Home – Home – Jornals – Russian Geology and Geophysics 2022 number 3

The early stages of basic-ultrabasic magmatism in Sarmatia are characterized by the appearance of ultrabasic rocks formed from the mantle with an abnormally high iron content. Therefore, it is important to study them as the source of information about the stages and causes of the activity of the mantle and its possible composition. This magmatism has been recorded in Sarmatia since the beginning of the Eoarchean. The relics of Eo- and Paleoarchean basic and ultrabasic rocks were found in the Dniester-Bug, Kursk, and Azov provinces, which underwent tectonic reconstruction in the Mesoarchean and Paleoproterozoic. Mesoarchean basic-ultrabasic magmatism is manifested in all provinces of Sarmatia and is represented by effusive and intrusive facies. The Mesoarchean greenstone belts composed of komatiites and basalts have been well preserved in the Middle Dnieper province; in other provinces, they are strongly deformed and form narrow linear structures. The Paleoproterozoic endogenous activity in Sarmatia differs from that in other regions in the almost complete absence of magmatism in the period 2.5-2.3 Ga and its significant manifestation 2.1-2.0 Ga. The magmatism in Sarmatia at this stage is similar in the ratios of basic-ultrabasic and granitoid complexes to the magmatism in South Africa but differs from that in Fennoscandia and Canada: The volume of granitoids coeval with basic rocks is larger than the volume of mantle magmatism. The igneous complexes formed 2.1-2.0 Ga in Sarmatia and South Africa are also similar in the presence of norites, the enrichment in Ni and platinum group elements, and the ratio of granitoids and basic-ultrabasic rocks. Magmatic activity (first of all, basic-ultrabasic magmatism in ancient cratons) is not a synchronous phenomenon on a planetary scale and varies greatly in the volume of produced material within the same time intervals. Early Precambrian basic-ultrabasic rocks (volcanics of greenstone belts, intrusions of large igneous provinces, and layered massifs) resulted from plumes, whose derivates formed within the lower and upper mantle and/or the upper mantle and crust, which determined the heterogeneous composition of igneous rocks. The spatial heterogeneity and nonsynchronic occurrence of basic-ultrabasic magmatism might have been due to impact events serving as the triggers of plumes.

The paper presents the results of studies of diamonds from Early Jurassic sediments making up the Nyurbinskoe buried placer of the Nakyn kimberlite field, unique in diamond reserves. The main task was to identify the patterns of diamond distribution in the deposits of the Dyakhtar strata (lower deposit) and the Ukugut Formation (upper deposit) within the placer. A comparative analysis of the typomorphic features of diamonds from the upper and lower deposits of the placer was carried out. Variations in the contents of crystals with certain properties that form the image of a diamond-bearing geologic object have been revealed. The zonal distribution of diamonds by characteristics in sedimentary deposits, regardless of their age, has been established. The properties of diamonds and their associations change within the placer, which is due to their redeposition during the Early Jurassic sedimentation.

The paper considers the geologic structure of the region and the current problems and prospects for the development of its energy potential and environmental safety. We provide grounds for the necessity of integrated projects aimed at studying the deep structure of the Caspian region as a single object by its five coastal states: Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan. The proposed Geokhazar project is aimed at obtaining the lacking parametric geological and geophysical information about the deep subsurface structure of the water area of a sedimentary basin in the unique intracontinental catchment of the Earth. The project provides for the development of a universal prospecting concept taking into account the emplacement and conservation of hydrocarbon fields under severe thermobaric conditions at great depths and the absence of regionally consistent drainage systems; the determination of the factors influencing the nature of long- and medium-frequency eustatic fluctuations in the Caspian Sea level; and the assessment of the energy (geothermal and hydrocarbon) resources of the deep subsurface in the Caspian, cis-Caucasian-Mangyshlak, and South Caspian oil and gas provinces.

We have studied the mineral composition of ores from the Pepenveem epithermal Au-Ag deposit, which is a promising new object of the Chukchi Peninsula. It has been found that the ore formation process was developed in the following sequence: Pyrite, arsenopyrite, and marcasite were deposited at the early stage, next were Pb, Zn, and Cu sulfides; at the late stage, native gold, pyrargyrite, stephanite, proustite, minerals of the pearceite-polybasite series, acanthite, and other Ag minerals were deposited. The results of fluid inclusion studies indicate that the Au-Ag mineralization formed from low-temperature (236-137 ºС) low-concentration chloride hydrotherms (0.18-1.57 wt.% NaCl eq.). The results of calculation of thermodynamic equilibria have shown that in the temperature range from 200 to 100 ºC, there were a decrease in the fugacity of sulfur (lg ƒ S2 from -10 to -21) and oxygen (lg ƒ О2 from <-36 to <-48) and a change from near-neutral to acidic solutions. Compared to other Au-Ag deposits on the Chukchi Peninsula (Corrida and Valunistoe), which are characterized by wide distribution of Se- and Te-bearing Au-Ag chalcogenides (naumannite, cervelleite, and hessite), ore formation with gold-silver-sulfosalt mineralization at the Pepenveem deposit took place at lower temperatures and lower selenium, tellurium, and oxygen fugacity. The data obtained permit us to refer the Pepenveem deposit to the group of epithermal low-sulfidation (LS) deposits.

The paper summarizes the results of study of the geologic position, composition, and age of basic igneous associations in Eastern Kazakhstan during the late Paleozoic (Carboniferous-Permian). At that time, the Altai accretion-collision system was developed here, which resulted from the interaction of the Siberian and Kazakhstan paleocontinents. The performed studies made it possible to establish three major stages of basic magmatism, corresponding to different stages of evolution of the collisional system: early Carboniferous, late Carboniferous, and early Permian. The chemical composition of ultrabasic-basic associations changed, with a successive increase in the contents of K2O, P2O5, TiO2, LREE, Rb, Ba, Zr, Hf, Nb, and Ta. The variations in magma compositions were determined by different compositions of mantle sources (harzburgites, spinel lherzolites, and garnet lherzolites) and different degrees of their melting. The early Permian ultrabasic-basic associations are the most enriched in TiO2 and incompatible components (P2O5, Zr, Hf, Nb, and Ta), which indicates the involvement of relatively enriched mantle sources in the partial melting. All manifestations of mantle magmatism were accompanied by subsynchronous crustal magmatism (granitoid intrusions or silicic volcanics). The major crustal magmatism was manifested in the early Permian; the area of its occurrence was dozens of times larger than the area of Carboniferous crustal magmatism. Possible geodynamic scenarios for magmatism are considered for each stage. The early Carboniferous (C₁s) magmatism of the early orogeny stage was manifested locally and was the result of the detachment of the subducting lithosphere (slab) beneath the margin of the Kazakhstan continent. The middle Carboniferous (C₂m) magmatism of the late orogeny stage was manifested throughout the area; it was caused by the activation of shear-extension motions along large faults and the orogen collapse. The early Permian magmatism was the result of the interaction of the Tarim mantle plume with the lithosphere, which comprised three stages: initial interaction, maximum interaction, and relaxation. This magmatism in the study area was caused by a combination of thermal disturbance in the upper mantle and the lithosphere extension processes.

The Hercynian mobile belts in Central Asia include the proper Hercynian and Late Hercynian (Indo-Sinian) belts, whose formation is associated with the evolution of the South and Inner Mongolian basins with oceanic crust. Within the South Altai metamorphic belt (SAMB), rock complexes compose tectonic slivers of different ranks. At the early stages, their metamorphic alteration occurred under conditions of the high-temperature subfacies of the amphibolite and, in places, granulite facies. Structurally, the band of the outcrop of these complexes is confined to the Caledonian North Asian continental margin and stretches along the southern slope of the Gobi-Mongolian-Chinese Altay Mountains from southeast to northwest (East Kazakhstan), where they occur in the Irtysh strike-slip zone. We assign these complexes to the Hercynian SAMB, running for more than 1500 km. The latter comprises poly- and monometamorphic complexes. Late metamorphic granitoids of the Tseel tectonic plate (Gobi Altay) in the southeast of the SAMB have been dated at 374 ± 2 and 360 ± 5 Ma. The previous data and these results show that the early (~390-385 Ma) low-pressure and late (375-360 Ma) high-pressure metamorphism proceeded almost along the entire belt. The interval between them was a short tectonic lull. These processes took place during the closure of a Tethyan basin of the South Mongolian Ocean (Paleo-Tethys I). The spatial position of the SAMB was controlled by the structural asymmetry of the basin, with an active continental margin at its northern edge and a passive one at the southern edge (in the present-day coordinates).

Magnetic viscosity is one of the aftereffects inherent in geologic materials. This phenomenon consists in the time lag of changes in the magnetic characteristics of ferromagnetic materials relative to changes in the external magnetic field. Magnetic viscosity in rocks is associated mainly with the magnetization of superparamagnetic particles of ferrimagnetic minerals. In the transient electromagnetic method, magnetic viscosity is manifested as a slowly decreasing voltage induced in a receiving loop or, in some cases, as a nonmonotone transient voltage response. Eddy currents and viscous magnetization arise and decay independently of each other; therefore, the induction transient response measured with a fixed-geometry TEM array gives no way of finding the vertical distribution of magnetic viscosity. To find this distribution requires geometric soundings. At late times, the voltage induced in the receiving loop due to the magnetization decay is vastly larger than that induced by the eddy currents. Therefore, the contribution of magnetic viscosity to the total transient response limits the sounding depth of the transient electromagnetic method.