New data are presented on the petrochemical, geochemical, and isotopic (Sr, Nd, and Pb) characteristics of volcanic rocks of the Dariganga Plateau, which is one of the largest volcanic fields in the Late Cenozoic volcanic province of Central and East Asia. Regular trends were observed in the composition of volcanic rocks versus time indicating cyclic changes in the chemistry of magma sources and conditions of melt generation during the formation of the Dariganga Plateau. During the early cycle, which included the middle Miocene and the late Miocene–early Pliocene stages, the metasomatically altered lithospheric mantle contributed significantly to magma formation. The magmatism of the late cycle was dominated by an asthenospheric (plume) source. Regional correlations confirmed a similar pattern of development in other volcanic areas of the province. The identified similarity indicates that the magmatic activity within the province was controlled by a single regulator; according to geophysical data, it was the Hangai plume. The sporadic occurrence of volcanism in various areas of the province reflects the pulsed activity of the mantle plume, and the absence of a strict correlation between the timing of eruptions and the scale of their productivity indicates a certain inertia in the propagation of the mantle signal from the source of activity to the periphery.

Abstract A model proposed for the formation of the Zashikhinsky Nb–Ta, Zr, and REE deposit is based on the results of geological, petrographic, and mineralogical–geochemical studies of rocks of the massif and ores hosted in it. Data are presented on the zonal structure of the massif and the distribution of ore minerals in the vertical section of the massif and between distinct zones identified in it. The behavior of ore components in the ore-forming process is elucidated. According to the proposed model, the deposit was formed by the interplay of two processes: (1) the in-chamber filtration differentiation of leucogranite melt, which was initially enriched in F, other volatile components (H2O, P2O5, CO2, and S), and trace elements: Nb–Ta, Zr, (Y + REE), U, Th, Pb, and Sn [Nb > 300 ppm, Zr > 300 ppm, (Y + REE) > 110]; and (2) autometasomatic alteration of the rocks under the effect of residual intergranular melt–aqueous solution without any significant input of material from the outside. This process operated during all evolutionary stages of the melt and led to the formation of multicomponent ores containing zircon, REE salts, and columbite of variable composition. The final phase in the formation of the ores was late hydrothermal and weathering processes. The high cryolite content in the rocks, and hence, the high fluorine concentrations in the solutions, caused secondary alterations of the rocks without significant changes in their bulk chemical composition.

Abstract The Bural alkaline granitoid complex is located in the southwestern part of the Eastern Sayan rare-metal province and makes up a compact field of dike and stock-shaped bodies of alkaline syenites, granites, and leucogranites with subordinate gabbromonzonites. The age of the rocks is estimated at 301 ± 4 Ma, which is close to the age of other alkaline granitoids in the region, including the Ulug-Tanzek and Ulan-Tologoy rare-metal deposits. The Bural complex comprises two groups of alkali-salic rocks, differing in contents of trace elements, primarily REE. The first group includes rocks whose REE contents exceeding 185 ppm. They compose a differentiated series enriched of REE, Nb, Ta, Zr, Th, and U in the final silicic (alkaline-granite) derivatives, formed as a result of fractionation of mafic melts. The other group includes leucogranites and syenites “depleted” in REE. The leucogranites correspond to eutectic melts and have features typical of rare-metal magmas of the metaluminous series: strongly reduced REE contents (<35 ppm) at relatively high concentrations of Rb, Cs, Ta, and Nb. The associated “depleted” syenites have higher contents of incompatible elements and resulted from interaction between leucogranite melts and melts of the differentiated series. A model proposed to explain the origin of the Bural complex assigns a leading role in the development of magmatic system to differentiated subalkaline mafic melts. On the one hand, they triggered the anatectic melting of the crust leading to the formation of metaluminous rocks, and on the other, they differentiated to alkali-salic melts enriched in rare elements.

Abstract To reconstruct the magmatic evolution of the bimodal association of Wangtian’e Volcano in the Changbaishan area (Northeastern China), we used a comprehensive petrological and geochemical approach. This approach included the study of the chemical composition of volcanic rocks, determination of their isotopic (Sr–Nd) composition, detailed investigation of melt and fluid inclusions in phenocrysts of the entire volcanic rock series, and thermodynamic modeling of melt fractionation and assimilation processes using the alphaMELTS program. The study of melt inclusions in minerals of the tholeiitic basalts and trachytes of the volcano showed that the basaltic melts crystallized at T = 1180–1200°C, whereas the trachytic melts of trachyte extrusions and trachyte lava flows crystallized at 1050–1060°C and 1070–1090°C, respectively. Based on the study of primary CO2 fluid inclusions in plagioclase of the basalts, the crystallization pressure was estimated as P = 0.1 ± 0.01 GPa, which corresponds to a depth of ~4 km. The analysis of the compositions of rocks and melt inclusions in minerals and their comparison with the results of thermodynamic modeling of assimilation–fractional crystallization allowed us to demonstrate the leading role of fractional crystallization of magmas in the formation of the entire rock series from basalt to trachyte and an insignificant contribution of crustal contamination (Ma/Mc = 15%). Magma fractionation at Wangtian’e Volcano occurred in two stages. The first stage of crystal fractionation of tholeiitic melts followed the Fenner trend, when FeO, TiO2, and P2O5 accumulated in the melts at a minor increase in SiO2 content. The fractional crystallization of tholeiitic magmas occurred at CO2 dominance over H2O in the fluid and was accompanied by extensive degassing of CO2 (up to 97.5–99%) and partial (up to 50%) H2O loss. The second stage of crystallization differentiation followed the Bowen trend and involved a change in the fluid regime: the dominant role of CO2 gave way to the predominance of H2O in both the fluid phase and the melt. The transition to the Bowen trend of melt differentiation occurred when FeO contents reached 15–20 wt % and titanomagnetite crystallization began. The change in the crystal fractionation trend from the Fenner to Bowen type was associated with a change in the fluid regime of the magmatic system: water accumulation in the melt promoted FeO oxidation to the ferric state and subsequent Fe3O4 crystallization. The fluid regime also played an important role in the formation of the bimodal distribution of rock compositions at Wangtian’e Volcano. Highly mobile mafic magmas with a predominance of CO2 over H2O in the fluid and trachytic magmas whose viscosity was reduced by melt enrichment in water and alkalis erupted on the surface. Magmas of intermediate composition remained buried in magma chambers owing to their high viscosity.

Abstract The continental crust along the southern edge of the Mongol–Okhotsk Foldbelt (MOFB) has been zoned according to the Nd isotopic composition of the magmatic rocks. Three terranes have been identified: Middle Gobi, Erendaba, and Idermeg. These terranes differ in the Nd isotopic composition of the granitoids and felsic volcanics and in their geological histories. The Middle Gobi terrane has a Precambrian basement and Mesoproterozoic isotopic model ages of its igneous rocks (TNd(DM-2st) > 1 Ga). The Erendaba terrane formed in the Early Paleozoic, incorporating recycled material with Mesoproterozoic isotopic signatures. The rocks of the Idermeg terrane possess predominantly Neoproterozoic isotopic characteristics [TNd(DM-2st) < 1 Ga]. The Mongol–Okhotsk Foldbelt formed via the accretion of these terranes to the Siberian continent at different time. The Middle Gobi terrane collided with Siberia at the beginning of the Triassic, forming the Khentei segment of the MOFB. The composite Erendaba–Idermeg terrane accreted to Siberia in the Middle Jurassic, completing the formation of the Jargantu segment of the MOFB. It can therefore be concluded that the Mongol–Okhotsk basin was a large embayment of the Paleo-Asian Ocean.

Abstract Using various methods of melt inclusion investigation, including electron and ion microprobe analysis, we estimated the composition and evolution of topaz rhyolite magmas of the Spor Mountain Formation (USA). Primary melt inclusions were found in quartz from the topaz rhyolite. Based on their phase composition, two types of melt inclusions were distinguished: (1) glassy and (2) partly or completely crystallized. The daughter minerals of the crystallized inclusions are albite, sanidine, fluorite, and fluorine-rich (up to 7 wt %) Li-annite. Heating experiments with second-type melt inclusions revealed high temperatures of their partial homogenization (to glass + gas) ranging from 1040 to 1100°C. The analysis of glasses of the homogenized melt inclusions in quartz from the topaz rhyolites distinguished two groups of melts. Melts of the first group are rhyolitic in composition and have high SiO2 and Al2O3 contents: 73–75 and 12–13 wt % respectively; their Na2O content is 4.8–5.8 wt %. The H2O content varies from 0.7 to 1.9 wt %. The melts of the second group correspond to the trachydacite composition. Compared with the melts of the first group, they are depleted in SiO2 (67–69 wt %) and enriched in Al2O3 (16.5–18.8 wt %) and Na2O (6.8–7.6 wt %). Their H2O content does not exceed 0.1–0.3 wt %. The F and Cl contents of the melts of both groups are 0.7–1.8 and 0.20–0.37 wt %, respectively. The analysis of trace elements in the glasses of homogenized melt inclusions of both groups showed high contents of Be, B, Li, Nb, Ta, Th, U, Rb, Y, and LREE. In addition to silicate glass, an unusual fluoride phase was found within the glassy (unheated) melt inclusions of the first type. This phase shows high contents (wt %) of F (34.93), Al2O3 (34.43), and Na2O (35.20) at 14.87 wt % SiO2 and 3.44 wt % K2O, which allowed us to suppose that a specific sodium aluminofluoride melt separated from magma enriched in alumina, alkalis, and fluorine. The data obtained from the study of melt inclusions in quartz from the Spor Mountain topaz rhyolite provided insight into the evolution of the rhyolite melt during rock formation. It was shown that quartz crystallized from an eutectic melt enriched in Be, B, Li, Nb, Ta, Th, U, Rb, and Y with elevated F (up to 1.8 wt %) and H2O contents (0.7–1.9 wt %). The injection of basaltic melt into a silicic magma initiated intense degassing and led to the melting of the mineral association that had crystallized by the time of the injection. During this process, fluorine-containing phases, such as fluorite, topaz, and F-annite, were melted. The residual melt became significantly enriched in fluorine and trace elements, leading to the separation of a specific aluminofluoride liquid. The further evolution path of the silicate melt included the fractional crystallization of feldspars.

Abstract The Uda, Yeravna, and Zaza riftogenic depressions belong to the Uda sector, which serves as the central fragment of the Western Trans-Baikal rift zone. New Sr–Nd–Pb–O isotopic data on the Cretaceous basaltic rocks from each depression in combination with geochemical data made it possible to estimate the compositions of mantle and crustal magmas and to propose a model for their mixing.

Abstract The first comprehensive study of a kimberlite weathering crust was carried out, and the vertical and lateral zonation of the crust was determined. Two types of weathering crusts on kimberlites were identified, with a complete change in the mineral and chemical composition of the original rock: (1) crusts with the preservation of the original volume, structures, and textures of the rock and (2) those with a sharp decrease in the original volume of the rock and its newly formed textures of crushing and foliation. The composition of the weathering crust is controlled by the petrographic composition of the original rocks and the intensity of the effects of magmatic, metasomatic, metamorphic, and sedimentary processes.

Abstract—Variably retrogressed eclogites and amphibolites from the North Muya block (Eastern Siberia) bear disseminated pyrite + chalcopyrite ± pyrrhotite mineralization of metasomatic origin. Fluid metasomatism led to the precipitation of pyrite with contrasting δ34S signatures (from −10 to +9‰) produced by oxidized ( $${\text{SO}}_{4}^{{2 - }}$$ ) or reduced (H2S or HS−) sulfur from a fluid. The data indicate contrasting sulfur reservoirs in the deep orogenic lithosphere, which were controlled by devolatilization of variably oxidized metasediments.