Home – Home – Jornals – Russian Geology and Geophysics 2019 number 1

The available body of geological and geophysical data indicates that the morphologic structures of the Central Arctic submarine elevations complex (CAE) form a single complex block of continental crust that broke away from the Barents-Kara continental margin in the late Paleocene. Seismostratigraphic interpretation of the multichannel seismic reflection data acquired within the CAE, based on seismostratigraphic benchmarks confirmed by drilling and continuous tracing of pre-Cenozoic unconformities from the offshore North Chukchi Trough to its deep-water extension (Vilkitsky Trough), makes it possible to draw the following conclusions: The sedimentary-basin depocenters of the Vilkitsky Trough and Chukchi basin include pre-Upper Jurassic sediments in addition to Cretaceous complexes. However, the former are not common in the rest area of the CAE. Synrift extension of the continental crust is the key factor that affected the tectonic evolution of morphologic structures of the Central Arctic basin. Multichannel seismic reflection data show the clearest signs of the synrift extension in the Lomonosov Ridge, Mendeleev Rise, Chukchi plateau, and their flanks sloping to the sedimentary basins of the Vilkitsky Trough and Chukchi basin. At the same time, the depocenters of these sedimentary basins formed by pre-Upper Jurassic deposits are characterized by an almost undisturbed bedding of all sedimentary complexes. Pre-Upper Jurassic deposits might be interpreted as a relic of the Ellesmerian structural stage preserved in the deep-water extension of the North Chukchi trough since the preoceanic evolution stage. Pre-Upper Jurassic complexes seem to be affected by deep rift activity only within the elevations of the Central Arctic area and near-flank zones of the depressions separating them. Pre-Upper Jurassic deposits in the sedimentary basin depocenters of the Vilkitsky Trough and Chukchi basin structurally linked to the shallow-water shelf were barely affected by the rifting processes. The tectonic evolution of the depocenters and their submergence relative to the flank zones might have been affected not only by crustal extension processes but also by compensation mechanisms.

Geological, geochronological, and geochemical isotope studies are carried out for metamorphosed volcanic rocks and dolerites of the Maltsevka sequence of the Elash Group in the Biryusa block of the Siberian craton. It is found that mafic igneous rocks (dolerites and basaltic andesites) are close in composition to intraplate basalts. Flat or slightly fractionated REE patterns ((La/Yb) _n = 1.3-2.3) and positive ε_Nd( T ) values of +3.7 and +4.1 are observed. It is assumed that the depleted asthenospheric mantle and, possibly, plume mantle were the sources of these rocks, while the lithospheric mantle had no significant effect. Meta-andesites of the Maltsevka sequence belong to the tholeiitic series and have high La, Th, and U contents. Pronounced negative Nb and Ti anomalies are observed in the multielement patterns of these rocks, along with negative ε_Nd( T ) values of -4.6. It is assumed that meta-andesites resulted from the late Archean crustal melting with the participation of the mantle material. Metarhyolites prevalent in the Maltsevka sequence are divided into two groups similar in REE composition to А-type and I-type granites. A-type metarhyolites show high contents of Zr, Y, Nb, Th, and REE (except for Eu) and positive ε_Nd( T ) values of +2.2 and might have resulted from the melting of a source with geochemical isotope parameters close to those of mafic igneous rocks of the Maltsevka sequence. I-type metarhyolites have low contents of Y, Yb, Zr, and Nb but high contents of Th and show negative ε_Nd( T ) values of -3.7. They might have resulted from the melting of lower crustal diorite-tonalite rocks with addition of juvenile mantle material to the magma generation area. U-Pb zircon dating of metarhyolites of the Maltsevka sequence corresponding to A- and I-type granites showed that they are close in age, 1872 ± 10 and 1874 ± 10 Ma, respectively, which agrees with the age estimated earlier for granitoids of the Sayan complex of the Biryusa block. The similar ages and structural positions, along with the localization within the same structure, made it possible to unite volcanic rocks of the Elash Group and granitoids of the Sayan complex of the Biryusa block into a Paleoproterozoic volcanoplutonic association. The rocks of the association form the Sayan-Biryusa volcanoplutonic belt stretching for about 300 km along the zone of junction of the Biryusa block of the Angara fold belt and the Archean Tunguska superterrane of the Siberian craton. The belt is part of the large Paleoproterozoic South Siberian postcollisional magmatic belt formed at the final formation stage of the Siberian craton, when it was possibly part of the Paleoproterozoic Columbia supercontinent.

The Inagli zoned ring massif is located in the Central Aldan ore district. The close localization of ultrabasic and alkaline potassic rocks, magmatism, and mineralization (platinum, gem Cr-diopside, and vermiculite) make this object unique from both geological and economic perspectives. Formation of alkaline rocks in the Inagli massif was related to the Mesozoic tectonomagmatic activation of the Aldan Shield and occurred in several stages. Intrusion of clinopyroxenites took place no later than 145.8 ± 3.2 Ma, whereas most of alkaline rocks of the ring framing formed at 133-128 Ma. The feldspar-Cr-diopside-mica veins in the dunite core are dated at 133.4 ± 1 Ma.

Composite dikes (mingling dikes) of Western Sangilen (southeastern Tuva) are believed to indicate a shift from tectonic contraction, related to the mountain fold system formation, to postcollisional extension that ended with the disintegration of the collisional orogen. However, the position of composite dikes in the general sequence of magmatic events was based only on their geologic location and has not been confirmed by U/Pb isotope-geochronological data. The proposed study is concerned with the geologic structure, composition, and isotopic age (485-490 Ma) of West Sangilen composite dikes. The isotopic age of the host granites from the Matut pluton is 509 Ma; thermal events of Riphean (679-934 Ma) and early Paleozoic (484 Ma) ages are reflected in zircons from gneiss-granites of the Erzin metamorphic complex. The obtained data are consistent with the earlier determined sequences of West Sangilen tectonometamorphic events. Basites from the examined composite dikes are strongly different in the contents of indicator elements (Rb, Nb, Ta, Zr, and Hf) and the Nb/Y, Zr/Y, Th/Ta, Zr/Nb, and Nb/Th ratios, which is indicative of different (probably, different-depth) mantle sources. The protolith of the salic component of the combined dikes might have been rocks similar in composition to the autochthonous/parautochthonous gneiss-granites of the Erzin metamorphic complex.

On the territory of the Egor’evsk district, 3500 results of probe microanalyses of surface gold particles taken from 17 placers, weathering crusts, and orebodies were processed and summarized. High-fineness and mercury-containing gold is predominantly found within mother lodes and placers, and medium- and low-fineness gold is distributed in subordinate and dramatically subordinate quantities, especially in the placers. A unique feature of the gold composition, rarely occurring in other districts, is the constant and commonly simultaneous presence of mercury and copper impurities. Analysis of Ag, Hg, and Cu content variations has enabled us to identify five main grades of gold. The mother lodes of the predominant gold grades are metasomatites with beresite and listwaenite compositions, which are developed primarily after lower Cambrian volcanoterrigenous-carbonate rocks and ore-bearing mafic dikes. Mercury-containing gold is characteristic of beresites, but copper-bearing gold is typical of listwaenites. The relationship between corresponding grades of surface and ore gold is confirmed by the presence of microinclusions in the gold grains. Nonconformity between the content of gold from mother loads, weathering crusts, and placers is explained by the losses of Hg and Ag impurities by endogenous gold under subsurface hypergene conditions. Identification of mineral-geochemical properties of surface gold is of exceptional practical importance in ore-grade gold mineralization prediction.

Integrated geological and geochemical studies were performed for lignite from the Yenisei middle reaches, represented by carbonized fragments of trees with high germanium contents. Geochemical characteristics of terrigenous sediments with Ge-bearing lignite are determined. The chemical and mineral compositions, textures, and structures of carbonized wood fragments were studied. Scanning of individual cross sections of lignite fragments has revealed a regular distribution of germanium and impurity elements. Consistent patterns of the formation of Ge-containing lignites have been established, as well as the processes of their posthydrothermal transformation, which led to the impoverishment of the primary contents of the valuable component and to the input of a number of impurity elements. The latter formed rims over the lignite fragments and microveinlets with sulfide mineralization. The hypothesis has been put forward that germanium mineralization formed in lignites of the Kas basin, in particular, the Serchanskoe deposit.

One of the main objectives of geoelectric prospecting is mapping of the consolidated low-conductivity basement geometry. To resolve the issue, it will be equally important to delineate local structures at the bottom of the sedimentary complex and to estimate the misinterpretation of results due to the presence of areas with nonuniform conductivity in the stratified geologic cross section. Another relevant objective of EM sounding is to resolve the problem of delineation of anticlinal oil and gas traps. The horizontally layered model with a local inclusion of 3D abnormal areas is certain to fit for the above-mentioned objectives. The study is concerned with the technique of nonstationary EM sounding. When applied to the solution of structural problems, this technique considers the uniform distribution of fields within relatively large volume units of the space explored. Consequently, it results in the more efficient application of the perturbation technique (Born approximation) to the solution of the forward electrodynamics problem. The study presents the findings of 3D tomographic inversion with the use of synthetic and physical modeling data. They definitely allow us to acknowledge that the proposed mathematical apparatus for 3D inversion based on Born linearization of the forward problem has proved to be quite applicable.

Results of profile magnetotelluric (MT) and magnetovariational (MV) soundings of the Pamir-Alai zone are presented. The problems of construction of a 2D geoelectric model of the lithosphere of the Pamir-Alai zone and its characteristics are considered. The results of the MT sounding inversion indicate the existence of a zone of lateral plastic flow in the Earth’s crust beneath the Alai depression, which manifested itself as a conductive lower-crust structure traced for at least 200 km in the E-W direction along the strike of the Alai depression. Analysis of the relationship between the parameters of the geoelectric structure and the seismicity distribution in the study region has revealed a spatial correlation between the location of the hypocenters of K > 11 earthquakes that occurred in the Pamir-Alai territory and the geoelectric structure of the Earth’s crust in this region. New data on the tectonic stratification of the Earth’s crust have been obtained, which permits us to supplement and refine the existing geological and geophysical data on the deep structure of the Pamir-Tien Shan junction zone. Conclusions about the nature of anomalous crustal conductivity in the Alai basin have been drawn.