Home – Home – Jornals – Russian Geology and Geophysics 2024 number 6

The paper presents data on a granulite xenolith from the Zarnitsa kimberlite pipe (Yakutia, Russia), which stores a record of two metasomatic events in addition to the main stage of metamorphism. The granulitic mineral assemblage consists of garnet, clinopyroxene, and plagioclase as main phases. The granulite xenolith contains kyanite-clinopyroxene and later orthopyroxene-plagioclase symplectites. Kyanite-clinopyroxene symplectites appear as short veins inside or between grains of rock-forming minerals. Orthopyroxene-plagioclase symplectites form kelyphite rims in all garnets or occur as veins in garnet grains. The P-T conditions for granulite in the lower crust reconstructed by Perple_X phase equilibrium modeling are 700-750 ℃ and 1.2-1.3 GPa. According to TWQ thermodynamic calculations, the kyanite-clinopyroxene symplectites were produced by Si-metasomatism at invariable Р-Т parameters. The growth of orthopyroxene-plagioclase symplectites after garnet was maintained by Ca inputs upon heating and decompression (200 ℃ temperature increase and 0.6 GPa pressure decrease) while the xenolith was transported by ascending kimberlite melt.

Sulfur isotope ratios (³³S/³²S, ³⁴S/³²S, and ³⁶S/³²S) archived in the rocks of volcanic and ferruginous formations of the Sumozero-Kenozersky (~2.9 Ga) and Kostomuksha (~2.7 Ga) greenstone belts of the Fennoscandinavian shield were studied. The data obtained provide new arguments in favor of developing ideas about the existence of bacterial activity in the Archean ocean and the involvement of sulfur in the biogenic cycle, and also indicates the absence of free oxygen in the Earth’s Archean atmosphere. Based on the isotopic data of sulfur-34 and -33, the active activity of sulfate-reducing and disproportionating elemental sulfur bacteria in the surface environments of Archean sedimentary rocks was identified. The discovery of isotopic anomalies of sulfur-33 and -36 in the studied rocks became a key evidence of the absence of free oxygen in the atmosphere during the formation of the Meso- and Neo-Archean deposits of Karelia.

The paper presents data on two groups of magnesian Cr-V spinel occurring as rock-forming minerals in various types of Cr-V bearing rocks of the Sludyanka metamorphic complex (South Baikal area, Siberia, Russia): (i) low-Al magnesiochromite-magnesiocoulsonite spinels and (ii) Al spinels varying from V-bearing aluminous magnesiochromite and Al-Cr coulsonite to spinel proper with minor contents of Cr and V, and with complex zoning patterns. Spinels in the Sludyanka rocks are also present as iron-magnesian accessories, among which high-Cr varieties are compositionally similar to mantle-derived spinel from igneous rocks and to inclusions in diamonds. Comparison of formation conditions for mantle spinels in igneous lithologies and spinel phases in metamorphic rocks shows that their mineral chemistry is mainly controlled by the bulk composition of the igneous and sedimentary protoliths, respectively. Detrital spinel may be a false indicator poorly applicable to paleogeodynamic reconstructions and diamond potential evaluation.

The work is devoted to the analysis of the geological structure and assessment of the oil and gas potential of the Anabar-Khatanga and Lena-Anabar oil and gas areas (OGA) of the Lena-Tunguska oil and gas province, which are located on the continental margin of the Siberian platform, on the coast of the Laptev Sea. Administratively, the study area includes the northeastern regions of the Krasnoyarsk Territory, the northwest of the Republic of Sakha (Yakutia), the offshore of the Khatanga Bay and the southern, transit part of the Laptev Sea (Khatanga-Lena interfluve). Based on integrated interpretation of geological and geophysical materials, we present regional-zonal models of the geology of the upper Proterozoic-Phanerozoic deposits of the Arctic regions of the Siberian platform, sets of seismogeological sections, structural and parametric maps, results of structural-tectonic analysis, potential estimates of oil-producing formations; analysis of reservoir properties and types of oil and gas promising objects, and a quantitative assessment of hydrocarbon resources. The study area is of interest in terms of oil and gas potential. In this region, the Central Olgin field was discovered, numerous bitumen, oil and gas shows were noted. A series of oil-producing formations enriched in organic matter occurs in the sedimentary cover, and in the sections of all sedimentary complexes, high-capacity reservoirs are distinguished. There are prerequisites for the formation of both anticlinal and complex oil and gas promising objects. The prospects for the oil and gas potential of the Anabar-Khatanga OGA are mainly associated with Permian terrigenous deposits and Cambrian-Devonian carbonates. In the Lena-Anabar OGA, all upper Proterozoic-Paleozoic sedimentary complexes from the Riphean to the Permian level are of interest in terms of oil and gas potential.

New and revised previous data based on detailed description of sections have provided better constraints on fauna taxonomy, lithology, and stratigraphy, as well as on biostratigraphic ages of sediments. The lithology and stratigraphy of several key Upper Ordovician sections in the northern Gorny Altai area show that the local units of Sandbian and Katian Savelieva and Bulukhta formations can be used as reference for the regional Ordovician division. It is suggested to include a newly distinguished Bulukhta facies zone, an area of Late Ordovician coarse clastic deposition, into the mapped Ordovician deposits of Gorny Altai. According to paleogeographic reconstructions, the deposition occurred in a large sea gulf surrounded with elevated landmasses exposed to erosion.

The wavefield structure is studied from explosions within the Mururoa atoll based on seismic records from four stations (BCAO, KAAO, GRFO, BRVK) located at distances 152.1º, 152.4º, 143.6º, 141.7º, respectively. The cross-correlation analysis was used as the main tool of the similarity of explosion seismograms conducted at different times. It has been concluded that changes in correlation similarity in time are reconcilable with the hypothesis of differential rotation of the Earth’s inner core in the period from 1977 to 1991. The evidence of structural anomalies not only in the inner core but also in the outer core has been demonstrated.

Ground penetrating radar method which was applied to study distribution and occurrence of polygonal wedge ice along with other structural features in the near-surface layers of the permafrost section on the western coast of the Taymyr Peninsula and Kotelny Island. The polygonal microrelief in the region under study is at a descending stage of development and is mainly placed in sand-gravel deposits and silts with rare interlayers of light gray sands. The ground penetrating radar (GPR) data were analyzed and interpreted by modeling of the polygonal wedge ice structures in gprMax - a software that simulates electromagnetic wave propagation. The effect of the seasonally thawed layer on the measurement results was assessed, and the results of ground penetrating radar sounding of the polygonal microrelief were interpreted. The ground penetrating radar studies performed in this work indicate that unmelted lower ends of ice wedges preserved under pseudomorphs can be observed in some spots. Repeated wedge structures of higher generation, which do not appear on the surface, have been identified. The propagation velocity of electromagnetic waves in the studied sediments was determined.