Home – Home – Jornals – Russian Geology and Geophysics 2015 number 5

Collisional granitoid magmatism caused by the Early Neoproterozoic orogeny in the west of the Siberian craton is considered. New data on the petrogeochemical composition, U–Pb (SHRIMP II), Ar–Ar, and Sm–Nd isotopic ages of the Middle Tyrada granitoid massif in the northwestern Yenisei Ridge are presented. Plagiogranites, granodiorites, and quartz diorites of the massif are of calcareous and calc-alkalic composition. The elevated alumina contents and presence of accessory garnet permit them to be assigned to S–type granitoids. Their spidergrams show Rb, Ba, and Th enrichment, minimum Nb, P, and Ti contents, and no Sr depletion. The granitoids formed through the melting of plagioclase-enriched graywacke source, obviously Paleoproterozoic metaterrigenous rocks of the Garevka Formation and Teya Group (T_Nd(DM) = 2.0–2.5 Ga), judging from the isotope composition of the granitoids (T_Nd(DM–2st) = 2200 Ma and ε_Nd(T) = –6.0) and the presence of ancient zircon cores (1.80–1.85 Ga). Formation of granitoids took place in the final epoch of the Grenville collision events in the late Early Neoproterozoic (U–Pb zircon age is 857.0 ± 9.5 Ma). In the Late Neoproterozoic, the granitoids underwent tectonothermal reworking caused by Vendian accretion and collision events on the southwestern margin of the Siberian craton, which explain the younger K–Ar biotite age, 615.5 ± 6.3 Ma.

Software modeling flow multireservoir systems was used to study the dynamics of carbonatization of lithospheric mantle with flows of magmatic fluids directed from a chamber in the upper mantle to the permeable zone, which dissects continental lithosphere. It has been shown that the region of physicochemical conditions of carbonatization in depleted mantle rocks corresponds to the narrow range of the compositions of hypothetical fluids. If the total content of the fluid is ~4 wt.% and the contents of SiO₂ and Ca are 0.5–0.1 moles, (1) the ratio of the molar fractions of Si to Ca is less than unity; (2) the ratios of molar fractions in the C–H–O system are 1 : 2 : 3 or 2 : 1 : 2; (3) –8 < lg p_O2 < –11; and (4) CO₂ content in the fluid is higher than H₂O content by a factor of 1.5–2, and chlorine significantly dominates over fluorine. If the content of the fluid phase is lower and this phase has a lower major-element content by an order of magnitude, the carbonatization becomes stronger as Ca content decreases.

Plagioclase-bearing garnet-omphacite (Grt–Omp) eclogites and garnet-augite (Grt–Aug) eclogite-like schists from the amphibolite and gneiss beds of the Belomorian Mobile Belt (BMB) have been studied. They are spread over a large area. In most of the studied objects, these rocks have preserved primary concordant relations with the host amphibolite and gneiss strata; they are not disturbed by late tectonic processes and are not genetically related to tectonic-melange zones. Their protoliths were amphibolite lenses in gneisses or large mafic zones composed of amphibolites. The Grt–Omp eclogites formed in the low-pressure field of the eclogite facies (P = 12.5–13.0 kbar, T = 600–630 ºC), and the eclogite-like Grt–Aug rocks, at the boundary between the amphibolite and eclogite facies (P = 9.6–11.1 kbar, T = 630–700 ºC), under the intense impact of metamorphic fluid on the amphibolites. The compositional evolution of the rock-forming minerals during the formation of Grt–Omp eclogites and eclogite-like Grt–Aug rocks followed the same scheme. The petrographic diversity of apoamphibolite rocks (Grt–Omp eclogites and Grt–Aug schists) might be due to the difference both in the bulk composition of the metabasic protolith and in the ratios of CaO and Na₂O activities in the metamorphic fluid. The relatively low content of CaO leads to the formation of Grt–Omp paragenesis in eclogites. Higher CaO contents give rise to eclogite-like Grt–Aug rocks containing jadeite-poor clinopyroxene.

Plates made of diamonds from the São Luiz province (Brazil) were investigated by confocal scanning luminescence microscopy. The samples have many macroinhomogeneities (cracks and inclusions), but there is a quasi-uniform distribution of luminescence centers in the bulk. At all investigated points of the crystals, the same group of centers was observed: N3, H4, 575, and a red band with a maximum at 690–700 nm. The visible nonuniformities in the distribution of luminescence over the area of the plates are determined by relatively small fluctuations in the ratio of the intensities of individual bands in the spectra. Nitrogen centers of different degrees of aggregation (H4, N3, and 575 nm, with four, three, and one nitrogen atom, respectively) coexist in these crystals. In the same zones of the samples, the distribution of blue luminescence (N3 centers) is diffuse (uniform), but the distribution of yellow-green luminescence is characterized by layering on (111). This might be a consequence of the tangential growth of octahedron faces or a result of plastic deformation of the crystals and dislocations along (111).

The paper is concerned with Jurassic calcareous nannoplankton from the Dobrudja foredeep. The regional Jurassic stratigraphy based on nannoplankton is considered. The NJ10/Stephanolithion speciosum, NJ11/Pseudoconus enigma, NJ14/Stephanolithion bigotii maximum, and NJ15/Cyclagelosphaera margerelii Zones, consistent with the International Nannoplankton Zonation Scheme, have been recognized in the deposits of the Dobrudja foredeep.

The biogeochemistry of organic lacustrine sediments (sapropels) has been poorly studied in Siberia. In this paper we show the specifics of sapropel formation caused by low pH and mineralization of water by the example of Lake Ochki in Cisbaikalia. The main sources of organic matter are zoo– and phytoplankton, which concentrate mostly basic chemical elements and also some alkaline, alkali-earth, and chalcophile elements and more them to the sediment. The calculated enrichment factors (EF) have shown that the lake plankton is strongly enriched with phosphorus and chalcophile elements. The calculations have also revealed a great contribution of the plankton to the elemental composition of sapropel (Me_bio, %): P (≈100), Cd (57), Hg (40), Se and Na (30), Zn (23), K (21), and Ca (15). Elements are rather uniformly distributed throughout the 3.2 m thick sapropel layer. Lithophile elements (Al, Sc, Ti, Y, Zr, Nb) and LREE are mostly from a terrigenous source. The high contents of mobile elements (Cd, Sb, Sn, Pb, Zn) in the upper part of sapropel are probably due to anthropogenic factors. The high Cu and Zn contents in some sapropel layers are probably related to the inflow of deep-level groundwaters, and the elevated contents of Hg, Cd, and Sb might have been caused by forest fires.

The present paper considers certain problems of gravity monitoring at oil and gas fields arising as a result of the inversion of repeat measurement data, when finding the positions of water-oil and water-gas contacts. The main sources of noise in gravity data are errors in vertical positioning of the tool, changes in atmospheric pressure, and variations in groundwater level and soil moisture. An algorithm based on using a multisensor borehole tool is proposed for a more accurate inversion. Examples of successful location of the water front while solving model problems with the help of this algorithm are provided.

We consider the acceleration spectra of S waves from M = 5–6.5 earthquakes that occurred in three regions of the Baikal Rift Zone. The main characteristics of the shape and level of the spectra are given, and the difference between the spectra in the relative positions of the focus and seismic station is shown. Average acceleration spectra have been obtained for all three regions with regard to the form and magnitude of the earthquakes. The spectra have been transformed so that they correspond to the accelerations of M = 6.5 and M = 7.5 earthquakes. The conclusion is made that the spectra not necessarily retain their shape even with the same movements in the focus.

The porosity and permeability of core rocks were studied by X-ray tomography. This method has a high potential for studying petrophysical properties of rocks, because it permits not only a general quantitative estimation of the void volume but also visualization of the rock texture, including pores, cavities, cracks, and zones of different densities in the matrix. X-ray tomography permits detailed studies of rock inhomogeneity, which are necessary for the elaboration of reliable porosity-permeability models for hydrocarbon pools. The investigations at Perm’ State National Research Polytechnic University have shown that X-ray tomography of core samples has a wide spectrum of applications in petroleum geology. Nikon Metrology XT H 225 X-ray computed tomography makes it possible to examine samples with a standard diameter (30 mm) and whole core samples (100 mm). The structure of voids in carbonate and terrigenous rocks was studied on samples with standard and full diameters; the results of hydrochloric acid treatment of carbonate reservoir rocks were visualized; and the mechanical properties of rock salts were studied. Three-dimensional models for the structure of voids and mineral matrix of the core samples have been constructed with the use of the Avizo Fire software.

This is an overview of theoretical research on solitons and other nonlinear waves induced by deformation in fault block (fragmented) geological media, including a brief description of available models with their geophysical and geodynamic applications. The cited models correspond to the classical and perturbed sine-Gordon equations and use the rotation angle and displacement of fault blocks as dynamic variables. The models simulate kinematics and dynamics of traveling deformation fronts (kinks and solitons) generated in faulted rocks. The sine-Gordon equation is presented in its seismological and geomechanic applications for description of earthquake data and modeling of strain waves, faulting patterns, and subduction processes, including slow earthquakes and periodic episodic tremor and slow slip (ETS) effects.