The paper presents data on the geological position, age and features of the material composition of the Saram massif granitoids, located in the northwestern part of the Malkhan ridge of Western Transbaikalia. The structure of the massif involves two-phase rocks corresponding to syenite, early-phase moderate-alkali granite and late-phase leucogranite families. The silica content (wt. %) varies from 63.8 to 71.2 in the early-phase granitoids and from 73.2 to 77.1 in the late phases. The early-to-late-phase rocks of the massif are mostly ferruginous (Fe*=0.77–0.88 and 0.80–0.93, respectively). Based on a high modified alkaline lime index (MALI) (8.75–9.97) and relatively low SiO2 contents, the early-phase rocks can be referred to as alkaline rocks, and the late-phase rocks – as calc-alkaline rocks. According to the aluminum saturation index, the early-phase rocks (0.93–1.07) correspond to moderate-to-high-alumina rocks, and the late-phase rocks (1.09–1.13) – to high-alumina formations. Granitoids are geochemically and mineralogically different from typical agpaitic A-type granites and correspond to a special group of aluminous A-type rocks. The two phase magmatic zircon U-Pb dating yielded the 175–177 Ma age (Early Jurassic). The formation of granitoids in the Saram massif is temporally synchronous with intensive orogenesis in Transbaikalia, probably caused by the closure of the Mongol-Okhotsk Ocean.

This paper presents new data on the composition, age, and formation conditions of granitoids from the Tamir molybdenum ore occurrence. The object of study is located on the area of the Tamir volcano-tectonic structure and within the ChikoyIngoda structural-mineralogical zone of Western Transbaikalia. In terms of mineralogy, petro- and geochemical characteristics, the granitoids of the ore occurrence are correspond to fractionated granites. They have an ultra-acidic composition, increased alkalinity and iron content, moderate alumina content, and subrare-metal geochemical specialization. Two groups of leucogranite-porphyry have been identified. One group of dikes has an asymmetric REE spectrum similar to that of leucogranite host rocks with a predominance of light lanthanides over heavy ones ((La/Yb) (N) = 16.2-36.7), and the other group is characterized by a V-shaped distribution spectrum ((La/Yb) (N) = 0.4-1.7), with a reduced content of all REE, compared with dikes of the first type. According to UPb LA-ICP-MS dating of zircon, the formation of granitoids occurred in two stages - Late Triassic (237 Ma) and Early Jurassic (176 Ma). The formation of leucogranites host rocks (enclosing ore) and dikes of leucogranite-porphyries of the first type is associated with the early stage, and the formation of ore-bearing leucogranite-porphyries of the second type is associated with the late stage. The ore prospect of the object has not been fully established. It is likely that the flat-dipping, short and thin quartz-molybdenum veins of the ore occurrence uncovered by mine workings are the apophyses of the main steeply dipping veins to which ore bodies richer in molybdenum can be associated. In addition, in terms of the mineral composition of ore veinlets, the salt composition of solutions, and the temperature of formation, the Tamir ore occurrence is comparable to the Pervomaisky molybdenum deposit, the largest in the region. For leucogranites, the established values of the primary Sr-87/Sr-86 = 0.70401 ratio, and for the Early Jurassic leucogranite-porphyries is 0.70568. The study of the Nd isotope composition in granitoids showed that the rocks of both stages are characterized by close positive epsilon(Nd)(T) = 1.9-1.7, T-Nd(DM-2) = 0.87-0.83 Ga values, which indicates the absence of the Tamir volcanotectonic structures of rocks of the ancient continental crust and the granitoids formation due to the melting of the source of the corresponding Caledonian isotopic province.

The article presents results of the biostratigraphic and U–Pb geochronological (detrital zircon) studies of the volcanoterrigenous lacustrine member of the upper subformation (Lower Paleozoic Oldynda Formation), which contains polymetallic massive sulfide ores of the Ozernoe deposit (Kurba–Eravna ore district, western Transbaikalia). The first, second, and “crystalline tuff” horizons of the first ore-bearing level of the Ozernaya member were studied. It is represented by an alternation of tuffs, calcareous, siliceous, carbonaceous tuffites, pelitomorphic limestones, calcareous gravelstones with interlayers and lenses of mineralized tuffaceous conglobreccia and layers of banded siderite pyrite ores. For the first time, bryozoans, algae, and palynoflora were recorded in calcareous tuffaceous siltstones and limestones of the second and “crystalline tuff” productive horizons. These data indicate the Early Carboniferous (Tournaisian) time of sediment accumulation. Results of the U–Pb geochronological studies of detrital zircons from the mineralized tuffaceous conglobreccia of the third productive horizon suggest that the lower boundary of rock formation is not older than Late Cambrian.

on new data on the geology, composition, U-Pb isotopic age, and paleomagnetism of the metavolcanic rocks of the Kataevo Formation, we consider the geodynamic conditions of their formation and alteration. The Kataevo Formation metavolcanic rocks belong to the K-Na-high-alumina andesite-andesibasalt-basalt volcanic series. Results for U-Pb analysis of magmatic zircon (SHRIMP II, 8 spots) from a metaandesibasalt sample of the stratotype section on Ungo River yielded and age of 852 +/- 9 Ma. Isotope systems for Sm- Nd yield a positive epsilon Nd(852) = +9.29, which indicates a juvenile magmatic source, close to depleted mantle (DM), with a Neoproterozoic protolith TNd(DM) model age. The content of the less mobile HFSE and REE (ppm) is consistently low for Nb (8-15), Ti (7074-12,410), Ta (0.32-0.93), Eu (1.80-2.29), Ce (50-79), Y (21-25), Yb (2.1-2.8), Rb (10-24) and elevated for Sr (1000-1500), Zr (170-270), La (25-41), and Ba (600-800). All studied parameters place the metavolcanic rocks close to the contemporary Kurile-Kamchatka type of developed island arcs. Paleomagnetic analysis of the section of metavolcanic rocks shows a complete remagnetization ca. 120 Ma. This is synchronous with manifestations of intraplate basaltoid magmatism in the studied region, the most typical example of which is the Lower Cretaceous Khilok Formation.

We present results of new mineralogical, geochemical, geochronological, and isotope (Sm-Nd, Rb-Sr, and O) studies of the volcanic rocks of the Tyya complex in the Olokit trough. These are normal tholeiitic basalts and scarcer basaltic andesites forming a fractionated series with mg# = 45-65, medium TiO2 contents (0.73-1.62 wt.%), low P2O5 contents (0.04-0.25 wL%), and a significant domination of Na over K (Na2O/K2O = 2.1-50.0). The rocks are metamorphosed to greenschists, which are composed of chlorite, actinolite, epidote, and albite with quartz, titanite, ilmenite, and magnetite impurity. The metabasalts have is an age of 915 +/- 5 Ma (zircon U-Pb dating) and are characterized by wide variations in epsilon(Nd) (T) (-3.5 to -11.9) and Sr-87/Sr-86 (0.70602-0.70732) and high delta O-18 values (9.0-15.2960) as compared to the mantle ones. According to the isotope-geochemical characteristics, the studied metabasalts have features of both IAB and E-MORB. The Tyya metabasalts might have resulted from the melting of the lithospheric mantle with a subductional component. Comparison of the studied rocks with volcanic rocks of recent geodynamic settings shows their similarity to basalts of back-arc basins. The Tyya metabasalts might belong to a back-arc basin of the late Mesoproterozoic Nyurundukan island arc system.

The formation of continental crust in the Mongolia-Transbaikalia region is researched to identify the mechanisms of interactions between the crust and the mantle in the development of the Neoarchean, Proterozoic and Paleozoic magmatic and sedimentary complexes in the study area. Using the results of his own studies conducted for many years and other published data on this vast region of Central Asia, the author have analysed compositions, ages and conditions for the formation of Karelian, Baikalian, Caledonian and Hercynian structure-formational complexes in a variety of geodynamic settings. Based on the geostructural, petrological, geochemical, geochronological and Sm-Nd isotope data, he determines the crustal and mantle sources of magmatism, conducts the identification and mapping of isotopic provinces, and reveals the role of island-arc oceanic, accretion-collision and intraplate magmatism in the formation of continental crust. Considering the formation of the bulk continental crust, three main stages are distinguished: (1) Neoarchean and Paleoproterozoic (Karelian) (almost 30% of the crust volume), (2) Meso-Neoproterozoic (Baikalian) (50%), and (3) Paleozoic (Caledonian and Hercynian) (over 20%). This sequence of the evolution stages shows the predominance of the ancient crustal material in igneous rocks sources at the early stage. During the subsequent stages, tectonic structures created earlier were repeatedly reworked, and mixed crustal-mantle and juvenile sources were widely involved in the formation of the bulk continental crust in the study area.

––Comprehensive studies of structural geology and metallogeny, taking into account the authors’ previous works started as early as the last century, have shown that the southeastern part of East Sayan formed mainly in the Neoproterozoic–early Paleozoic in the settings of multistage thrust and nappe tectonics and tectonomagmatic restructuring of autochthonous and overthrust allochthonous oceanic (ophiolitic), island arc, and ocean-marginal terranes as well as amalgamation of accretion–collision and postcollisional igneous complexes that formed during the opening and subsequent closure of the Paleoasian Ocean marginal structures. In the middle and late Paleozoic, active intraplate volcanic and plutonic processes continued in the thrust/overthrust fault setting, which led to the formation of new dome-shaped nappe structures and the redistribution of ore matter (gold etc.) in large mineral deposits. The final structure of the East Sayan region formed during the late Cenozoic as a result of mountain uplifting and volcanic eruptions, including those in the valley of the Zhombolok River.

In our research we present the results of studies of Fe-Mn-Ti oxide minerals from granitoids of the Khamnigadai massif, which is part of the peripheral zone of the Early Mesozoic Khentei-Daurian magmatic area. The granitoids of this massif are represented by medium-grained biotite two-feldspar leucogranites and aplites. Leucogranites consist of fine grains of pink potassium and sodium feldspar, plagioclase (albite oligoclase, albite), dark or black quartz, and a small amount of lamellar biotite (phlogopite annite). Aplites are composed of potassium feldspar, albite, quartz, biotite, and muscovite. According to the petrological and geochemical criteria, the leucogranites of the central and marginal parts of the massif differ insignificantly, which is due to the more "differentiated" nature of the marginal varieties; moreover, the latter are more albitized. Accessory minerals of leucogranites in the central part of the massif are represented by magnetite, titanite, zircon, fluorapatite, ilmenite, thorite, phosphotorite; in the leucogranites of the marginal part, in addition to the above-mentioned minerals, fluorite is often found. Magnetite, ilmenite, zircon, monazite, thorite, columbite, ishikawaite and fluorite are detected as accessory phases in aplites. The study of ilmenite in the granitoids of the Khamnigadai massif showed a very high content of manganese oxide in them, more than 16.60 wt. %. It was found that minerals of the ilmenite group have an unstable composition, varying from manganoilmenite (Ilm(61-47)Prph(37-46)Hem(1-8)) to ferruginous pyrophanite (Prph(72-64)Ilm(24-32)Hem(0-4); Prph(58-50)Ilm(33-) (43)Hem(5-9)) and pyrophanite (Prph(93-87)Ilm(0-4)Hem(4-14)). Pyrophanite and its ferruginous varieties are the first finds of this mineral in Central Transbaikalia due to their discovery in moderately alkaline granitoids. It was found that thrace-element content of these minerals differ significantly, as well as a certain dependence of the composition on the facies variety of granitoids was detected. Minerals of the ilmenite group are found both in the intergranular space of rock-forming minerals and in the form of lamellar-acicular and irregular bodies in magnetite, which indicates different conditions and time of their formation. It is assumed that primary ferruginous pyrophanites were formed during the crystallization of a differentiated silicic melt at relatively low temperatures, increased total alkalinity, under oxidizing conditions. Regularly oriented lamellar and acicular buildups of pyrophanite in magnetite seem to be a decomposition product of the initial solid solution. Pyrophanite found in the most leucocratic and albitized granites of the marginal part of the massif occurs in the form of irregular bodies replacing large grains of low-titanium magnetite from the edges and along cracks, and most likely formed as a result of metasomatic processes under the influence of an alkaline fluid. The relatively low manganese content in the minerals of the ilmenite group of aplites is caused by the redistribution of manganese between the coexisting ilmenite and columbite.

The Central Asian orogenic belt (CAOB) is one of the largest orogens on Earth characterized by complex and longterm history resulted from the development of the Paleo-Asian Ocean. In Late Cambrian?Ordovician, closure of Paleo-Asian basins and accretion of microcontinents, island arcs and back-arc basins to the southern margin of the Siberian continent resulted in large-scale magmatism recorded in Southern Siberia and Mongolia. The timing of terminal stage of subduction and start of accretion-collision stage is a matter of discussion for different parts of this extensive collage of terrains. In this paper, we report new data on whole-rock and mineral composition and 40Ar/39Ar age of calc-alkaline dolerites, lamprophyres and gabbros of Bayangol river area (Northern Mongolia) from the Dzhida terrain of CAOB. The Bayangol calc-alkaline mafic rocks were previously considered as a member of boninite-basaltic sequence originated in a primitive island arc. We found that the Bayangol calc-alkaline mafic rocks compose late mafic dikes, cutting terrigenous sediments and serpentinite m?lange with boninite blocks, and, therefore, calc-alkaline rocks are not related to boninites. We obtained 40Ar/39Ar age of 498.9 ? 7.7 Ma for amphibole from Bayangol lamprophyre. The calc-alkaline rocks preserve clinopyroxene (diopside, Mg# of 76?90) and amphibole (mainly pargasite and magnesiohastingsite, Mg# of 38?92), which crystallized at 1170?1210??, 2.2?9.5 kbar and 760-1030 degrees, 2.0?7 kbar, respectively. Dolerites, lamprophyres and gabbros show variations in major oxides (3.3?14.9% MgO, 10.1?19.4% Al2O3, 0.09?0.92% P2O5) and immobile trace element systematics (e.g., Th and LREE enrichment, HFSE depletion) corresponding to island arc calc-alkaline series. Isotope compositions of Nd (epsilon Nd(T) = -0.7 to +2.5) and Sr (87Sr/86Sr(T) = 0.7049-0.7078) suggest 1-5% addition of recycled terrigenous sediment melts to mantle wedge source of the Bayangol mafic rocks. Relations of trace element and SiO2 abundances and Nd\\Sr isotope ratios are resulted from fractional crystallization and do not support in situ crustal contamination of Bayangol magmas. Within the Dzhida terrain, the Bayangol mafic rocks have Nd isotope compositions transitional between those of Late Cambrian island arc intrusions with juvenile signatures and Early Ordovician collisional granitoids with crustal characteristics. Therefore, the Bayangol calc-alkaline mafic rocks mark transition from subduction to the accretion-collision regime during accretion of Dzhida island arc to the Siberian continent as a result of Paleo-Asian Ocean closure in SW Transbaikalia and Northern Mongolia. (c) 2021 Elsevier B.V. All rights reserved.

Research subject. This article presents data on the age, material composition, petrogenetical and geochemical properties of morion-containing granites in the Etytei and Khamnigadai massifs of Central Transbaikalia.Materials and methods. The composition of rock samples was investigated using the methods of chemical analysis, XRD and ICP-MS; the age was determined by the zircon U–Pb method using SHRIMP and LA-ICP-MS; the mineral composition was studied using an LEO-1430 electron microscope.Results. The massifs composed of morion-containing granites belong to Early Jurassic (190–185 Ma) and form the peripheral area of the Early Mesozoic Khentei-Dauria magmatic region.Conclusion. According to their petrochemical and mineralogical characteristics, the morion-containing granites under study differ from typical intra-plate granitoids and correspond to the “oxidized” A-type granites. The black smoky colour of quartz contained in granite samples is associated with a relatively high radioactivity of rocks caused by the presence of accessory thorium and uranium-containing minerals.

— Integrated structural geological, minerogenic, and metallogenic studies with allowance for previous topical surveying, geological mapping and prospecting, and mineral exploration data has revealed that Upper Paleozoic and Early Mesozoic tectonomagmatic structures are widespread in the Selenga ore district. They are associated with the evolution of the transregional Upper Paleozoic Selenga–Vitim rift-related volcanoplutonic belt and the formation of the Early Mesozoic West Transbaikalian region of intraplate magmatism. Late Paleozoic–Mesozoic igneous activity accounts for the bulk of mineable mineral resources in the Selenga ore district concentrated inside and outside the ore clusters (Kunalei, Kizhinga, Cheremshana–Oshurkovo, Tashir, etc.). It is demonstrated that the main mineable metals in the district are molybdenum and beryllium, which determine the minerogenic specificity of the ore district. New compositional characteristics of the Upper Paleozoic and Early Mesozoic intraplate magmatic complexes and the associated mineral deposits (Mo, Be, Ti, quartz, fluorite, and apatite ores), as well as other promising gold, uranium, and REE–Ba–Sr ore occurrences were obtained. The geodynamic settings of their formation and the ages of the main ore-forming processes have been established; the viability of the mining industry in the Selenga ore district and the feasibility of involving its ore potential in the district’s economic modernization program have been assessed.

This paper is focused on the relationship between plate‐ and plume‐tectonic processes during the formation of Neoproterozoic and Vendian‐Paleozoic island‐arc systems and active continental margins in the interaction zone of the Siberian continent and Paleoasian Ocean (PAO). In this study, we use our own materials collected in the long‐term research of the Central Asian Orogenic Belt (CAOB) and the published models showing convection in the asthenosphere and mantle, subduction‐related and plume magmatism at the Cenozoic active margins of the Western Pacific and California types. It is clearly shown that subduction‐related magmatism of the Paleoasian Ocean active margins should not be considered separately from plume magmatism. These interrelated processes played a major role in the CAOB formation. Based on the reconstructed Neoproterozoic, Vendian – Early and Middle Paleozoic paleogeodynamic features, 25 island‐arc systems of PAO are characterized. These island arcs are related to the occurrence of more than 30 plume magmatism areas. At the active margin of the Siberian continent, there are numerous fields of intraplate magmatism in riftogeneous structures. Such fields of various scales not related to subduction zones, especially at the final Late Paleozoic stage. All the major stages of the CAOB development, including the Cambrian‐ Ordovician collision stage, are clearly correlated with plume magmatism. Considering a revealed combination of the island arcs and the plume magmatism areas, there are grounds to suggest that the development of the entire Neoproterozoic‐ Paleozoic region of Central Asia was related to the activity of mantle plumes.

—The paper presents a new understanding of the geologic composition and geodynamic evolution of the Mongol–Okhotsk Fold Belt. It considers the issues related to recognition and substantiation of the Amur composite microcontinent (Amuria superterrane). We analyze the latest data on the geologic composition, age, and paleomagnetism of the Neoproterozoic–Paleozoic complexes, such as the Argun terrane and neighboring structures of Transbaikalia and Mongolia, as one of the key elements of Amuria. In particular, we have refined the age of a number of Precambrian and Paleozoic stratified and magmatic stages and demonstrated the absence of an Archean–Paleoproterozoic crystalline basement. Using a set of our own paleomagnetic and paleontological data, we have substantiated the equatorial position of the Argun terrane in the close proximity to Siberia at 560–525 Ma. The results of our study and the performed analysis of available geological data on the Argun terrane and neighboring Transbaikalia and Southeastern Asia territories point to the fallacy of previous arguments about the Amur composite microcontinent as a single tectonic unit, whose collision led to the formation of the folded structures of the Mongol-Okhotsk belt. This conclusion is of crucial importance for reconstructing the geodynamic evolution of the eastern part of the Central Asian Fold Belt in the Neoproterozoic, Paleozoic, and Mesozoic.

This paper presents new data on the age, composition and structure of the Paleozoic deposits of the Zag-Kharaa Terrane of the Khangay-Khentey Megazone (Northern Mongolia). New palynological data have made it possible to define the formation time of the terrigenous deposits of the Azhnay Formation of the Dzun-Mod volcano-tectonic structure (VTS) as early Frasnian. The palynological assemblage present here has been compared with the assemblage of the Contagisporites optivus-Spelaeotriletes krestovnikovii miospore Biozone [1]. In addition, rocks which were previously considered as part of the Lower Paleozoic Kharaa Formation have been referred to the Azhnay Formation. These deposits have been dated as middle Frasnian using conodonts and miospores. The palynological assemblage has been compared with the complex of the Geminospora semilucensa-Perotrilites donensis miospore Biozone [1]. It has been established that the sedimentation at the beginning of the Frasnian most likely took place in terrigenous shallow basin settings in combination with the delta plain settings, and at the beginning of the Middle Frasnian - in conditions of a deepening paleobasin adjacent to a zone of volcanism.

The geostructural, petrological, geochemical, geochronological and biostratigraphic studies were conducted in the Hentei-Dauria fold system of the Mongolia-Okhotsk orogenic belt. This Paleozoic system is composed mainly of three heterochronous rock associations related to the onset and development of oceanic basins and active margins in the conjugation zone of the Siberian continent and the Mongolia-Okhotsk ocean. This region developed in three stages: (1) Late Caledonian (Ordovician – Early Silurian), (2) Early Hercynian (Late Silurian – Devonian), and (3) Late Hercynian (Carboniferous–Permian). In the Late Caledonian, oceanic seafloor spreading was initiated, deep-sea siliceous deposits were formed, basaltic and andesitic pillow lavas were erupted, and layered and cumulative gabbros, gabbro-dolerite dykes and subduction zones with island-arc magmatism were formed. After a short quiescence period, new zones of spreading and subduction occurred at the active margins of the Mongolia-Okhotsk ocean in the Early Hercynian. In the Late Hercynian, large back-arc sedimentary basins, accretionary prisms and connecting intraplate magmatic complexes were formed in all structures of the Hentei-Dauria fold system. As a result of our studies, we propose a comprehensive model showing the geodynamic development of the Hentei-Dauria fold system that occurred in the area of the Mongolia-Okhotsk Ocean and its margins.

This article presents new geochronological and isotope-geochemical data on ultramafic–mafic rocks of the banded complex of the Dzhida zone of the Caledonides ophiolite association.

Based on complex structural, rheological, and metallogenic studies, taking into account the results of earlier subject-specific, prospecting, mapping, and exploration works, it has been established that the geological structure of the district was caused by the ensimatic evolution of the Vendian–Early Paleozoic Dzhida island-arc system, in which oceanic and island-arc complexes served as a melanocratic basement for Late Paleozoic–Mesozoic active within-plate (riftogenic) processes, which gave rise to the formation of ore deposits and occurrences of strategic mineral commodities (Mo, W, Au, Pt, Ag, and rare elements, including REE). Mantle plumes and flows of deep-seated transmagmatic solutions (ore-forming fluids) played a critical role in these processes, the significance of which increases in upper crustal swarms of dikes and fault systems. The forecasts and development prospects of the Dzhida ore district envisage the expansion of geological prospecting and exploration, scientific research, and technological testing of ore for insight into strategic mineral commodities, as well as reanimation of mining within the areas of the Dzhida’s large territorial and industrial complex (TIC) in eastern Siberia.

In this study, we present new palaeomagnetic and geological data obtained from Ediacaran and Cambrian sedimentary rocks of Argun terrane, which is traditionally considered a key element of the hypothetical Amuria composite continent. Since 1990, when Amuria was first proposed in palaeogeographic reconstructions, it became one of the principle members in the global palaeotectonic schemes. A scenario when collision of Amuria with Siberian margin resulted in formation of the Mongol-Okhotsk Ocean is universally accepted and supported by majority of researchers. However, time of Amuria's final assembly and relative position of the blocks within Amuria before the collision with Siberia is still a topic of debate. Questions about principal allocation of Argun terrane and its relation to Amuria during the late Neoroterozoic-Cambrian are addressed in this study. Palaeomagnetic poles for the Ediacaran-early Cambrian rocks of Argun terrane differ within an error from coeval poles from Siberia indicating that Argun terrane could have been located similar to its present-day position with respect to Siberia already at 560-525 Ma. This observation calls into question association of Argun terrane with Amuria, which in classic reconstructions is usually placed close to the North China Craton. It also questions our current understanding of the Amuria palaeocontinent and consequently, accuracy of global palaeogeographic reconstructions for the late Neoproterozoic-Cambrian in general, and those of the eastern part of the Central Asia in particular.