Volume 481, Nº 2 (2018)

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas producing the the comendites of the Sant bimodal volcanic association (Central Mongolia). The mechanisms of the formation of melts were determined. The primary melt and coexisting crystalline inclusions in quartz from three samples of comendites collected from different parts of the volcanic section were studied. Among the crystalline inclusions, sanidine, zircon, and the REE diortosilicate–chevkinite were identified. The phenocrysts of the comendites were determined to crystallize at temperatures of 880–960°C. The homogeneous glasses of melt inclusions have both trachydacite and rhyolite compositions. They are characterized by high concentrations of Zr, Nb, Rb, Y, Th and REE. Significant differences were determined in concentrations of Li and volatile component (H₂O and F) in the glasses: some of the melts are enriched in these components, whereas other are depleted in them.

Analysis of the composition of the glasses of the homogenized melt inclusions in quartz of comendites from the Sant bimodal association allowed us to recognize magmatic processes responsible for formation of the comendite melts. The dominant role among them belongs to crystallization differentiation of the magma, accompanied by a process of liquid immiscibility with participation of fluoride melts.

An analysis of the development of Late Devonian reefs in a number of regions worldwide shows that reef formation decreased gradually and discontinuously during the second half of the Frasnian and was completely terminated in some places. These events were associated with regression stages. The final Frasnian regression and Kellwasser event, which led to the biotic crisis, resulted, first, in the quantitative reduction of reef formation in the Famennian and, second, in the change of the frame reef formation by the formation of microbial mud mounds in the Famennian.

Cambrian igneous formations in the Northern Urals were identified and described for the first time. It was established that the Timanide orogeny terminated in the Early Cambrian and was followed by continental rifting above a mantle plume in the Late Cambrian.

New results of dating of detrital zircons from the Kema and Silasa Formations (Albian, Sikhote–Alin Orogenic Belt) permit determination of the provenance sources and geodynamic settings of sediment deposition. The Silasa Formation was deposited in outer island arc settings, separated by a forearc basin from the inner island arc, where the Kema Formation sediments were accumulated. The significant depth and width of the forearc basin hindered input of continent-derived debris to the area of sedimentation of the Silasa Formation.

Obtained data shows that high-potassic dyke rocks of the Ryabinoviy massif (Central Aldan) belong to low-titanium lamproite series (Mediterranean type) and are distinct with “classic” high-titanium lamproites. Based on Al-in-olivine thermometer, temperature of olivine–chrome-spinel pair crystallization varies in range between 1100 and 1250°C. This suggests lithospheric mantle source for the parental melt and makes role of mantle plume insignificant. High-precision data on olivine composition and bulk rock traceelement composition imply mixed source for the parental melt, consisted of depleted peridotite and enriched domains, originated during ancient subduction.

Accessory zircon from the polyphase Raygorodok stock (Northern Kazakhstan) has been dated by local U–Pb analysis (SIMS and LA-ICP-MS). This Au-bearing intrusion has been dated to 442–447 Ma, suggesting its emplacement at the very end of the Ordovician and Early Silurian, allowing the stock’s correlation with the Stepnyak Complex of small intrusions. Thus, control of small intrusions of the Stepnyak type over gold mineralization has been corroborated permitting their use as a regional prospecting characteristic.

For the first time, the age of magnesite in the Lower Riphean Bakal Formation of the Southern Urals is determined by the U—Pb (Pb—Pb) method: it is equal to 1366 ± 47 Ma (MSWD = 18). The stage of magnesite formation of the Bakal ore field was associated with the Mashak rifting pulse and took place prior to the formation of industrial deposits of the Bakal siderite.

The chemical composition of rock-forming minerals from unusual olivine-mica rocks from the Udachnaya-East kimberlite pipe (Yakutia) was studied. It is shown that all these rocks were formed as a result of the influence of deep metasomatic fluids. Two ilmenite-bearing rocks show the similarity to polymict breccia genetically representing analogs of protokimberlite melts, which did not reach the Earth’s surface. Two other rocks (one with K-bearing amphibole, another with clinopyroxene) are most likely rocks from the zones of high permeability, which were formed upon consolidation of the Siberian Craton and were subjected to the influence of aggressive fluids from the early portions of the protokimberlite melt.

The results of local U–Th–Pb-SIMS-SHRIMP-II analysis of the collection of pyrochlores from Nb rare-metal deposits of the Ilmeny–Vishnevogorsky alkaline complex are discussed. Independently of the chemical composition, the host rocks, and the stage of carbonatite formation, the studied pyrochlores are characterized by an almost undisturbed U–Pb isotope system. This allowed to estimate the age of ore mineralization on three deposits (Vishnevogorsky, Buldymsky, and Potaninsky) with an accuracy of ±1–6 m. y. The U–Pb system of the studied pyrochlore samples suggests three epochs of the mineral formation: 380, 230–240, and 216–218 Ma. Convergence of the results obtained by the different methods of U–Pb dating for one of the samples from our collection (±0.5–1.0 Ma) allows to use it as an in-house standard for local dating.

Based on complex mineralogical and geochemical analysis, high concentrations of dissolved and metallic native forms of Au were registered in the tailing store near-surface horizon of the Komsomol’skii gold-extracting factory. According to the physicochemical characteristics, this layer is distinguished by contrasting variations in the environmental parameters and oxidizing medium. The processes of dissolution, growth, and reprecipitation of authigenic Au were detected, and its predominant formation at the expense of redistribution from the lower horizons of gold-bearing sediments and accumulation in the oxidation zone were supported.

The joint analysis of current data on the topography and nonequilibrium part of the Martian gravity field allowed us to obtain a detailed pattern of the distribution of nonhydrostatic stresses in the Martian interior. The criteria for selection of possible sources of marsquakes were large values of shear stress against the background of significant tensional stresses. The rheological models considered were the elastic model, the one with the lithosphere, and the model with the lithosphere and possible melting zones in it. Independently from the rheological model chosen, zones of high shear and tensional stresses in the Martian crust and mantle were revealed beneath the Hellas and Argyre impact basins, Mare Acidalium, Arcadia Planitia, and Valles Marineris.

The traveling of surface waves in a model geophysical medium with a nonsmooth boundary is studied. It is shown that relief significantly distorts the microseismic noise field recorded at the day surface. This result should be taken into consideration when interpreting the data of passive surface-wave geophysical studies of deep crustal inhomogeneities.

The results of instrumental observations have shown that the spectra of geomagnetic variations exhibit a quasi-harmonic component, the frequency of which is close to the basic spheroidal mode of the Earth ₀S₂. In periods 15 days long after three large earthquakes and in the absence of strong magnetic disturbances, the fine structure of this mode in geomagnetic variations is identified in the form of singlets. The frequencies of these singlets are similar to the theoretical values calculated using an ideal elastic isotropic model of the Earth of spherical symmetric form without its rotation.

Lagrangian calculations based on satellite altimetry data (1993–2015) statistically demonstrated the existence of stable transport pathways of subtropical waters to the southern coasts of Russian Primorye through the subarctic front of the Sea of Japan. This transport is mainly provided by a peculiar mesoscale circulation north of the front as eddy streets, which facilitate the formation of Lagrangian intrusions of warm water penetrating further northward. The transport of warm waters in recent decades could be one of the factors responsible for the prolonged warm autumns observed in Primorye since the 1990s and explains the penetration of thermophilic fishes and other tropical and subtropical marine organisms and animals to the Primorye coasts.

This paper reports on the results of the seismic studies on the disastrous water-stone flow passing down the Kyngarga River (Republic of Buryatia) on June 27–29, 2014, according to data of the Arshan seismic station. The spectrum-polarization analysis of velocigrams showed that the period of the flow activity of debris was accompanied by a change in the properties of microseisms. Two components appeared in the velocity spectrum: the high-frequency component (22–48 Hz) associated with the motion of the solid fraction of the debris flow and the low-frequency component (0.35–0.45 Hz) associated with the water displacement. The registered elastic vibrations are the surface waves polarized in a horizontal plane with an apparent northeastern orientation.

This paper shows the important role that circulation can play in the long-term variability of thermohaline properties of the bottom layer in the Irminger Sea, and the increase in salinity in the 21st century in spite of intense ice melting in the Arctic and freshening of the upper and intermediate layers that started over the last 5–7 years in the North Atlantic.

Experimental data have shown that the cyclonic circulation in the Middle Caspian is a seasonal contour current surrounding the Middle Caspian megabasin along the slope during the cold half of the year. This current and the enormous influx of suspended particulate matter on the western slope of the Middle Caspian Basin provide conditions for the formation of contourites near the slope foot. In addition, the joint action of the contour current and slope gravity flows led to the formation of specific accumulative structures, sedimentary waves, on the western slope of the Middle Caspian.

This paper reports that in the period of global warming continuing over 150 years, there are the glaciers growing in size.