Home – Home – Jornals – Russian Geology and Geophysics 2017 number 2

We present geochemical characteristics of rocks and results of local dating and Lu-Hf isotopic analysis of zircons from two massifs of Paleoproterozoic collisional granitoids in the northwest of the Sharyzhalgai uplift. The rocks of the Alar intrusion in the Bulun terrane correspond in major- and trace-element composition to I -type potassic granites. The Alar granites formed at ~780 ºC and <5-8 kbar through melting of predominantly graywacke (volcanosedimentary) source rocks with the contribution of plagiogneisses of tonalite-trondhjemite complex. The age and Lu-Hf isotopic similarity between inherited zircon core (3.3-3.0 and 2.85-2.6 Ga) in these granites and zircons from the Paleo- and Mesoarchean rocks of the Bulun terrane suggests that the latter are the most likely crustal sources of the granites. The more radiogenic isotope composition of the Paleoproterozoic (1.85 Ga) igneous zircons from the granites as compared with the zircons from the Archean crustal rocks of the Bulun terrane testifies to the contribution of juvenile material to the granite formation. Highly ferroan granodiorites and granites of the Shumikha intrusion in the Onot terrane are enriched in HFSE and correspond to A -type granites. They probably derived by the melting of crustal sources of intermediate-felsic (tonalitic) and mafic composition at ≥860 ºC. The Hf isotope composition of igneous and inherited zircons indicates that the granites formed from ancient crustal source (model Hf age is >3.0 Ga) with the contribution of Neoarchean juvenile, probably mafic material.

The paper reports on seafloor terrains mapped to a high resolution, the structure of shallow-water sediments, and the extent of barite mineralization in the Deryugin Basin, Sea of Okhotsk. The data were collected by different methods of remote sounding of the bottom and a Comanche submersible in the area of the Barite Ridge deposit. The sedimentary cover, including Holocene sediments, is strongly deformed in this area and in some other areas of the basin. Several small mud volcanoes and new barite build-ups along the ridge axis are evidence of active fluid venting. Precipitated barite forms columns (chimneys) or grows over older layered barite build-ups. The research confirms the hypothesis of Ba supply by deep cold hydrotherms.

The Noion-Tologoi polymetallic deposit corresponds to the Akatui type of polymetallic deposits of eastern Transbaikalia. These deposits are associated with volcanoplutonic rocks of the Akatui complex. In element contents the polymetallic ores of the Noion-Tologoi deposit are similar to ores of volcanohydrothermal (VHMS type) deposits of southern Siberia. The orebodies are tabular, stockwork- and vein-like. They are localized in fault zones. The ores contain, on the average, 4.9% Pb, 6.8% Zn, and 272 ppm Ag. The most common ore minerals are pyrite, galena, arsenopyrite, and sphalerite. The δ¹⁸O (SMOW) values in the ore quartz range from -3.9 to +10.6‰. The isotopic composition of oxygen in the fluid (at 250-275 ºС) varies from -12.80 to +2.79‰, indicating the participation of meteoric waters in mineralization. The enclosing rocks are the volcanoterrigenous deposits of the Mulino Group (J₂-₃) and syenite porphyry of the Akatui complex (J₂-₃). In geochemical characteristics the syenite porphyry is similar to adakite intrusions. The Rb-Sr isotopic age of syenite-porphyry is 150 ± 9 Ma. The initial ratio of strontium isotopes in these rocks is (⁸⁷Sr/⁸⁶Sr)0 = 0.70698 ± 0.0004, which indicates their mantle-crustal source.

The Afontova Gora II archaeological site located within two geologically and geomorphically different areas has been studied in terms of geology and Quaternary deposition history. Both areas lie upon alluvial terrace II of the Yensei River. At the time when terrace I alluvium was deposited, aeolian and slope-wash facies accumulated on the surface of terrace II and a slope-wash apron formed on the gently sloping surface of terrace III. It is at these different levels of the Yenisei River bank that the latest Paleolithic Afontov culture of Siberia has been found. At the Late Glacial-Holocene boundary, the slope-wash apron upon terrace III was involved in landsliding and thrust over terrace II, which deformed the subaerial deposits over both terraces II and III. As shown by detailed analysis, the landslide deposits are either strongly deformed to the degree of losing of their primary sedimentary textures and structures or form imbricated blocks with deformed margins but minor deformation in their interior. The preserved textures and structures of sediments inside the imbricated landslide blocks provide stratigraphic and genetic constraints and prove the in situ origin of anthropological, archaeological, and paleontological finds. This division also explains the archaeological sterility and abnormal thickness of the subaerial cover upon the landslide blocks.

Morphostructural modeling of the block structure of part of the Scandinavian crystalline shield has shown that the ring structure of the Siljan Ring impact crater is located in the center of a morphostructural node, a ring structure with a diameter of 300 km, marking a large disjunctive knot. The crater area consists of a central block, which is a granite massif, and of a surrounding mobile morphostructural boundary forming a wide small-block ring depression zone, where oil and gas shows have been revealed. This zone is regarded as the most promising one for the search for migration channels and atypical shows of hydrocarbons.

The Bathonian Stage in the Sokur Tract section on the periphery of the city of Saratov (Central Russia) contains both Tethyan and Boreal fossils. This makes the section a key object of research aimed at solving the problems of detailed correlation of the Bathoman between different paleobiogeographic provinces and calls for integrated studies involving both paleontological (based on different groups of fauna) and physicochemical (paleomagnetic and isotope-geochemical) methods. The results of study of carbon isotopes in belemnite (Cylindroteuthididae) rostra from the Sokur Tract section show that the Subboreal Oraniceras besnosovi Zone and Boreal Arcticoceras harlandi Subzone are correlated with the standard Zigzag Zone, whereas the Boreal A. ishmae Subzone is totally or mostly comparable to the standard Tenuiplicatus Zone. We have recognized analogs of the chrons of the geomagnetic-polarity time scale (GPTS): e-Bath N, M-Bath R, and lt-Bath N, corresponding to the Lower-Middle Bathonian and, possibly, lower Upper Bathonian.

We present results of pyrolysis (Rock Eval) and pyrolysis-gas chromatography-mass spectrometric methods of analysis of organic matter (OM) in the Holocene sediments of the Black Sea. The studied samples represent two parts of the sediment column: coccolith ooze with an average Corg = 2.8% at the top and sapropelic silts with an average Corg = 8.5% at the bottom. Sedimentation of the Holocene sediments might have proceeded in euxinic conditions, and in ancient Black Sea time they were more favorable for OM safety and the formation of sapropelic silt. The performed research has shown that the degree of diagenetic OM transformation is higher in the upper part of the core as compared with the lower one. This is probably due to the contemporary processes of anaerobic bacterial oxidation of methane supplied from deeper layers of the Black Sea deposits into the ancient Black Sea sediments.

We present a review of the literature on the methods of modeling of sedimentary rocks containing gas hydrates with the aim of a subsequent study of their physical and mechanical properties. The attention is focused mainly on the non-Russian researches carried out in the last 15 years. The review includes description of corresponding setups and modeling techniques. The dependence of the morphology of hydrate present in the specimen on the specimen preparation technique is reviewed in detail. In addition, the structure and texture of synthetic and natural hydrate-containing rock specimens are briefly reported, as well as the regularities of formation and dissociation of gas hydrates in the Earth’s crust.

The seismic effective quality factor ( QC ) and its frequently dependences or the frequency parameter ( n ) and attenuation coefficient (δ) for the Earth’s crust and upper mantle of the North Tanzanian divergence zone (East African rift system) were estimated from an analysis of the earthquake coda waves recorded in the SEISMO-TANZ’07 French-Tanzanian seismic experiment. The QC values increase and the n and δ values decrease with increasing frequency and length of the lapse time window. This behavior of the attenuation parameters may be evidence that the degree of heterogeneity of the lithosphere decreases with depth. Comparison of the depth variations in the attenuation coefficient δ and the frequency parameter n with the velocity structure of the region shows that there is a distinct change in the behavior of seismic wave attenuation at velocity discontinuities. The obtained attenuation parameters were compared with the same parameters obtained in our previous studies for other continental rift systems - the Baikal rift system (Eurasia) and the Basin and Range Province (North America).

This paper presents the results of physical and mathematical modeling performed to evaluate the potential of transient electromagnetic sounding in areas of salt-dome tectonics. Two geoelectric arrays are considered: an array with an inductive source (a horizontal loop) and an array with a mixed-type source (a horizontal current line). It is shown that the transient electromagnetic method provides important information on the relief of the top of salt deposits.