Dating of magmatic rocks from paleo-island arcs of orogenic belts helps to define the precise timing of subduction processes that took place during the formation of the orogen. Within the central (East Trans-Baikal) part of the Mongol-Okhotsk orogenic belt, the Kamensk island-arc terrane is an example of such paleo-island-arc complex. Its intrusive part is included into the Bereinsky complex, represented by a gabbro-diorite-tonalite-plagiogranite series of rocks demonstrating subduction geochemical characteristics. The dating of zircons from acidic rocks of this complex by the U–Pb classical method showed that they were produced in a narrow time interval – 203±1–205±1 Ma, which corresponds to Norian/Rhaetian boundary of the Late Triassic. Taking into account the previously obtained age of the diorites (254±5 Ma), the timing of formation of the entire series of intrusive rocks is about 50 Ma, thus indicating in the Late Permian – Late Triassic the subduction along the northern (in modern coordinates) margin of the Mongol-Okhotsk Paleocean beneath the Siberian paleocontinent. Diorites of the first phase have positive values ɛND(254MA) = 3.2–3.6 (TNd(DM) = 879–994 Ma), and plagiogranites – ɛND(205MA) = 2.3–3.5 (TNd(DM) = 859–1028 Ma), which points to the connection of these rocks with the substance of the depleted mantle source and is consistent with the Sm-Nd isotope characteristics of the juvenile crust of the Central Asian orogenic belt. This study was supported by Russian Science Foundation, grant no. 22-27-00775. The resources of the Shared Use Center for Isotope-Geochemical Research (Vinogradov Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk) were used in this work.

Dating of igneous rocks from paleo-island arc complexes of orogenic belts makes it possible to determine the precise timing of subduction processes that occurred during the formation of the orogen. Within the central (East Transbaikal) part of the Mongol–Okhotsk orogenic belt, the Kamenka island-arc terrane is an example of such a complex. Its intrusive part is included into the Bereya complex, represented by a gabbro-diorite-tonalite-plagiogranite series of rocks demonstrating subduction geochemical characteristics. The dating of zircons from acidic rocks of this complex by the U–Pb classical method showed that they were formed in a narrow time interval of 203 ± 1–205 ± 1 Ma, which corresponds to the Norian–Rhaetian boundary of the Late Triassic. Taking into account the previously obtained age of the diorites (254 ± 5 Ma), the timing of formation of the entire series of the intrusive rocks is about 50 Ma, thus indicating the probable subduction along the northern (in modern coordinates) margin of the Mongol–Okhotsk Paleocean under the Siberian paleocontinent in the Late Permian – Late Triassic. The diorites of the first phase have positive values of εND(254 Ma) = 3.2–3.6 (TNd(DM) = 879–994 Ma), and the plagiogranites have ɛND(205 Ma) = 2.3–3.5 (TNd(DM) = 859–1028 Ma), which points to the relationship between these rocks and the substance of the depleted mantle source and is consistent with the Sm–Nd isotope characteristics of the juvenile crust of the Central Asian orogenic belt.

The geochronological, geochemical, and Sr–Nd isotopic features of the Late Mesozoic volcanic rocks associated with the development of the Nercha Depression in the Eastern Transbaikalia are presented. The structure of the depression involves the sequences of K-rich subalkaline volcanic rocks with content of 55 to 73 wt

We have first determined the petrogeochemical, Sm-Nd isotope, and U-Th-Pb geochronological (detrital-zircon LA-ICPMS) characteristics of terrigenous rocks from a thick sequence in the south of the Sangilen block, which is part of the Central Asian Orogenic Belt (CAOB). The rocks are heterogeneous in facies composition: From west to east, there is a transition from graywacke sandstones with an admixture of pyroclastics to much more silicic lithoid arenites. Geochronological data on detrital zircons from the predominant graywacke sandstones mark the following intervals of concordant ages (Ma): 787-907, 1870-2236, 2613-2725, and 2900-2980. With regard to the oldest determined age (early Cambrian, 520 Ma) of the igneous rocks intruding these sandstones, the possible period of accumulation of terrigenous sequences in the south of the Sangilen block is estimated at 790-520 Ma. The obtained geochemical, Sm-Nd isotope, and U-Th-Pb geochronological data point to Neoproterozoic and early Precambrian island arc and continent-marginal complexes as probable provenances for the terrigenous deposits of the "southern band". The relative proportions of rocks of these complexes in the deposits changed with distance from the sedimentary basin (from west to east, in modern coordinates): The portion of early Neoproterozoic juvenile rocks decreased, and the portion of early Precambrian metamorphic complexes increased. The Paleoproterozoic and Mezoarchean detrital zircons found in the terrigenous rocks of the Sangilen block could not originate from any known Precambrian complexes of the Tuva-Mongolian microcontinent. This gives grounds to regard other craton blocks of the CAOB as their sources.

The article presents the results of mineralogical, geochemical, isotopic, and geochemical (Sm-Nd) studies on siltstones from the upper Bystraya subformation of the Argun continental massif, as well as U-Pb dating of detrital zircon grains from them. The main goal of the research was to identify the main sources of clastic material, reconstruct the paleogeodynamic environment of accumulation, and constrain the lower age limit for the formation of terrigenous rocks of the upper Bystraya subformation. The mineralogical composition of the studied rocks indicates their accumulation in an environment associated with subduction processes. The trace element composition of siltstones from the subformation suggests the presence of rocks of mixed composition in the source area in terms of their silica content. Based on Sm-Nd isotopic and geochemical data, it was established that the main sources of clastic material for siltstones of the stratigraphic unit were Paleoproterozoic rocks and (or) younger rocks whose formation is associated with the reworking of the Paleoproterozoic continental crust. Most of the zircons from the studied sample of siltstones of the upper Bystraya subformation are Neo- and Paleoproterozoic in age. Their sources are probably Neo- and Paleoproterozoic igneous rocks widespread within the Argun massif in China. The age of the youngest zircons (556 ± 9 and 566 ± 10 Ma) isolated from siltstones of the upper Bystraya subformation was used to constrain the lower limit of their accumulation in the middle of the Ediacaran.

New major-, trace-element and Sr-Nd-Pb isotope data are presented on the Holocene high-potassium basic lavas of Alaid volcano, which is located in the north of the Kuril island arc, in the junction zone with the Kamchatka volcanic segment. According to the petrochemical criteria, two groups of coeval rocks are distinguished: Ne-normative shoshonites and high-potassium subalkaline basalts, which have many similar geochemical characteristics. Chondrite-normalized REE distribution patterns show LREE enrichment, with flat HREE pattern, and the absence of Eu and Ce anomalies. MORB-normalized incompatible element patterns show LILE enrichment and a well-defined negative Ta–Nb–Ti anomaly typical of suprasubduction volcanics. The high K2O/Rb and Rb/Sr ratios indicate the presence of biotite and amphibole in the magmatic source, while the low Sr/Y ratios and flat MREE and HREE distribution patterns indicate the absence of residual garnet. Significant variations in the contents of major- and trace elements at similar MgO concentrations indicate a heterogeneous magma source, while linear mixing trends in isotope and discrimination diagrams, as well as experimental data, suggest the involvement in magmogenesis of not only peridotite mantle, but also amphibole–clinopyroxene mineral paragenesis. An analysis of literature data shows that the manifestations of potassium alkaline magmatism in “cold” island arcs are frequently, if not always, confined to local extension zones. Since such zones are associated with the adiabatic rise of a hot and ductile asthenosphere, it can be assumed that melting involved subduction mélange, which is formed along the boundary of the slab and supra-subduction mantle and consists of hydrated fragments of ultrabasites and metamorphosed oceanic crust transformed into amphibole-bearing pyroxenites. This mechanism makes it possible to logically explain the geochemical and isotopic features of the anomalous alkaline magmatism of the Kuril island arc and the relation of its northern segment with anomalous tectonics. The results obtained may be important in discussing the genesis of potassium alkaline magmas occurred in subduction geodynamic settings.

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23050082

The geochronological, geochemical and Sr-Nd isotopic features of Late Mesozoic volcanic rocks associated with the development of the Nerchinsk depression in Eastern Transbaikalia are presented. Its structure involves the sequences of high-potassium subalkaline volcanic rocks containing of 55 to 73 wt.% SiO2.40Ar/39Ar isotope dating of bulk rock samples was performed. 40Ar/39Ar dating results from high-K basaltic andesite gives value of 150.8±1.8 Ma and from latite is 131.0±1.6 Ma. Volcanic rocks are characterized by depletion of hydrophobic Ti, Nb and Ta, slightly negative εNd(T) values, but increased in εSr(T), that probably indicates contamination processes of primary melts by crust component. In terms of their geological and geochemical characteristics, the volcanic rocks of the Nerchinsk depression belong to the shoshonite-latite series of the rear part of the Greater Khingan volcanic region, which formed in the subduction setting of a continental active margin.

The publication presents the results of geochemical, isotopic-geochemical (Sm–Nd) and isotopic-geochronological (U–Th–Pb) studies of terrigenous rocks of the Middle Riphean (?) Nadarov formation and the Upper Riphean (?) Nortui formation of the northwestern part of the Argun continental massif. Features of the material composition of the deposits indicate the presence of formations of various silicia acidity in the source area. According to Sm–Nd data, sedimentary rocks of the Nadarov and Nortui formations are characterized by negative values åNd(t) = –6.6…–3.5 at Early Proterozoic values of Nd model age (tNd(DM) = 2.0–1.8 Ga). According to U–Th–Pb dating of grains of detrital zircon, it was defined that the lower age limit of accumulation of terrigenous deposits of the Nadarov and Nortui formations falls at the Late Riphean (~775 and ~ 744 Ma, respectively). The main provenances area for them were Late Riphean igneous rocks with the participation of Early Proterozoic rocks, extended in the structure of the Argun massif.

Existing and recently acquired data on the isotope age and composition of volcanogenic Au-Ag deposits and occurrences of the Evensk group (Okhotsk-Chukotka volcanogenic belt, Northeast Russia) are summarized. The K-Ar isotopic and Rb-Sr isochron ages of Au-Ag ores from the deposits and occurrences studied is 82.7 ± 3–77.5 ± 3 Ma and 84.1 ± 2–79.7 ± 5, respectively. Au-Ag mineralization formed during a 5 million-year time interval. Au-Ag mineralization, typical of productive ore stages and practically unaffected by later thermal processes, is characterized by low values of primary 87Sr/86Sr(0) = 0.7055–0.7059 ratios close to primary 87Sr/86Sr(0) ratios in wall-rock metasomatites (0.7033–0.7082) and unaltered host rocks (0.7045–0.7048) indicative of a pronounced predominance of mantle Sr in the ores. The composition points of ore Pb on the 207Pb/204Pb – 206Pb/204Pb evolution diagram are grouped in close proximity to the Pb isotope composition points in the exhausted (depleted) mantle with a trend towards the volcanic rocks formed in subduction zones typical of the continental margin. A close genetic relationship between ore and magmatic processes is confirmed. It is assumed that this relationship is due to the functioning of a single mantle basaltoid chamber, a source of metal-bearing fluids and, probably, the primary source of Au and Ag.

The palingenic calc-alkaline granitoid massifs of the Olekminsky complex form a magmatic belt stretching within the Western-Stanovoy terrane in the northeastern direction for more than 700 km. New U-Pb LA-ICP-MS dates for zircons from the granodiorites of the Marekta-Bereinsky massif of the Olekminsky complex and the granodiorites of the Yamninsky massif of the Krestovsky complex were obtained, amounting to 371±4 Ma and 364±5 Ma, respectively. These geochronological data are well consistent with the 355–358 Ma ones, therefore suggesting the Late Carboniferous age of quartz-diorite-granodiorite-granite rocks of the Olekminsky complex. However, these dates are not correlated with the existing legends of geological maps covering the area of the Western-Stanovoy structural-formation zone or the Western-Stanovoy terrane, as the intrusive formations of the Olekminsky complex are dated as the Early Paleozoic. In addition, new geochronological data call into question distinguishing of a separate Early Paleozoic Krestovsky granitoid complex.

This paper presents the first geochemical, isotope-geochemical (Sm–Nd) and geochronological (U–Pb, LA–ICP–MS) data on acid volcanic rocks collected in the Bogdanikha River basin of the Priamursky fragment of the Nora–Sukhotino terrane in the northeastern South Mongolia–Khingan orogenic belt. In terms of the contents of rock-forming components, the studied volcanic rocks correspond to high-silica and high-alumina rhyolites. The elevated contents of alkalis, Ga, Zr, Nb, and Y, lower concentrations of Ba, Sr, Ti, and Eu, and mantle values of εNd(t) = +3.0…+3.6 allow the rhyolites from the Bogdanikha River basin to be classified as A2-type rhyolites. The concordant age of the youngest zircon population from the rhyolite, according to geochronological (U–Pb, LA—ICP–MS) studies, is 301 ± 4 Ma, which corresponds to the Late Carboniferous. Taking the geochemical features of the studied rhyolites and the existing models for the formation of the South Mongolia–Khingan orogenic belt into account, it is most likely that they formed in a collisional setting as a result of slab break-off.

The Archean Olondo greenstone belt (OGB) is located on the Aldan shield, the largest basement of the Siberia craton. With well-preserved abundant mafic-ultramafic rocks, ≥30% in volume, the OGB is unique among other greenstone belts in the world. In this study, we present the most up-to-date geochemical and isotopic data for the ultramafic-mafic rocks of the OGB, in order to better constrain their mantle sources and the plate tectonic process involved in the formation of OGB at ca. 3 Ga. The ultramafic rocks vary from fresh to serpentinized dunites, and are highly refractory as residual mantle phase as indicated by depletion in P-Platinum Group Elements (PGE) relative to I-PGEs for highly siderophile elements (HSE). Fresh dunites show U-shaped rare earth element (REE) patterns, with positive to negative Nb anomalies, indicative of late metasomatism in their mantle source. Rhenium-Osmium isotopic compositions of these dunites yield mantle model age (TMA) of 2960–3020 Ma, comparable to the formation age of the OGB at ca. 3 Ga. Together, the data suggest that, unlike mantle cumulate origin for most of the Archean ultramafic rocks, the OGB dunites were mantle residuals after a high degree of partial melting (>30%), which subsequently interacted with the subduction-related melt/fluid. On the other hand, the OGB mafic rocks including komatiitic and tholeiitic basalts show geochemical characteristics relative to the ultramafic residuals that reinforce a subduction-related regime as their formation setting, despite extra mid-ocean ridge and plume settings. Tholeiitic basalts yield variable REE patterns from depleted, chondritic, to enriched light rare earth elements (LREE) patterns, with variable Nb-Ta anomalies, indicating their similarities with modern N-MORB and boninites, comparable to mafic rocks in typical supra-subduction zone (SSZ) ophiolites. Such mafic rocks with combined lower εNd(t) and negative Nb-Ta anomalies were most likely the result of mixing with subducted components, consistent with the observed Nb depletion in the residual dunites. The Al-depleted komatiitic basalts may have originated from deep mantle source, corresponding to garnet stability field, confirmed by their depletion in HREE and requiring a mantle plume to transport and melt at such a depth. The OGB ultramafic-mafic rocks could be a record to witness plume-induced subduction initiation processes such that mantle plume, sea-floor spreading and subduction were all in operation in the Mesoarchean time.

The paper presents the results of the mineralogical and geochemical studies of sandstones and siltstones of the Ernichny formation of the Argun series in the Argun continental massif, and the results of U–Pb (LA‑ICP-MS) dating for the detrital zircons from these rocks. It is established that the youngest detrital zircons from sandstone of the Ernichny Formation have age of 549–570 Ma. A maximum on the relative probability curve of zircon age corresponds to 566 Ma. These data determine the lower age limit of their accumulation on the border of the Ediacaran and Paleozoic age. According to the results of U–Pb dating of detrital zircon, the majority of detrital zircons from sandstones of Ernichny Formation are characterized by Neo- and Paleoproterozoic ages. The sources of zircons were Neo- and Paleoproterozoic igneous and metamorphic rocks that were widely developed within the Argun continental massif. In turn, the geochemical features of the terrigenous rocks of the Ernichny Formation, together with the presence of the poorly sorted and rounded clastic material in the studied samples, as well as the presence of interlayers of gravelstones, enabled us to establish that they were formed in a subduction-related setting.

To investigate the process and chemistry of mineral reaction zone formation, we conducted detailed petrographic observations and chemical analysis of rocks and minerals of spinel lherzolite xenoliths from basanites of Tumusun volcano (Baikal Rift Zone). The reaction zones gradually disappear from contact toward the center of the xenoliths. The influence of basanite melt on major and trace element composition of secondary minerals of reaction zones is notable only at a distance up to 100–200 μm from the contact. At a distance of 0.3–1.0 mm from the contact, the major and trace composition of secondary clinopyroxenes from the orthopyroxene reaction zone indicates their formation from a melt formed by dissolution of orthopyroxene and influenced by the element diffusion from basanite melt. Inside xenoliths, the secondary minerals have Mg# values equal to or higher than Mg# of primary minerals, and secondary clinopyroxenes inherit their depleted or enriched REE pattern from primary pyroxenes. The compositional variations in secondary clinopyroxenes testify melt heterogeneity. Clinopyroxene rims have slightly higher LILE and similar abundances of other trace elements compared to clinopyroxene cores. This is consistent with the model developed from experimental studies: due to the interaction with basanite, incongruent dissolution of orthopyroxene occurs to form a melt which circulates in lherzolite and leads to pyroxenes and spinel dissolution. Diffusion of elements from basanite results in lherzolite enrichment in K, Na, Rb, Ba, La, and Ce, which are incorporated in feldspars and clinopyroxene of reaction zones as well as in feldspar veinlets. Non-dissolved mineral cores are homogenous and similar in major and trace element composition to primary minerals without reaction rims.

The paper presents the first results on Sm-Nd isotopic-geochemical studies of the Paleozoic sedimentary rocks in the Nora-Sukhotino terrane at the northeastern flank of the South Mongolia-Khingan orogenic belt. According to the studies, the sedimentary rocks of the Zeya-Selemdzha and Amur fragments of the Nora-Sukhotino terrane are characterized by the two-stage Mesoproterozoic Nd-model age (T Nd(DM2) =1.62–1.08 Ga) at negative ε Nd(0) =–9.5…–3.0 and ε Nd(T) =–5.8...–0.2. Based on previous geochemical and isotopic (U-Pb, Lu-Hf) studies of sedimentary rocks of the Nora-Sukhotino terrane, as well as on the available models for the formation of the South Mongolia-Khingan orogenic belt, it can be assumed that most of the Paleozoic sediments of the Nora-Sukhotino terrane were carried from the Mamyn terrane of the Argun superterrane with the participation of the island arc formations.

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23050082

Cenozoic alkali basaltoids are developed in southern Transbaikalia (Dauria-Khentei Ridge). They are represented by thin flows and sheets. Unlike the volcanic fields of the South Baikal Volcanic Region (SBVR), the investigated fields are not associated with rifting. Volcanics from the flow of the Zharnichikha River were studied. Compositionally, they belong to basanites. For the first time, the mineral compositions of the volcanics have been determined. In their geochemical parameters, the basanites correspond to basic volcanics of oceanic islands. The P-T parameters of the equilibrium crystallization of olivine and clinopyroxene phenocrysts have been determined. For the calculated parental melt, the temperature of formation was 1536 degrees C, and the pressure was 3.28 GPa. The obtained original data on Sr, Nd, and Pb isotope composition have shown that the isotope composition of the magmatic melt formed from a PREMA source with a minor contribution of the latter. The mantle source for the studied basanites is assumed to be olivine pyroxenites (Ol + Cpx + Grt). The generation of basaltic melts occurred under the influence of a mantle plume, which caused Cenozoic activation.

The isotopic data showed that there are two stages distinguished in the Cenozoic history of the Darkhad depression volcanic activity, the Late Oligocene initial stage (~28.0–26.6 Ma) and the final Late Miocene – Early Pliocene stage (~5.8–4.2 Ma). It has been stated that the rocks of the initial stage are only represented by trachybasalts; however, among the final-stage basaltoids there are series of shield-volcano hawaite-basanite-phonotephrite rocks and compex trachybasaltic "valley" lava flows, the formation of which is the last stage in the territorial volcanic evolution. It has been shown that the initial-stage trachybasaltic andesites are characterized by their enrichment of TiO 2 , P 2 O 5 , Sr, Zn, Ga and low concentrations of Al 2 O 3 , MnO, CaO, Sc and HREE (La/Yb=27.2–30.2). Basaltoids of the final stage have a similar rare-element distribution and show an increase in the contents of TiO 2 , Al 2 O 3 , P 2 O 5 , LILE, HFSE, Th, U and in the degree of fractionation of REE (La/Yb from 12.2 to 20.9) towards the rocks alkalinity enhancement. Modeling of eclogite, pyroxenite and peridotite melting processes in the La/Yb – Sm/Yb system shows that trachybasaltic andesite melts could be formed at ~7–8 % melting of eclogitic matter or at ~10–11 % melting of Grt-containing pyroxenites, with trachybasalt formed at ~3 % melting of Grt-containing peridotites. The composition distribution of rocks in coordinates (Mg# – Fe/Mn) indicates that the parental magmas are the initial-stage trachybasaltic andesite magmas as well as the Early Pliocene trachybasaltic "valley" lava flows. Sr, Nd, Pb isotope characteristics of the Darkhad depression basaltoids show significant shift of isotopic ratios in time towards the relatively enriched mantle as compared with the depleted MORB mantle. The initial formation of trachybasaltic andesite melts occurred in the Late Oligicene at the pre-rift stage of the territory development involving metasomatized mantle matter, with the pyroxenite or eclogite component contained in the magma formation source. The origin of trachybasalt magmas of the final stage is associated with the processes of decompression melting of peridotites in a weakly metasomatized lithospheric mantle at the rift stage of the Darkhad structure development.

The intraplate alkaline-granite magmatism essentially contributes to formation of rare-metal strategic raw materials. In the Major Sayan Fault of the East Sayan Mountains, the rocks of the Zashikhinsky (Pz3) massif were studied through the isotope-geochemical analysis to identify probable sources of alkaline-granite magma and mechanisms of their evolution resulting in ore accumulations, up to the formation of Nb-Ta deposits. The Nd isotopic characteristics of its alkaline granites were obtained for the first time. Together with the results of mineralogical and geochemical studies, they were applied for modeling its formation, in which crystallization differentiation of alkaline granite melts proceeds simultaneously with their assimilation of the enclosing granite-metamorphic formations.