agrees with geological data.

Anisotropy of electrical conductivity in the lower mantle is regarded as a possible consequence of the phase transition of magnesiowϋstite from semiconductor into metallic state. We propose a method for obtaining information about the tensor of electrical conductivity of a deep anisotropic layer from geomagnetic data of the global network. It is possible to estimate the parameters of the electrical-conductivity tensor which are tangential components of the electromagnetic field by analysis of the characteristics of a magnetic mode. We present the results of processing of real data by the proposed method for the period from 1920 to 2009; the data were borrowed from the Net databank of the monthly values ​​of components X, Y, and Z of the geomagnetic field.

In recent years, the Hustai, Gunji, and Kerulen paleoseismic structures have been discovered and examined within the Henteyn uplift. Their investigations revealed zones of probable earthquake foci, presenting a seismic hazard for Ulaanbaatar. We report the first data on the quantitative parameters of prehistoric earthquakes in the Meso - Cenozoic Upper Kerulen basin. The absolute age of two paleoevents determined by radiocarbon dating is within 1152 - 1702 BC and 5466 - 7201 BC. Seismogeological study of surface ruptures made it possible to estimate the type of tectonic stress during the formation of structures within the Henteyn uplift and to explain their nature. The horizontal-compression stress is mostly due to endogenous processes, which lead to the enlargement of the uplift area at the expense of the peripheral Mesozoic sedimentary basins.

In 2012, the renewable energy organization of Iran (SUNA) performed a gravity survey around hot springs of the Mahallat geothermal field in the Markazi Province of Central Iran, as part of the explorations and developments of geothermal energy investigation program in the region. The Mahallat region has the greatest geothermal field in Iran. This work presents interpretation results of various gravity maps and a calculated 3D inversion model. The residual gravity map shows three negative gravity anomaly zones (A1, A2, and A3) associated with the geothermal reservoirs in the region. The horizontal gradient maps reveal a complex fault system. In order to attain more information about the Khorhe geothermal reservoir, a 3D density contrast model was calculated using the Li–Oldenburg method. The attained 3D model provides an in-depth image of the evolution, showing the density contrast and the A1 zone having a high potential for the geothermal reservoir in the region. The results also show that the rocks which exist between 1000 and 3000 meters under the Earth’s surface in the Al zone are the most suitable aquifers for utilization of geothermal energy.