Home – Home – Jornals – Russian Geology and Geophysics 2017 number 10

We provide new geochemical and isotope-geochronological evidence for gabbro intrusions of the Krestovsky zone in the Olkhon composite terrane. The intrusions belong to the Birkhin (~500 Ma) and Ust’-Krestovsky (~470 Ma) complexes, which underwent several pulses of magmatism. Rock structures and textures record magma crystallization in tectonically turbulent conditions. The compositions of gabbro are similar to those of subduction-related basalts, but the Ust’-Krestovsky gabbro is richer in incompatible elements than the Birkhin rocks. A similar composition trend of gabbro is observed in other areas of the Central Asian Orogenic Belt (CAOB): southeastern Tuva, Gornaya Shoria, and western Mongolia. We suggest a model of regular composition changes in parental magmas during the interaction of a mantle plume with suprasubductional lithospheric mantle.

Based on new geostructural, mineralogical, geochemical, and isotope (Sm-Nd, Rb-Sr, and U-Th-Pb) data for igneous intrusions and metamorphic rocks of the Zimoveinyi massif, an adakite-gabbroid association has been identified for the first time in the Yenisei Ridge (southwestern framing of the Siberian craton). This study demonstrates that the adakites (quartz diorites, tonalites, and plagiogranites) and associated gabbro-anorthosites of this massif formed in the interval 576-546 Ma (U-Th-Pb zircon SHRIMP-II analysis) at the final Neoproterozoic evolution stage of the active continental margin of the Siberian craton. Our results point to a genetic relationship between the adakites and the host Nb-enriched metabasites of the Zimoveinyi massif. The studied late Neoproterozoic adakites might have formed from both crustal and mantle-crustal sources in the setting of transform strike-slip of lithospheric plates during a halt in subduction, similarly to Cenozoic igneous complexes of the transform margin in the eastern framing of Eurasia.

U-Pb geochronological studies have shown at least two stages of Neoproterozoic magmatism, 940-933 and 804-789 Ma, throughout the geologic evolution of the Bureya continental massif in the east of the Central Asian Fold Belt. The first stage (940-933 Ma) was marked by the formation of the protoliths of metagabbroids and amphibole-biotite gneiss-granites of the Bureya River basin. The geochemical features of these rocks suggest that they formed, most probably, in an island-arc setting on the continental basement or on an active continental margin. The second stage (804-789 Ma) of magmatism was manifested as the formation of biotite leucogranites and hastingsite-lepidomelane granites (A-type granites) in the Chepkan River basin. The latter rocks are similar in geochemical features to within-plate granitoids. The established stages of magmatism are the most ancient for the continental massifs in the east of the Central Asian Fold Belt. It is not ruled out that these magmatism stages were related to the stages of formation and breakup of the Rodinia supercontinent.

New data have been obtained on major- and trace-element compositions and Sm-Nd isotope systematics of clastic metasediments that belong to the Kornilova and Shubutui Formations in the Khamar-Daban terrane. The Kornilova Formation flysch derived from felsic mature crust, while the Shubutui Formation, with 4.5-7.5 wt.% MgO and higher Cr and Ni contents, had a mafic-ultramafic protolith of an island-arc setting. Isotope data indicate significant inputs of Precambrian crustal material (ε_Nd( T ) = -10 to -6.2) to the protolith of the Kornilova rocks and of high amounts of synsedimentary juvenile material to the protolith of the Shubutui Formation (ε_Nd( T ) = -2.9 to -1.5). Different model ages (1.7-2.0 and 1.3-1.4 Ga, respectively), compositions, and provenances of the Kornilova and Shubutui rocks, as well as a prolonged deposition gap (>100 Myr), evidence that the two formations were deposited in different basins and tectonic settings. The rock compositions and Nd isotope signatures suggest an island arc setting for the Shubutui deposition in the Dzhida terrane and a continental-margin setting for the deposition of the Kornilova Formation, which is the uppermost straton of the Khamar-Daban terrane.

The diversity of the chemical composition of water in lakes, even those located at a few hundred meters from each other, has long attracted the attention of researchers. Detailed hydrochemical testing of two small soda lakes in the Kulunda steppe, located at 14 km from each other, provided answers to many questions. Although the lakes have pH > 9 and thus are assigned to the soda type, they differ in the composition of saline waters (HCO3-Cl-Na and Cl-SO4-Na-Mg) and bottom sediments. The indicator minerals in the sediments of the lake near Severka Village are pyrite, disordered Ca-smectites, and dolomite (the latter is strongly predominant, especially in the lower part of the sediment section). The minerals in the bottom sediments of the other lake are dominated by terrigenous quartz, feldspars, excess-Ca dolomite, and Mg-calcite, with the portion of carbonates increasing in the lower part of the section. Based on the reported facts, the assumption is made that the landscape position, the influence of groundwaters, and technogenic factors (land plowing) are the main reason for the differences between the two lake systems. The task is set to study biocoenoses and their effect on the diagenesis of bottom sediments.

For experimental modeling of the formation of pentlandite-bornite ore during fractional solidification of sulfide magma, we performed directional quasi-equilibrium crystallization of a melt containing (mol. %) 17.19 Cu, 19.05 Fe, 19.66 Ni, and 44.10 S, from which pentlandite and bornite can cocrystallize. Based on the results of measurement of the average composition of the solid phase, the distribution curves of the components in the ingot were constructed and the change in the melt composition during directional solidification was calculated. The obtained data show that the crystallized sample consists of six zones, each of which has characteristic phase and chemical compositions. In the Cu-Fe-Ni-S composition tetrahedron, the melt composition path is a broken spatial curve, each segment of which corresponds to a point showing the average composition of one of the zones. These points form a disordered set, indicating a complex fractionation process. Investigations of characteristic microstructures along the ingot, using data on its average chemical composition, made it possible to determine the probable set of primary phases crystallizing from the melt (bornite solid solution bnss, three types of pentlandite with different cation compositions: cfpn, npn, and cnpn, and quaternary solid solution tss) and to establish the sequence of separation of these phases and their associations from the melt in six zones: cfpn (zone I) / cfpn + bnss (zone II) / cfpn + bnss + npn (zone III) / tss + bnss (zone IV) / cnpn + bnss (zone V) / npn + bnss (zone VI). A scheme of phase reactions along the crystallization path has been compiled, and it has been shown that high-temperature pentlandite forms have a significant effect on the fractionation process. These results are consistent with previous data on the fractional crystallization of multicomponent sulfide melts and the theoretical features of this process. In previous studies, samples with a small number of zones were obtained. In this study we first obtained a sample containing six zones. The zoning observed in the sample belongs to the complex second type of low-sulfur zoning of orebodies. The same data confirm the possibility of different types of zoning within the major two types of zoning of copper-nickel orebodies.

We present results of U-Pb (SHRIMP II) and Ar-Ar geochronological study of the rocks of the Chuktukon massif, which is part of the Chadobets alkaline-carbonatite complex, and of the weathering crust developed after them. Perovskite from picrites and monazite from the weathering crust were dated by the U-Pb (SHRIMP II) method, and rippite from carbonatites, by the Ar-Ar method. Rippite has first been used as a geochronometer. The estimated ages (252 ± 12 and 231 ± 2.7 Ma) testify to two magmatism pulses close in time (within the estimation error) to the stages of alkaline magmatism in the Siberian Platform (250-245 and 238-234 Ma). These pulses characterize, most likely, the processes accompanying and completing the activity of the mantle superplume that formed the Siberian Igneous Province at 250-248 Ma. The monazite-estimated age (102.6 ± 2.9 Ma) is the time of formation of the ore-bearing weathering crust on the massif rocks.

During ⁴⁰Ar/³⁹Ar dating, the age spectrum of the sample can be seriously distorted because of the averaging of its Ar isotope composition over the portion of the released gas. New methods of age spectrum approximation with suppression of distortions have been elaborated. The efficiency of these methods is experimentally demonstrated. A new method for measurement of age spectra with high resolution by the portion of the released gas is proposed. The method is based on the experimental data and permits refining of the age spectrum or its fragments. The applicability and efficiency of this method are also shown experimentally. The proposed method can be used not only to measure the age spectra during ⁴⁰Ar/³⁹Ar dating but also to measure other isotope characteristics of the rock and mineral samples during the stepwise separation of the analyte.

The Igol’sko-Talovoe oilfield is one of the largest oilfields in southeastern West Siberia, which provides reference for Upper Jurassic reservoirs of the area. The reported studies included integrated interpretation of seismic and well-log data, mapping of tectonic and paleotectonic patterns and sediment thicknesses, as well as seismic-geological and basin modeling. The results were used for tectonic analysis and allowed reconstructing the history of the Igol-Talovoe dome uplift that traps Upper Jurassic oil accumulations. Local uplifts on the Jurassic surface within the Igol-Talovoe dome originated in the Neocomian above basement uplifts produced by erosion and tectonic activity. The local domes merged into a large anticlinal trap as a result of regional-scale vertical crust movements in the Cenozoic. Analysis of the Upper Jurassic hydrocarbon system and basin modeling show that oil generation by the Bazhenov Formaton source rocks began in the Turonian, culminated in the Cenozoic, and continues at present. The process has led to complete filling of the anticlinal trap and formation of the large Igol’sko-Talovoe oilfield. Geological and geophysical data suggest the same scenario for all Upper Jurassic reservoirs in southeastern West Siberia.

This study presents the results of geochemical analysis of organic matter and paleogeographic reconstructions supplemented by data from well log interpretation and cyclostratigraphic analysis for the Vasyugan, Georgiev, and Bazhenov Horizons of the Callovian-Lower Berriasian section in the western part of the Yenisei-Khatanga regional trough and adjacent areas of the West Siberian geosyneclise. It has been found that each horizon contains zones dominated by terrigenous, mixed, and aquatic organic matter. The distribution of different types of organic matter over the area and throughout the section was examined. It was shown that the accumulation of aquatic organic matter took place in the deepest parts of the trough during the Callovian-Oxfordian. The area of accumulation of aquatic organic matter expanded considerably and reached its maximum extent within the Bolshaya Kheta megasyneclise during the Kimmeridgian and Early Volgian and in the west Yenisei-Khatanga regional trough during the Volgian and Early Berriasian.

At present, sounding methods based on the effect of electromagnetic pulses on the environment are widely used for marine exploration of hydrocarbon deposits. The exploration is performed using research systems with special equipment fixed in the water column. The goal of this work is to develop equipment and methods for marine electrical prospecting that would allow reliable predictions of petroleum fields in the underlying environment with reduced labor intensity of the necessary surveys. For this purpose, a subsurface array for marine electrical prospecting during vessel movement is proposed. The effective frequencies, current strengths in cables, and the size and efficiency of the array are determined using both the theoretical knowledge of the operation of similar arrays in similar environments and numerical simulation of the developed array. The 3D finite element method is used for mathematical modeling.

Thermal regime in borehole is simulated by water-filled vertical channel passing through the rock massif. A constant temperature gradient is maintained at the external borders of the massif. Results of numerical simulation of natural thermal convection arising in the channel are presented. Analysis of the results shows the existence of two types of convective thermal effects disturbing the natural temperature field: transient effect, which manifests itself as temperature oscillations around the mean temperature value at a given depth, and quasi-stationary effect causing distortion of natural temperatures and temperature gradient. The obtained statistical relations for estimation of the characteristics of thermal effects and convection flow velocities agree with the data recorded in real boreholes.