Home – Home – Jornals – Russian Geology and Geophysics 2019 number 7

Our study of mantle xenoliths in the Cretaceous lamprophyre diatremes and late Cenozoic plateau basalts in western Syria has shown that the ancient lower crust that existed in the Cretaceous and was composed of garnet granulites and eclogite-like rocks was replaced by mantle peridotites in the late Cenozoic. We conclude that the heads of the local (secondary) plumes of the present-day Afro-Arabian thermochemical mantle plume responsible for the regional basaltic magmatism reached the basement of the ancient upper sialic crust, where they spread, leading to a displacement of the mafic lower crust.

The Bayan-Kol gabbro-granite association has been recognized within the West Sangilen fragment of collision zone in the northwestern framing of the Tuva-Mongolian massif, and its composition, age, and tectonic and geodynamic settings have been studied. The association includes the Bayan-Kol pluton and composite (mingling) dikes, which formed in the late collision period (495 ± 5 Ma), during the transition from transpression to extension mode with left-lateral strike-slip kinematics. The Bayan-Kol gabbro-granite association is spatially confined to the penetrating tectonic zones of the West Sangilen shear system. The position of gabbroid and granite bodies is controlled by local zones of tectonic extension. Basic magmas have a similar petrogeochemical composition, which indicates their intrusion from a single chamber of basic composition and differentiation of ascending magma. The melting, transfer, and formation of crustal granitoids of the Bayan-Kol association are genetically related to the thermal effect of basic melt and syntectonic drop in lithostatic pressure. The intrusion and formation of basic and acid melts of the Bayan-Kol association took place at the lower and middle crustal levels in the settings of the reactivation and subsequent fragmentation of the tectonic zone.

We present results of investigation into the composition and parageneses of pyrrhotite at the Sovetskoe gold-quartz deposit (Yenisei Ridge, Russia) have been studied. The variability of parameters (temperature T and sulfur fugacity f_S2) during the stage crystallization of pyrrhotite-containing assemblages has been assessed from the composition of this mineral (Fe_0.873±0.02S-Fe_0.885±0.02S) and its parageneses. The compositions Fe_0.873-0.875S close to Fe₇S₈ (Apy + Po + Rut + Qz), for which the estimated formation parameters are T = 486-465 ºС and lg f S2 = -4.71 to -5.28, are typical of early pyrrhotite in the form of microinclusions in arsenopyrite, associated with rutile and quartz. According to the composition of inclusions of pyrrhotite microcrystals (Fe_0.873-0.881S) associated with pyrite in native gold (950 ‰) (Au + Po + Py), the formation parameters are T = 489-410 ºС and lg f S2 = -4.63 to -6.98. Coarse pyrrhotite grains containing microinclusions of relict arsenopyrite and galena, sometimes, in aggregate with siderite (Po+Apy+Ga+Sid), and pyrrhotite in aggregate with pyrite and siderite (Py + Po + Sid) have composition Fe_0.874-0.878S and form at 479-443 ºС and lg f S2 = -4.9 to -5.9. The xenomorphic pyrrhotite microinclusions present together with galena and native gold (950 ‰) in pyrite crystals (Py + Po + Ga + Au) are characterized by higher contents of iron (Fe_0.878-0.885S) and, correspondingly, lower temperatures of formation, 432-382 ºС, and lg f S2 = -6.27 to -7.95. The lg f S2-Т diagrams have been calculated for the systems Fe-S and Ag-Au-S in the temperature range 25-700 ºС with regard for the stability fields of iron sulfides (pyrite FeS₂, troilite FeS, and pyrrhotite Fe₇S₈), phases Fe₁₁S₁₂, Fe₁₀S₁₁, and Fe₉S₁₀, metallic iron, native sulfur, uytenbogaardtite, petrovskaite, and solid-solution phases Fe_{1- x} S (0 < x < 0.125), Ag_1-zAu_z (z = 0, 0.25, 0.5, and 1), and Ag_{2- y} Au _y S (y = 0, 0.5, 1, and 2). The calculation results have demonstrated that there is a field of petrovskaite and uytenbogaardtite solid solutions and Au-Ag alloys (>670 ‰, Ag_0.5Au_0.5-Au) in the stability field of the pyrrhotite-pyrite parageneses of the Sovetskoe deposit. The gold and silver contents in iron sulfides of the Sovetskoe deposit show that the Au/Ag ratios in pyrrhotites (0.002-2.4) and pyrites (0.004-13) are lower than those in high-fineness (950-980 ‰) gold (19-50). The difference in the Au/Ag ratios in these minerals and the results of thermodynamic calculations show the possible presence of Au-Ag sulfides and Au-Ag alloys of lower fineness in the pyrrhotite-pyrite ores of the studied deposit. The absence of visible mineral forms of gold sulfides from the ores suggests that these sulfides are present in finely dispersed or invisible microscopic forms. The pyrrhotite compositions in pyrite-containing parageneses as well as Au/Ag in pyrites, pyrrhotites, and visible native gold in sulfide ores of other gold and gold-silver deposits can be used to assess the possible presence of nanosized solid microinclusions of sulfide and other gold and silver forms.

The minerals of the Maslyanino iron meteorite and their trace-element composition are described in detail for the first time, and the meteorite classification is substantiated. The meteorite is a fine-structural octahedrite. Its metallic matrix consists of kamacite, taenite, and schreibersite. Large troilite segregations are associated with silicate inclusions; in addition, rare minerals altaite and dobreelite are found. The silicate inclusions contain olivine, orthopyroxene, clinopyroxene, plagioclase, apatite, merrillite, chromite, and graphite. A detailed trace-element analysis of the metal matrix permits the Maslyanino meteorite to be assigned to the narrow Pitts subgroup of the IAB group. It is also similar to meteorites of the Udei Station subgroup. Both subgroups include meteorites with silicate inclusions and are intermediate between the sLL (low Au and Ni contents) and sLM (low Au and medium Ni contents) subgroups. According to the metallographic data, the cooling rate of the Maslyanino meteorite is 30-60 ºC/Myr. The data obtained are consistent with the formation of the meteoritic material under impact of a parent asteroid resulting in the removal of its outer chondrite-winonaite shell. Subsequent weaker impacts led to the formation of IAB group meteorites (including meteorites with silicate inclusions) and winonaites from the asteroid remnant.

Paleoenvironmenal reconstructions have been made from a multidisciplinary study of a borehole permafrost record on Kurungnakh Island (Lena delta). According to data on palynomorphs and ostracods, the clay silt units from the 10.58 to 13.54 m and 1.58 to 10.3 m core depth intervals were deposited in the Late Pleistocene (during the Karginian interstadial) and Early-Middle Holocene, respectively. The sediments were studied in terms of moisture contents, grain size distribution, mineralogy, and magnetic susceptibility, and the results were compared with published evidence from nearby natural outcrops. Quite a cold oligotrophic lake existed in the area during the Karginian period, and the deposition was interrupted by a gap recorded at a core depth of about 11 m. In the Early and Middle Holocene, the area was covered with shrub tundra vegetation.

Middle Mesozoic and Cenozoic tectonic events on the periphery of the West Siberian Plain and in the flanking mountains of the northwestern Altai-Sayan province produced highland topography over a part of southeastern West Siberia. The activity stages were separated by a long lull from Late Cretaceous through middle Paleogene, when the Mesozoic mountains were denuded to the base level corresponding to the level of the West Siberian epicontinental sea. The sea of that time was connected to the World Ocean, and its level fell in several successive events. The periods of stable sea level are marked by surfaces at 200, 250, and 300 m above sea level (in the present reference of elevations) and correlate with global sea level changes according to Haq and Vail. The stability surfaces were revealed during geomorphological surveys in the Salair Range and in the Bugotak-Sokur upland. Their elevations have not changed since the origin in the studied part of the Bugotak-Sokur area, but the SW tilting Salair block delineated by thrust faults in the north and in the east has been uplifted at 0.1 mm/year. In the course of neotectonic activity, the line of mountain growth shifted notably to the southeast, leaving behind the Fore-Altai plain and the Bugotak-Sokur upland, which were occupied by high mountains in the Jurassic. The lack of post-Mesozoic molasse in the Kuznetsk Basin and in the Chulym plain indicates that the present Kuznetsk Alatau and Salair Ranges are considerably lower than their middle Mesozoic precursors.

Ground-penetrating radar (GPR) profiling is applicable to study peatlands and swampy areas in permafrost but have some limitations in summer time. Theoretical calculations and field experiments show that estimating attenuation of electromagnetic waves is required for planning GPR survey. Radar images acquired with a 300 MHz antenna fail to resolve reflections from below the permafrost if the thaw/permafrost boundary is deeper than 1.5 m and the attenuation coefficient is 0.7, as in water-saturated peat. GPR data allow high-resolution lithological division of permafrost and provide reliable constraints on the depths to interfaces and physical properties of the ground. Thus, GPR can fully or partly substitute for the time- and labor-consuming direct measurements. The inferences have been confirmed by field results.

Analysis of the geologic structure of the Hettangian-Aalenian deposits of the Ust’-Tym megadepression was carried out. The history of the formation of traps in the Hettangian-Aalenian complex has been reconstructed on the basis of a comprehensive interpretation of seismic materials and deep-drilling data. The oil and gas potential has been estimated. The time of the subsidence of the Togur Formation into the oil window has been determined, and the history of the generation of liquid hydrocarbons (HC) by the organic matter (OM) of the Togur Formation has been reconstructed. The Lower Jurassic and Aalenian deposits overlap the rocks of the pre-Jurassic basement with disagreement and are distributed almost over the entire study area. The complete section of the Hettangian-Aalenian deposits is in the most submerged parts of the territory. The section includes the Urman, Togur, and Salat/Peshkov Formations and the lower Tyumen Subformation. Three oil and gas subcomplexes - Hettangian-Early Toarcian (U_16-17), Toarcian-Aalenian (U₁₅), and Aalenian (U_11-14) - are distinguished within the Hettangian-Aalenian sediments. Closed positive structures that can serve as hydrocarbon traps have been identified in each of the subcomplexes. Positive structures developed in the Jurassic, Berriasian-Early Aptian, and Aptian-Albian-Turonian time were inherited and finally formed at the post-Turonian stage only. The authors carried out a quantitative assessment of the total oil resources of the D₀ category for all promising objects, taking into account the success rate. The heterogeneous organic matter of the Togur formation is the main source of hydrocarbons in the Hettangian-Aalenian complex. The Togur Formation began to enter the oil window (OW) about 115-110 Ma and fully entered it about 5 Ma. The escape of rocks from the zone of the oil window began about 48 Ma and still continues. The history of the generation of liquid hydrocarbons by the organic matter of the Togur Formation has been reconstructed for types II and III of kerogen. For type II, the generation began about 94 Ma, at the beginning of the Late Cretaceous (Cenomanian), and for type III, it began in Turonian time (89.8 Ma). The most significant volumes of liquid HC were generated in the last 5 Myr. Potential hydrocarbon traps existed throughout the generation process, which allowed accumulation of the generated hydrocarbons in the Hettangian-Aalenian complex. Comparison of the estimated oil resources of the D₀ category in the traps of the Hettangian-Aalenian complex with the volumes of generated hydrocarbons leads to the conclusion that the traps might have been filled. The results obtained in the course of the study suggest that the Hettangian-Aalenian complex is oil- and gas-promising and the Togur Formation is the main source of hydrocarbons.

Temperature logging furnishes the essential part of geothermal data. Its applications are progressively expanding owing to advanced temperature loggers and data acquisition systems that ensure precise and stable measurements at high spatial and temporal resolution. However, it may be hard to achieve the full effect of the available logging facilities because of noisy temperature oscillations produced by natural convection in water-filled boreholes. A new laboratory method is suggested to study the structure of convection flows and their thermal effect by infrared thermography in conditions close to those of real temperature logging. Thermographic cameras image temperature anomalies on the outer wall of a water-filled pipe, which are imprints of the convection processes in the water column. The temperature gradient on the pipe wall maintains flow of warm air ascending from a toroidal heater. It is shown experimentally, using a pipe 20 mm in inner diameter, that convection of fluid in the pipe forms a helical system rotating around a vertical axis at Rayleight numbers in the range of 280 to 2800. As the Rayleigh numbers increase from 280 to 2800, the helical pitch decreases from 270 to 130 mm, while the angular velocity increases from 0.7×10^-2 to 3.4×10^-2 rad/s. The experiment confirms the theoretically predicted dependence of the standard deviation of temperature fluctuations on the temperature gradient and inner radius of the logged borehole: σ _T = 3 G·r .

Seismic-moment tensor solutions for earthquakes in Azerbaijan for 2012-2015 have been calculated with a new method by full waveform inversion of broadband data fr om modern digital seismic stations and processed statistically. The results are used to model main faulting elements in the region, to correlate the seismicity and fault patterns, and to compile a map of fault plane orientations for large events. The principal compression stress (Р) directions are NW to SE in the Zaqatala area and N-S in the Shaki area but then gradually change clockwise toward NE-SW in the Caspian Sea. The directions of principal extension are mainly NE-SW and N-S within the zone wh ere the Kura basin is subsiding beneath the Great Caucasus.