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Russian Geology and Geophysics

2022 year, number 2

1.
THE RUPTURING PHENOMENA IN THE DELTAIC DEPOSITS OF CAPE RYTYI ON THE NORTHWESTERN SHORE OF LAKE BAIKAL (based on aerial-photography data)

O.V. Lunina, A.A. Gladkov
Institute of the Earths Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: Rupture zone, scarp, parameters, aerial photography, UAV, orthophoto, digital terrain model, Cape Rytyi, Lake Baikal

Abstract >>
We present materials of aerial photography of Cape Rytyi, a unique and most mysterious place on the northwestern shore of Lake Baikal. Photogrammetric survey was carried out using a DJI Phantom 4 Pro V2.0 UAV and provided an orthophoto and a digital terrain model of an 11.074 km2 area. When deciphering the images obtained in the Rita River deltaic sediments composing the cape, surface ruptures trending north and northeast at 30-150 m from the shore of Lake Baikal were discovered. The ruptures are a clearly localized zone 2.9 km in total length. The performed analysis showed that the structural features of the zone obey the general laws of the development of faults resulted from prevailing extension. It has been established that the formation of the ruptures was predetermined by tectonics and is a secondary effect of resonant oscillations from the M = 5.2 earthquake of 08.13.1962, the epicenter of which was located ~35 km southeast of Cape Rytyi, in the Morskoi fault zone. The seismic event initiated the formation of surface ruptures, which led to a gravitational subsidence of coarse deposits of the fan in the shore zone. It is concluded that the development of modern geomorphologic forms in the peripheral part of the Rita River fan on land is similar to the formation of an underwater topography in the region of the Selenga River delta. It occurs under the influence of seismogenic rupturing and following gravitational movements, which intensify in a water-saturated environment and are subsequently complicated by erosion processes.
																								



2.
EARLY PRECAMBRIAN CRUSTAL EVOLUTION IN THE IRKUT BLOCK (Sharyzhalgai uplift, southwestern Siberian craton): SYNTHESIS OF U-Pb, Lu-Hf, AND Sm-Nd ISOTOPE DATA

O.M. Turkina1,2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
Keywords: Archean, Paleoproterozoic, crustal growth and recycling

Abstract >>
The paper presents a synthesis of zircon U-Pb and Lu-Hf and whole-rock Sm-Nd isotope data from main early Precambrian (3.4 to 1.85 Ga) metamorphic and magmatic units of the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). The Archean complexes consist of relict Paleoarchean (3.4 Ga) melanocratic granulites and predominant Neoarchean mafic and felsic granulites (2.7-2.66 Ga), paragneisses (≤2.75 Ga), and gneissic granites (2.54 Ga). The Paleoproterozoic complexes include paragneisses (1.95-1.85 Ga), granitoids and charnockites (1.86-1.84 Ga), as well as mafic intrusions and dikes (1.86 Ga). Few detrital zircons with Hf model ages of ≥3.6 Ga mark the Eoarchean onset of crustal growth in the Irkut block. Isotopic data record two major stages of crustal growth in early Precambrian evolution of the Irkut block: Paleoarchean (3.6-3.4 Ga) and Neoarchean (~2.7-2.66 Ga). The Paleoarchean crustal growth was most likely associated with plume magmatism fed from depleted and primitive mantle sources. The spatial distribution of Paleoarchean crust is traceable in isotopic signatures of magmatic and detrital zircons from most of Mesoarchean to Paleoproterozoic units. The Neoarchean crustal growth from a depleted mantle source was due to subduction magmatism. Moderate crustal growth occurred in the Paleoproterozoic from 2.3 to 1.85 Ga. At the turn of 1.86-1.85 Ga, mafic magmas and products of their fractionation formed from both depleted and enriched sources under postcollisional extension; the latter sources were the subcontinental lithospheric mantle formed during Neoarchean subduction. Three major stages of crustal recycling have been established: Mesoarchean (~3.0 Ga), Neoarchean (~2.55 Ga), and Paleoproterozoic (1.86-1.85 Ga), which are characterized by near-coeval intracrustal melting and metamorphism. The recycling during the ~2.55 Ga and 1.86-1.85 Ga events apparently occurred in a collisional setting. The 2.7 Ga subduction-related felsic magmas also formed through the recycling of the Paleo-Mesoarchean crust. The hypothesized scenario for the geological evolution of the Irkut block is the dominant vertical growth and crustal recycling for about two billion years. Available isotope data record similar major crustal growth in the Paleoarchean and growth combined with recycling during the Neoarchean and Paleoproterozoic events in both the southwestern and northern and central parts of the Siberian craton. The Irkut block in the southwest differed in a long and continuous recycling during the Mesoarchean and pronounced Neoarchean crustal growth.
																								



3.
ULTRAPOTASSIC ALUMINOSILICATE MELTS: SPECIFICS OF FORMATION BY THE EXAMPLE OF SYNNYRITES FROM THE SYNNYR MASSIF

L.I. Panina, E.Yu. Rokosova, M.A. Ryabukha
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: Synnyrite, melanocratic rocks, apatite segregations, melt inclusions, alkali-basaltic magma, volatile components, water, closed chamber

Abstract >>
To reveal the formation conditions of synnyrites in the Synnyr alkaline pluton, we studied melt inclusions in the minerals of shonkinites and pseudoleucite syenites, in apatite segregations in pyroxenites, shonkinites, and synnyrites, and in the minerals of later monchiquite-camptonite dikes. Based on the obtained and earlier published data, a conclusion has been drawn that all plutonic rocks of the massif formed from the same parental alkali-basaltic magma during long-term crystallization differentiation and fractionation in a closed system excluding a release of volatile components. Similar minerals in the rocks crystallized at similar temperatures in the same sequence: clinopyroxene (1280-1150 ) → leucite (1250-1200 ) → K-feldspar (1200-1180 ) ↔ apatite (above 1180-1050 ) → nepheline and kalsilite. The composition of the parental magma during crystallization evolved toward an increase in Si, Al, and K contents and a decrease in Fe, Mg, and Ca contents, i.e., toward melaphonolite and phonolite melts. The differentiation and fractionation processes led to the separation of minerals according to their specific gravity: Heavy minerals (clinopyroxene, ore minerals, and apatite) descended to the bottom of the magma chamber, forming the lower melanocratic series, and light minerals (leucite, K-feldspar, and foids), together with the residual melt, accumulated in the upper horizons of the chamber, forming the upper leucocratic series of rocks. During crystallization, the amount of fluids increased. At 920-830 , the fluids contained 3033-4051 mg/kg 2, 397-644 mg/kg 2, and 42.7-83.7 mg/kg . At the early high-temperature stage, when the amount of fluids was insignificant, the trend of magma transformation coincided with the trend of basaltoid crystallization. This fact is evidenced by the homogenization temperatures and chemical composition of inclusions in the minerals of monchiquite-camptonites and alkali basaltoids, similar to those in the plutonic rocks of the massif. Clinopyroxene crystallized in dike rocks at 4.58 kbar at a depth of 10-12 km. At the stage of crystallization of feldspars, when the amount of fluids in melts significantly increased during the formation of plutonic rocks and drastically decreased during the formation of basaltoids, the formation trends of these rocks became different. The trend of basaltoid crystallization was directed toward trachyte melts with an increase in Si contents and a decrease in Fe, Mg, Al, and alkali contents. During the formation of plutonic rocks of the massif, the high water pressure prevented the formation of plagioclase, and the melts became more enriched in Al and K and acquired a high-alumina ultrapotassi composition, forming kalsilite-nepheline-K-feldspar synnyrites at the final stages of transformation. A conclusion has been drawn that synnyrites crystallized from the residual products of differentiation and fractionation of alkali-basaltic magma in the temperature range slightly above 1050-1180 in a closed system excluding a release of volatiles. The occasional occurrence of synnyrites is due to the limited natural occurrence of closed magma chambers, macroanalogs of inclusions of mineral-forming media in minerals.
																								



4.
GEOCHEMISTRY OF BRINES AND OIL OCCURRENCES IN THE UDACHNAYA KIMBERLITE PIPE (Siberian Platform)

D.A. Novikov1,2, A.V. Ilin3, V.A. Kashirtsev1,2, A.V. Chernykh1, A.N. Pyryaev4,2, F.F. Dultsev1, A.A. Maksimova1, I.N. Zueva5, O.N. Chalaya5
1Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
3Vilyui Exploration Expedition (Alrosa PJSC), Mirnyi, Russia
4V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
5Institute of Oil and Gas Problems of the Siberian Branch, Siberian Branch of the Russian Academy of Sciences, Yakutsk, Russia
Keywords: Hydrogeology, geochemistry of brines, geochemistry of oil occurrences, metamorphic grade, stable isotopes, oil and gas content, Udachnaya kimberlite pipe, Siberian Platform, Arctic

Abstract >>
Results of a geochemical study of brines and oil occurrences in the Udachnaya kimberlite pipe are presented. Like other intrusions in the Daldyn-Alakit diamondiferous region, this diamond deposit is a unique cryohydrogeologic microstructure differing from the host sedimentary rocks and other diamond pipes of the Yakutian diamond-bearing province. Two waterlogged zones distinguished in the section of orebodies at the explored depths of the deposit correspond to the upper and middle Cambrian aquifers. Predominantly acidic (average pH = 5.5) Cl-Ca and Cl-Ca-Na brines with TDS from 94.3 to 391.3 g/dm3 are widespread within the orebodies and host rocks. The brine mineralization and contents of major salt-forming components increase with depth, to the horizon at the -365 m elevation, where TDS reaches 391 g/dm3, while below, at the -650 m level with noted hydrogeochemical-field inversion, TDS is 253 g/dm3. The mineralization of Cl-Ca, Cl-Ca-Na, Cl-Ca-Mg, and Cl-Ca-Mg-Na brines in the upper Cambrian rocks varies from 102.9 to 192.9 g/dm3, and the pH values, from 4.9 to 6.2, averaging 5.6. Among the microcomponents, the highest average concentrations (mg/dm3) are found for Br1292.8 > S875.7 > Sr453.7 > Fe79.7 > Li53.4 > B32.7 > I13.3 > Si10.8 > Mn6.4 > Se3.6 > Rb2.3. The values of genetic coefficients vary widely: The r Na/ r Cl coefficient ranges from 0.18 to 0.31; r Ca/ r Mg, from 1.03 to 3.60; Ca/Cl, from 0.2 to 0.3; and the integrated metamorphism index S (according to S.L. Shvartsev) varies from 193 to 277. The middle Cambrian rock complex, containing more saline brines, has been examined in much more detail. It hosts Cl-Ca, Cl-Ca-Na, Cl-Ca-Mg, and Cl-Na-Mg brines with TDS from 94.3 to 391.3 g/dm3 and high average concentrations (mg/dm3) of microcomponents: Br2224.9 > Sr1024.9 >S500.1 > B202.9 > Li147.1 > Fe97.0 > I33.2 > Rb11.4 > Si9.6 > Se9.5 > Mn3.6 > Ni1.7. As compared with brines in the overlying rocks, the middle Cambrian brines show a wider variation in element ratios: r Na/ r Cl from 0.14 to 0.34, r Ca/ r Mg from 0.66 to 9.71, and Ca/Cl from 0.03 to 0.45. These brines are also characterized by a significantly higher metamorphism grade, which is indicated not only by the r Na/ r Cl and r Ca/ r Mg ratios but also by the S index varying from 278 to 316. The composition of stable isotopes (δD and δ18O) and dissolved inorganic carbon (δ13C) of the brines was investigated. The studied waters are assumed to be of sedimentary-metamorphic origin. Their isotopic composition reflects the climatic conditions existing at the time of their burial, which were probably aggravated by the contribution of the oxygen isotope exchange with water-bearing rocks. The δ13C values of carbon dioxide dissolved in water allow an inference about its biogenic origin. The biogenic carbon isotope exchange is governed by the relationship between methanogenic and SMT processes. Analysis of the 87Rb/86Sr and 87Sr/86Sr isotope ratios of the studied brines has revealed affinity between the isotopic compositions of waters in the Cambrian deposits and in ancient seawaters. The mass chromatograms of saturated-hydrocarbon (HC) fractions show at least two individual types of oils and malthas (naphthides). The third variety resulted from their mixing at different stages of migration. The fourth is from the contact zone; it changed during the explosion of kimberlites. The first, most common, type of naphthides («postexplosive) is similar in all geochemical parameters to oils from the Nepa-Botuobiya anteclise, in particular, to those from the Mirnyi arch. Oils of the second (pre-explosive) type are found only in the Udachnaya Formation, within the depth range 1130-1430 m.
																								



5.
THE AGE OF GOLD MINERALIZATION OF THE UNGLICHIKAN DEPOSIT (Russian Far East): RESULTS OF 40Ar/39Ar DATING

A.Yu. Kadashnikova1, A.A. Sorokin1, A.V. Ponomarchuk2, A.V. Travin2, V.A. Ponomarchuk2
1Institute of Geology and Natural Management, Far Eastern Branch of the Russian Academy of Sciences, Blagoveshchensk, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: Mongol-Okhotsk Orogenic Belt, Unglichikan gold deposit, gold, Ar/Ar geochronology

Abstract >>
The isotope-geochronological studies of ore metasomatites from the Unglichikan gold deposit have shown their age of 136-140 Ma. There are no data on magmatism of this age within the study area; therefore, it is impossible to associate the ore mineralization of the Unglichikan deposit with magmatic processes. The thermal event superposed on the host rocks of the Zlatoustovsk Formation beyond the ore zone is dated at 140 2 Ma. Thus, the last stage of regional metamorphism and deformation and the formation of ore metasomatitesare are of the same age. We believe that the orogen deformation processes accompanied by hydrothermal activity played a significant role in the mobilization and redistribution of ore matter and in the formation of the Unglichikan deposit.
																								



6.
Low Geomagnetic Field Paleointensity on the Permian-Triassic Boundary from Study of the Kuznetsk Basin Traps (Southern Siberia)

A.A. Eliseev1,2, V.V. Shcherbakova3, D.V. Metelkin1,2, N.E. Mikhaltsov1,2, G.V. Zhidkov3, V.V. Abashev1,2, A.M. Rogov4
1Novosibirsk State University, Novosibirsk, Russia
2Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
3Borok Geophysical Observatory, department of Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Borok, Russia
4Kazan Federal University, Institute of Geology and Petroleum Technologies, Kazan, Russia
Keywords: Paleointensity, Thellier-Coe method, Mesozoic Dipole Low, Kuznetsk basin, Maltsevskaya Formation, Siberian trap province

Abstract >>
Here we report the first data on the absolute intensity value of the geomagnetic field on the Permian-Triassic boundary from basalts of the Kuznetsk basin. The latter are considered as one of the manifestations of the initial stage of trap magmatism during the formation of the Siberian large igneous province. The good preservation of information on the ancient geomagnetic field in the Maltsevskaya Formation basalts is due to the presence of small single domain and pseudo-single domain grains of primary magmatic titanomagnetite in the groundmass. The paleointensity values obtained following the Thellier-Coe method correspond to the generally accepted criteria of reliability and indicate that the geomagnetic field intensity during the formation of the Kuznetsk basin traps on the Permian-Triassic boundary was almost an order of magnitude lower than the present-day one. Moreover, the mean values of the virtual dipole moment for the Kozhukhta and the Vlasov units in the lower and middle Maltsevskaya Formation ((1.9 0.6) × 1022 A × m2 and (1.1 0.7) × 1022 A × m2, respectively) are in good alignment with determinations of the paleointensity during the accumulation of the Ivakinsky Formation of the Norilsk Region in the Siberian province, which confirms the accuracy of traditional regional correlations.
																								



7.
RECORDS OF LOCAL EARTHQUAKES AS A BASIS FOR THE CORRECT ESTIMATION OF STRONG GROUND MOTION (the area of the second Severomuisk tunnel)

O.V. Pavlenko
Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
Keywords: Stochastic simulation, stress parameter, quality factor, geometric spreading, local site amplification, duration of strong ground motion, soil response, local earthquakes

Abstract >>
To obtain realistic and correct estimates of strong ground motion in the area of construction of the second Severomuisk tunnel, the records of local earthquakes obtained by the Severomuisk seismic station (SVKR) in 2005-2010 are studied (magnitudes Mb of ~4.2-5.5, hypocentral distances r of ~15-220 km). Stochastic-simulation methods are used to estimate the regional parameters of radiation and propagation of seismic waves in northeastern Buryatia: the stress parameter Ds, shapes of source spectra, wave attenuation at high frequencies (k), parameters defining the shape and duration of the acceleration time history, etc. Beforehand, the duration of strong motion as a function of the hypocentral distance was evaluated from the records; the local site amplification A ( f ) was estimated based on the velocity structure of the region; and the envelope method was applied to estimate the quality function Q ( f ). The following data were obtained: Ds ≈ 100 bars; k ≈ 0.012 s; Q ( f ) as ~ 60 f 1.05 for r of ~15-30 km, ~ 80 f 1.0 for r of ~40-60 km, ~ 90 f 0.9 for r of ~100-110 km, and ~ 150 f 0.7 for r of ~160-220 km; geometric spreading as 1/ r for r < 50 km, 1/50 for 50 < r < 150 km, and 1/ r 0.5 for r > 150 km; and duration as 0 for 0 < r < 5 km, 0.222 r for 5 < r < 50 km, and 10 + 0.015 r for r > 50 km. These estimates can be used to predict ground motion parameters during future strong earthquakes in the region.