Vol 12, No 6 (2018)

Based on the map of the gravitational-field anomalies with respect to the existing structural data, a single structural scheme of the Sanjiang–Middle Amur sedimentary basin is constructed. It reflects the Paleogene rift-related stage of the basin’s evolution, when numerous grabens oriented mostly to the northeast formed under the action of NW–SE tension forces. The regularities in the location of the grabens indicate that the basin formed under conditions of interaction of right-lateral northeastern and left-lateral sublatitudinal and west-northwestern strike-slip faults. The ancient northeastern faults activated in the Paleogene formed marginal Western and Eastern zones of right-lateral strike-slip faults. En-echelon series of grabens in the central part mark the position of the rudimentary zones of the left-lateral strike-slip faults. The internal structure of the grabens is analyzed using the materials of the previous and new seismic and well data. The common and individual features of their structures are revealed. The structure of the grabens of the western and eastern marginal zones is demonstrated. It is noted that some grabens are prospective for raw hydrocarbons. An analysis of the deep structure of the territory of East Asia, including the basin, assumes the presence of an upwelling zone by the results of geophysical studies of recent years, primarily seismotomography.

The Transbaikal–Mongolia–North China province of East Asia is ranked as the most productive uranium province. Its area contains hundreds of occurrences, dozens of deposits of different (endo-, exo-, and polygenic) types, and several large uranium-bearing clusters and districts. The largest F–Mo–U clusters were revealed in the Mongol–Argun and Inshan–Liaohe volcanoplutonic zones (VPZ) in the western and southwestern periphery of the Great Xingan belt, respectively. Tectonic data on the Late Mesozoic (J₂–K) Tulukuev (Southeastern Transbaikalia), Dornod (East Mongolia), and Guyuan–Duolung (Inshan–Liaohe, China) depressions of the above-mentioned VPZ are analyzed to distinguish their important structural details, which are responsible for the distribution of uranium-bearing fields and deposits in the volcanic rocks. Some of the deposits are associated with paleovolcanic necks, extrusive rocks, subvolcanic bodies, and dikes and have complex morphology. Other deposits are localized in stratified volcanic–pyroclastic rocks or within the basement rocks of the depressions. Recent geochronologic, geophysical, and seismotomographic data indicate not only the synchronous formation of the ore clusters in the Early Cretaceous (Valanginian), but also their localization within a crust of moderate (36–42 km) thickness above the periphery of a stagnant slab in the mantle transition zone. The coincidence of the slab boundary projections and area of their influence with the spatial position of large F–Mo–U ore clusters (Streltsovka, Dornod, and Guyuan–Duolung) separated from each other by hundreds and thousands of kilometers is regarded as evidence for the possible influence of deep geodynamics on the formation of the corresponding mineralization. Some of the aforementioned and similar clusters of the province seem to be insufficiently studied. This refers, in particular, to the exploration of the thick volcanic–pyroclastic fields associated with volcanic paleocenters within the clusters, as well as the basement rocks of the basin.

The work presents the K–Ar isotope age (54–65 Ma) and isotope–geochemical composition of the post-subduction volcanics which compose the basalt nappes in the Bolshaya Garmanda River valley (North Okhotsk Region). It is established that the age of the Garmanda volcanics differs from that of the rocks of the Kytyima Formation, in the composition of which these volcanics were earlier studied. The isotope contents of Sr and Nd and low concentrations of heavy rare earth elements (HREE) relative to the MORB composition indicate the moderately depleted composition of the mantle source. The enrichment of the volcanics with large-ion lithophilic elements (LILE) can be explained by the role of fluids captured by the melts in the course of melting of the continental crust transformed as a result of the subduction metasomatism.

The Kalakan volcanic area in the Vitim region was formed in three stages: 137–121, 119–117, and 110 Ma. The first two stages were responsible for the formation of the mild-alkaline basaltic lavas of the Vitim Group and a complex of minor intrusions of the dolerite–granite porphyry series. The Early Cretaceous sequence is completed by a terrigenous sequence, which is intruded by a differentiated teschenite sill. The differences between the teschenite and the earlier volcanic rocks are best expressed in the distribution of incompatible elements. In the teschenite, their content steadily increases with SiO₂ increase (crystallization differentiation trend). The early volcanic rocks show no such trend, evolving chaotically, which can be explained by the contamination of mantle melts with crust and/or anatectic melts. The differences in both groups of the Kalakan magmatic area are caused by a change of magmatic sources prior to the late Early Cretaceous.

The main features of the lithofacial conditions of formation of the sedimentary complex of the Aldan–Maya Sedimentation Basin (AMSB) are discussed. It is shown that different facies (continental, coastal–marine, and marine) were formed under the conditions of one sedimentation basin of the type of one-side syneclise, which was formed on the eastern slope of the Aldan anteclise of the North Asian Craton, at its contact with the Okhotsk Massif. We distinguish and characterize the main sedimentation stages separated by inversions, magmatic layers, and weathering crusts (from the bottom upward): Pre-Riphean (Late Karelian, Ulkanian); Early Riphean (Uyan–Uchur); Middle–Late Riphean (Kerpyl–Lakhanda–Uisk); and Vendian–Cambrian. Areas of bituminous lithocomplexes are revealed in the sedimentary cover of the AMSB. Structural and sedimentary events in the AMSB are related to the geodynamics of the southeastern zone of the North Asian Craton and to the hydrodynamic regime of the Okhotsk–Pacific sector of the World Ocean. The article is devoted to the problems of comprehensive study of the geology, deep structure, and geodynamics and evaluation of hydrocarbon potential of the AMSB.