The article presents the results of an analysis of data obtained on the Yamm–Torzhok regional profile, which crosses the Ilmen anomaly of electrical conductivity along its main axis. These works continue the long-term study of the structure of the Precambrian basement complexes of the western part of the East European Platform by the staff of the Department of Geophysics of the Faculty of Geology of Moscow State University and GEMRC IPE RAS. Based on the obtained data, together with the analysis of potential fields, a deep geoelectric model was constructed, and a model of the sedimentary cover structure was presented separately.
Kuzmin I.A, Tolstykh N.D.
Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the RAS, Novosibirsk, Russia;
Keywords: Talnakh intrusion, main layered series, picritic gabbro-dolerites, inverse zoning, hybrid magma.
Norilsk-type intrusions are characterized by unique reserves, disseminated copper-nickel mineralization, which is localized in picrite and taxitic gabbro-dolerites. At the same time, if picrite and taxitic gabbro-dolerites are usually genetically separated from each other, then the mechanism of formation of picrite gabbro-dolerites themselves is still under discussion. Most often, they are considered as a cumulative part of a layered series. The work shows a sharp geochemical contact between the layered series and picrite gabbro-dolerites, in the section of which, in turn, we have identified an inverse geochemical zoning, expressed in the patterns of accumulation of petrogenic elements that do not fit into a single trend of crystallization differentiation with the rocks of the main layered series. The discreteness of rocks within the horizon of picritic gabbro-dolerites is demonstrated - two intervals are distinguished: the lower one with low chromium concentrations, with stable europium and strontium anomalies, and the upper one, which is characterized by abnormally elevated Cr2O3 contents, reduced LILE concentrations and the absence of a pronounced europium maximum, typical of the lower part of picritic gabbro-dolerites. Based on the known models of formation of inverse zonality of marginal zones of layered massifs, the article presents a new genetic scheme for the formation of picritic gabbro-dolerites as products of pulsation intrusion, which suggests that the lower part of picritic gabbro-dolerites was formed from hybrid magma, and the upper one - from primitive magma, due to pulsation filling of the formation chamber, which, in our opinion, is associated with inverse zonality and accumulation of chromium in the upper part of picritic gabbro-dolerites.
V.V. Lapkovsky, M.V. Lebedev, E.A. Ianevits 1Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia
2OOO Tyumen Petroleum Research Center, Tyumen, Russia
3Branch of Lukoil-Engineering LLC "KogalymNIPIneft",Tyumen, Russia
Keywords: Stochastic modeling of structural uncertainties; probabilistic estimation of resources; estimation of seismic velocity anomalies, probability distribution of structural trap areas, lognormal distribution of structural trap areas
In the new paradigm of the Russian oil and gas industry, much attention is paid to small hydrocarbon fields. This statement increases the requirements for the accuracy of forecasts of total hydrocarbon reserves in small-scale anticlinal traps. Such forecasts can be made by probabilistic estimation of resources, taking into account the probabilities of the field existence. The objective of the present research is the development of the scientific approach to quantifying the uncertainties associated with the existence and geometric parameters of small-scale anticlinal hydrocarbon traps mapped by modern 3D seismic exploration. The main method of solving this problem is stochastic modeling of structural uncertainties based on errors of structural mapping. The results of the study are summarized as follows. The type and parameters of the probability functions of the structural area are determined by the intensity and size of the velocity anomalies, as well as the parameters of the trap itself and the nature of its structural environment. The distributions of the structural closure areas can be described by functions in which the Mean value is greater than the quantile P50 value, the Mean value is approximately equal to the P50 value, and the Mean value is less than the P50 value. The lognormal distribution, which is commonly used to model structural uncertainty, is just one of the possible options. For small-scale hydrocarbon traps, their amplitude is comparable to the thickness of a productive reservoir; therefore, a change in the amplitude of the trap determines a change in the weighted average oil-gas-saturated thickness. As a result of the modeling, a positive relationship was established between variations in the anticline traps areas and their amplitudes. Accordingly, this relationship must be taken into account during the probabilistic assessment of the resources of this kind of the hydrocarbon traps. Otherwise, there may be a significant reduction in the range of uncertainty in resource estimates. Stochastic modeling of structural uncertainties is also a method of estimating the probability of the existence of anticlinal traps mapped by seismic exploration.
A.A. Tsygankov, G.N. Burmakina
Dobretsov Geological Institute of Siberian Branch of Russian Academy of Sciences, Ulan-Ude, Russia
Keywords: Angara-Vtitm batholith, U-Pb isotopic age, Lu-Hf, sources of magma sources, geodynamics, Western Transbaikalia.
The Late Paleozoic granitoid province of Transbaikalia (Angara-Vitim batholith, AVB, Russia), located in the northeastern part of the Central Asian Orogenic Belt (CAOB), covers an area of about 200,000 km2 and is composed of rocks ranging in composition from monzonites and quartz syenites to leucocratic granites. This work is aimed at: 1) determination of the total duration and dynamics of the formation of granitoids of the Angara-Vitim batholith; 2) finding the causes that determined the spatial and temporal heterogeneity of granitoids; 3) reconstruction of the sources of salic (granitoid) magmas, assessment of the contribution of mantle-crustal interaction processes to the petrogenesis of granitoids. The article is based on new petro-geochemical, isotopic (Lu-Hf) and isotopic-geochronological (U-Pb) data on the northern part of the AVB. Combined with the results of previous studies, it has been established that one of the largest granitoid provinces on Earth (AVB) was formed ~ 45 million years ago (from 320 to 275 million years). About 90% of the batholith rocks were formed during this time. Mainly crustal metagravaccian protoliths were the source of salic magmas. Formation of monzonitoids, quartz syenites, and granodiorites is associated with melting of mixed protoliths, in which the proportion of juvenile mafic material could reach 40-50%. The Late Paleozoic granitoid magmatism of Transbaikalia began with the actual introduction of calcareous-alkaline granites, granodiorites, and quartz syenites, which make up the bulk of the first stage of magmatism. At the second stage, magmatism was concentrated in a relatively narrow (200-250 km) permeable zone of the northeastern strike. This zone drained crustal foci of salic magmas and favored the entry of mafic mantle melts into the upper horizons of the Earth's crust. The granitoids of the Angara-Vitim batholith were formed at the post-collisional stage of the evolution of the eastern segment of the CAOB under the influence of mantle plume on the crust of the young orogen.
D.V. Metelkin1,2, E.V. Vinogradov1,2, A.A. Eliseev1,2, M.E. Luzan1,2, V.V. Abashev3 1Novosibirsk State University, Novosibirsk, Russia 2Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia 3Geological Institute Russian Academy of Sciences, Moscow, Russia
Keywords: Hotspots, mantle plumes, geodynamo, geomagnetic reversal frequency, paleointensity, geocentric axial dipole, equatorial dipole, anomalous geomagnetic field, global magnetic anomalies, absolute plate reconstructions
We present an analysis of modern paleomagnetic data from large igneous provinces and palaeorift structures in Siberia and the High Arctic that are potentially related to mantle plumes. The interrelationship between plume magmatism, geomagnetic reversal frequency, and field intensity over the last 600 Myr exhibits a periodicity of 70–100 Myr. Periods of mantle plume activity were preceded by an increase in geomagnetic reversal frequency, accompanied by a decrease in geomagnetic field intensity. Our proposed hypothesis explains this effect by changes in the thermal convection in the Earth’s outer core while mantle plumes are regarded as regulators of the state of the hydromagnetic dynamo. “Overheating” of the core increased the turbulence of convective currents, and therefore, the amount of reversals. During reversals, the value of the main component of the geomagnetic field – the geocentric axial dipole – first fell to zero and returned to high values only afer a full reversal of the poles. Reduction of relaxation time in periods of frequent reversals led to prolonged low values of the absolute intensity of the geomagnetic field. Mantle plumes forming during such periods could remove the excess heat and stabilize the state of the geodynamo, even almost completely stopping reversals. We link the Vendian and Devonian geomagnetic phenomena to periods of ultra-frequent reversals. During these extended periods of low value of the axial dipole, the configuration of the geomagnetic field was determined by low-order non-zonal harmonics and by the global magnetic anomalies. We observe a qualitative coincidence of the position of paleopoles with centers of lower mantle gravitational and magnetic anomalies and postulate that the anomalies were stationary. This is the basis for substantiating a new reference framework for paleotectonic reconstructions in absolute coordinates. Examples of reconstructions made using this system also agree with the hypothesis of stationary hotspots. From the terminal Precambrian to the Mesozoic inclusively, the Siberian paleocontinent was located in the area of effect of the African mantle hot field, migrating northwards along the 0° meridian from the latitude of Tristan da Cunha to that of Iceland.
V.S. Shatsky1,2,3, A.L. Ragozin1, V.N. Reutsky1, V.V. Kalinina1 1 V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Science, Novosibirsk, Russia 2 Novosibirsk State University, Novosibirsk, Russia 3 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: inclusions in diamonds, diamond formation, mantle, subduction zone
Evidence of the involvement of Fe-C-O melts in diamond formation from placers in the northeastern Siberian Craton, along with our previously obtained data on iron carbide inclusions and literature sources, has provided a new perspective on processes diamond formation in subduction zones. Iron carbide and oxide inclusions in diamonds, along with the presence of moissanite and carbonates in polyphase inclusions, indicate a heterogeneous diamond environment. Extreme variations in oxygen fugacity during diamond formation processes can be caused by the generation of hydrogen and hydrocarbons during the interaction of carbonated rocks of the subducting oceanic lithospheric plate with aqueous fluids. Separated hydrocarbon fluids can create local areas of ultra-reduced mantle, where silicon carbide can be formed under conditions of the absence of equilibrium with the surrounding rocks.. A characteristic feature of the diamonds studied is that they were subjected to brittle deformation, followed by crack healing, accompanied by the formation of polyphase inclusions of iron carbides and oxides, which we interpret as melt inclusions. Brittle deformations of diamonds, in the conditions of the lower lithosphere, are explained by high deformation rates, which can be realized in the hypocenters of deep-focus earthquakes of the subducting lithospheric plate, during dehydration processes or the formation of carbonatite melts.
A.N. Didenko1,2, M.Y. Nosyrev2, G.Z. Gilmanova2, M.D. Khutorskoy1 1Geological Institute of the Russian Academy of Sciences Moscow, Russia 2Institute of Tectonics and Geophysics FEB RAS Khabarovsk, Russia
Keywords: Amur Plate, Curie Point Depth, plate boundaries, thermal anomalies and tectonic structures
Based on the results of spectral analysis of the anomalous magnetic field, the depths of the roof and the sole of the magnetically active layer of the Amur Plate and adjacent territories are calculated. The causes of variations in the depth of the sole of the magnetoactive layer (CPD) from 14 to 38 km (average 24 km) have been determined. Maximum CPD depths are observed within sedimentary basins (Erlian, Songliao, Sredneamursky) in the southwest and in the central part of the plate. The areas of minimum depths in the continental part are located in the northwest within the giant granitoid batholiths (Angara-Vitim, Khentei) and in the northeast within the Bureinsky province. The third area of minimum CPD values is located within the waters of the Sea of Japan.
The relatively high elevation of the bottom of the magnetically active layer in the Sea of Japan is associated with rifting processes in the back-arc basin, which began at the end of the Oligocene, and the generation of fluids and magma chambers above the Pacific slab, sinking under the Amur Plate. Two areas of high CPD standing in the continental part of the plate are associated with the presence of two thermal anomalies. The north-western one is explained by the presence of a thermal crustal anomaly due to the process of radioactive heat generation by granitoids of the giant Angara-Vitim, Khangai and Khentei batholiths. Northeastern Bureinskaya – the presence of an anomalous temperature of the mantle here.
A comparison of the newly constructed CPD map with the boundaries of the Amur Plate, previously determined mainly from seismic data, shows that the surface boundaries of the plate coincide mainly with the zones of the greatest CPD gradients. All of them are associated with areas of increased seismic energy generation, with the exception of one small area on the southern border of the Amur Plate at its junction with the Yangtze Plate. In our interpretation, plate boundaries are not just lines on the surface, they are fairly wide zones from tens to the first hundreds of kilometers that encircle the plate.
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