Home – Home – Jornals – Russian Geology and Geophysics 2022 number 6

The Tuva segment of the Central Asian Orogenic Belt is characterized by the ubiquitous presence of conglomerates few tens of meters to a kilometer in thickness in early Paleozoic volcanosedimentary sequences. We present the first results of geochemical, isotope-geochemical (Sm-Nd and Rb-Sr), and U-Pb geochronological studies of granitoid boulders and pebbles from the conglomerate sequence of the early Cambrian Bayan-Kol Formation of the Systyg-Khem depression. These studies made it possible to establish several sources of clastic material as a result of the destruction of granitoids of different ages and isotope-geochemical compositions. At least two complexes of granitoids were denuded in the pre-Ediacaran tectonic block in the early Cambrian: (1) middle Ediacaran (~590 Ma) and (2) early Ediacaran (~630 Ma); the latter resulted from the melting of pre-Ediacaran island arc crust formed from a depleted mantle source (εNd(T) = +8.0 to +8.6). At present, no granitoids of this age and with such isotope-geochemical characteristics have been found within the Tuva segment. Probably, the granitoid complexes reconstructed from the results of study of clastic conglomerates are eroded or buried beneath younger deposits and do not expose. Thus, the study of clastic conglomerates from the Bayan-Kol Formation provided the first information about the Precambrian history of the tectonic block whose destruction led to the accumulation of this terrigenous sequence.

Marly limestone, marble, and gabbro samples from the Kochumdek contact aureole are studied in order to estimate to which degree the spurrite-merwinite ( T = 750-900 °C) metamorphism at the site was isochemical. The 3 m wide zoned Kochumdek aureole is located at the contact between the Early Triassic trap intrusion of the Kuzmovka complex (νβT₁kz) and the overlying Llandovery marine marly limestones (S₁ln) of the lower Kochumdek subformation in the right side of the Kochumdek River (tributary of the Podkamennaya Tunguska, East Siberia). The Kochumdek rocks are remarkable by the lack of prominent skarn or vein mineralization in marbles and autometasomatism in gabbro. The samples have been analyzed for major, chalcophile, rare-earth, and high-field strength elements. The Kochumdek marbles and marly limestones share similarity in major-element patterns that record mixing of fine siliciclastic material with biogenic and chemogenic CaCO₃ components. The major-element chemistry of marly limestones has been controlled by the compositions of fine clastic material transported from the provenance (Trans-Angara part of the Yenisei Ridge) and by the deposition environment of the early Silurian marine basin. The contents of Cu, Ni, Co, V, Mo, Se, Cr, and U in marly limestones and marbles correspond to marine sediments deposited in a low-productive oxigenated environment. The Ni/Co, V/Cr, Cu/Zn, V/(V + Ni), and U/Th ratios in spurrite marbles at the intrusive contact remain within typical limestone ranges but differ from those in gabbro, which have higher V and Cu enrichment and moderate contents of Zn, Ni, and Co. Marly limestones and spurrite marbles show identical REE+Y patterns and average ΣREE contents. The major- and trace-element similarity between marbles and the protolith limestones, along with the absence of gabbro trace element fingerprints in the metamorphic rocks, indicate that the thermal metamorphism was nearly isochemical. The reactions occurred without any large-scale transport of major and trace elements across the sill/sediment boundary. The conditions were unfavorable for metasomatic alteration due to low fluid saturation of the Kochumdek intrusion and its structural setting, as well as to the lithological, mechanical, and thermal properties of the sediments.

Two cores of the Dulikha peat bog deposits, 4.0 and 5.2 m thick, were studied in detail. The reduction type of diagenesis has been established. Decomposition of the organic matter of the bog deposits begins already in the upper intervals of the section at the earliest stages of diagenesis. Microbiological studies of the peat bog revealed numerous organotrophic, ammonifying, nitrifying, and phosphate-mobilizing microorganisms as well as Fe-Ox and Mn-Ox microorganisms and showed the almost complete absence of sulfate-reducing bacteria (except for the lower intervals of the peat section). The processes of the nitrogen, phosphorus, and carbon cycles are more active than the sulfur cycle processes. The chemical composition of the bog water changes during diagenesis. The contents of HCO₃^-, NH₄⁺, PO₄^3-, NO₃^-, and TOC (Total organic carbon) increase in the peat bog section, which is the result of the destruction of organic matter, leading to the transition of the most mobile and labile components into the bog water. The results of physicochemical modeling show that the high content of organic matter significantly changes the chemical composition of the bog water. The enrichment of the latter in Cu, Zn, Sr, Ba, As, Si, Al, and Pb indicates a diagenetic transformation of the bog deposits. A change of oxidizing conditions by reducing ones in the peat bog leads to the reduction of oxidized Fe (III) and Mn (IV) forms to mobile Fe (II) and Mn (II) forms and their subsequent migration from the solid phase of deposits into the bog water. With depth, the reduced S (II) forms become predominant in the peat, which indicates more reducing environmental conditions. The concentration of SO₄^2- decreases in the pore water of the peat-underlying mineral deposits, which marks the beginning of sulfate reduction. This process, however, is very slow in the bog deposits because of the acid environmental conditions and the lower degree of transformation of organic matter.

More than a hundred lakes, groundwater, rivers, and precipitation of their catchments have been studied in Transbaikalia. The chemical composition of natural waters and the mineral composition of rocks and lacustrine bottom sediments have been analyzed. The isotope composition of waters, the isotope ratios in dissolved and deposited carbonates and in bacterial mats, and the content of heavy oxygen in the aluminosilicate fraction of bottom sediments and rocks in catchments have been determined. It is shown that all secondary minerals inherit the oxygen isotope composition of water, but their oxygen is isotopically much heavier than the water oxygen, because the water-rock interaction leads to the fractionation of oxygen and the transition of its heavy isotope into secondary carbonates and aluminosilicates. Bacterial mats using oxygen of carbon dioxide ions for their vital activity are also enriched in its heavy isotope. We have established that the water of soda lakes, being less saline than other lake waters, is more enriched in heavy oxygen isotope. Mineral analysis of lacustrine bottom sediments and material from sedimentation traps revealed chemogenic carbonates and secondary aluminosilicates. The performed thermodynamic calculations confirm their possible formation in lakes. It is shown that the lake water is in equilibrium not only with carbonates, clays, and hydromica but also with zeolites, and the most alkaline and mineralized water is in equilibrium with albite. The bottom sediments of soda lakes have a greater portion of clays and carbonates than those of fresh, chloride, and sulfate waters. The enrichment of salt lake waters with heavy isotopes is due not only to evaporation but also to the water-rock interaction. The contribution of aluminosilicate hydrolysis to oxygen fractionation is confirmed by the direct dependence of the “oxygen shift” value on pH, whose value is determined by the degree of the above interaction. We have established that the diversity of the chemical and isotope compositions of lake waters is due to their multifactorial formation related to the different degrees of lake water evaporation, the different mineralization of organic matter, and the different duration of the interaction of lake waters and groundwater with rocks.

The pattern of current crustal stress in central and southeastern Asia has been reconstructed from earthquake focal mechanisms by inversion Rebetsky’s method of cataclastic analysis. The inversion provides constraints on principal stress directions, as well as on relative magnitudes of maximum shear and effective isotropic pressure. The crust of the High Asian mountainous province is subject to horizontal extension or shear in plateaus (East and South Pamir and Tibet) and to horizontal compression or shear in mountain ranges. The relatively high horizontal compression in the ranges is apparently due more to denudation and exhumation than to the pressure from the Indian indentor. Denudation and removal of clastic material from the Himalayan slopes has been a key agent that may compensate for the N-S crust shortening in central Tibet and the W-E stretching in eastern Tibet. The current stress field of High Asia has been mainly controlled by vertical buoyancy forces that arise by thickening of lighter crust and detachment of heavier lithosphere.

Lacustrine sediments, related subaerial aeolian sands, as well as fluvioglacial sands enclosed in ablation moraines in the Chuya River valley between the Chuya and Kurai basins in Gorny Altai, have been dated by optically stimulated (OSL) and infrared stimulated (IRSL) luminescence. The OSL and IRSL dates indicate that the formation of the lake, which spread into the Chuya Basin, was caused by the Sukor rockslide that dammed the Chuya valley. The rockslide was triggered by a large earthquake (shaking intensity at least 9-10) generated by an active fault in the Kurai Fault Zone about 16 ka, which is the oldest known date for its activity. Thus, large landslides or rockslides can remain geomorphically expressed for at least 16 ka in the climate and landscape conditions as in the Chuya and Kurai basins. The lake existed till ~10 ka BP and then was gradually drained. The Kuyuktanar glacier reached its greatest extent in the early marine isotope stage (MIS) 2, and its ablation moraines formed about 25 ka. Judging by the age and good preservation of moraines left by the glacier, the hilly terrain in the area of the Sukor rockslide, and the presence of lacustrine sands in the vicinity of the lake dam, no catastrophic flooding events in the Chuya and Katun valleys have occurred since the Last Glacial Maximum (LGM; ~20 kyr BP). The water table of the glacier-dammed lake that occupied the Kurai Basin around the LGM could not have exceeded 1750 m above sea level (a.s.l.). Higher lake terraces traceable till elevations of 2100-2200 m a.s.l. should be attributed to more ancient lake stages. Archaeological sites in the area of the junction between the Chuya and Kurai basins were formed in the post-lake period, the oldest dating back to the latest Late Paleolithic time.