Mineralogical and U–Pb (ID-TIMS) geochronological studies of garnets from three types of scarn associations of the Khovu-Aksy Ag–Bi–Cu–Ni–Co deposit (Republic of Tuva) are carried out. The U–Pb garnet age estimate (404 ± 2 Ma) is close to the Early Devonian stage of basite and alkaline-basite magmatism identified within the Tuva trough of the Altai-Sayan folded area. The age of the scarn formation and the associated early stage of mineralization of the Khovu-Aksy Ag–Bi–Cu–Ni–Co deposit has been established for the first time.

The issue of the age of rare-metal granites of the Zashikhinskoye field is discussed. To obtain U–Pb (ID TIMS) geochronological data, a modified “chemical abrasion” technique with preliminary high-temperature annealing was used for metamictic zircon. Estimates of the age of alkaline leucogranites and albitites coincide and correspond to the age of formation of rare-metal granites of the Zashikhinskoye field 267 ± 1 Ma. Within East Sayan, igneous rocks with close ages are not yet known. The closest area of magmatic activity of this time was the large zonal Khangai magmatic range, which arose under the influence of the mantle plume and is characterized by widespread development in its peripheral part of alkaline and bimodal associations, including rare-metal magmatic ones. The Zashikhinskoye field is more than 350 km from the edge of its range. Nevertheless, on the basis of geochronological and geochemical data, it was assumed that this field was associated with the activity of the Khangai plume.

New evidence of the Early Silurian sublithospheric magmatic activity in the eastern part of the Altai–Sayan orogen has been obtained. This activity occurred between large-scale mantle-derived magmatic episodes of the Middle–Late Ordovician and Devonian. It involved high-Mg (15–22 wt

The age relationships between the alkaline rocks from the region of the rare-metal (Y, REE, Nb, Ta) Aryskan deposit have been determined. Metamict zircon with high content of U, Th was used as a geochronometer. A dedicated treatment protocol was used for sample preparation for geochronological U–Pb (ID TIMS) studies. Dating of zircon from alkaline granites of the Aryskan and nearby Astyg massifs showed that their formation occurred almost simultaneously, 448 ± 1 and 446 ± 1 million years ago, respectively. The supposed genetic relationship of alkaline granites with the accomodating alkaline-feldspar syenites formed 472 ± 2 million years ago (zircon, U–Pb, ID TIMS), has not been confirmed.

The temporal relations between alkaline rocks of the Aryskan rare metal (Y, REEs, Nb, Ta) deposit are determined using metamict zircon with high U and Th contents, which was prepared for geochronological U–Pb (ID TIMS) studies following a special procedure. Dating of this zircon from alkali granites of the Aryskan and neighboring Astyg plutons showed that they formed synchronously: 448 ± 1 and 446 ± 1 Ma, respectively. The suggested genetic link of alkali granites with country alkali feldspar syenites formed at 472 ± 2 Ma (zircon, U–Pb, ID TIMS) is not confirmed.

Early Devonian Altai-Sayan rift system (ASRS) has spread to the structures of East and West Sayan, Kuznetsk Alatau, and Mongolian Altay. Its largest fragments are the Tuva, Delyun-Yustyd, Kan, Agul, and Minusa basins as well as depressions in north-western Mongolia. The paper summarizes the geologic, geochemical, and Sr-Nd isotope characteristics of the ASRS mafic rocks represent-ed by nappes of moderately alkaline and alkali basalts and their subvolcanic and intrusive rock analogues. They are present in all magmatic associations and are divided into low-Ti (TiO2 = 0.2-2.2 wt.%) and high-Ti (TiO2 = 2.2-4.3 wt.%) subgroups. These rocks are characterized by wide variations in Sr isotope characteristics (ESr(T) = -16 to +30). High-Ti mafic rocks are common at the southern segment of the ASRS; they show a weak positive Ta-Nb anomaly (La/Nb = 0.8-1.1) and are relatively enriched in LREE ((La/Yb)N= 6-14) and radiogenic Nd (ENd(T) = 3.8-8.7). Low-Ti varieties are confined to the northwestern segment of the ASRS; they are enriched in Ba but depleted in Th, U, Nb, Ta (La/Nb = 1.2-2.2), Zr, Hf, LREE ((La/Yb)N = 3-7), and radiogenic Nd (eNd(T) = 2.0-6.0). Taking into account the existence of different terranes, which were combined in the structure of the Altai-Sayan folded area during accretion (ca. 500-480 Ma), we propose a model suggesting different environments of magma formation at the southern and northwestern segments of the ASRS and the relationship of magmatism with a mantle plume within the ASRS. In composition the plume corresponds to the sources of high-Ti magmas. The effect of the melted lithospheric mantle of different compositions beneath different groups of terranes led to the observed isotope-geochemical heterogeneity of mafic rocks within the ASRS, in particular, the absence of high-Ti mafic rocks from the Minusa basin.

Late Mesozoic carbonatites of Central Asia are developed within the Central Asian Orogenic Belt and adjacent territories of the Siberian and North China platforms. In terms of their structural position, age, geochemical characteristics, and other parameters, they differ from other carbonatite occurrences of Central Asia and are distinguished as the Late Mesozoic carbonatite province in Central Asia. The province includes separate areas of carbonatite magmatism, the geological position of which is determined by the relation with Late Mesozoic rift zones of intracontinental Asia. The carbonatites were formed within a relatively narrow time range (between 150 and 118 Ma) at the early evolution stages of these zones. The carbonatite-bearing complexes of the province are represented by subolcanic and volcanic associations of silicate rocks, carbonatites, magmatic non-silicate rocks (phosphates, sulfates, and others), as well as products of hydrothermal activity. The carbonatites are characterized by diverse composition and include calciocarbonatites, magnesiocarbonatites, and ferrocarbonatites. The silicate rocks are dominated by K–Na and K intermediate rocks. All these rocks have similar geochemical features determined by the elevated contents of LREE, Sr, Ba, and Pb, at low Nb and Ta contents. The typomorphic minerals of carbonatites of the province, in addition to carbonates, are fluorite, Ba and Sr sulfates or carbonates, LREE F-carbonates, and apatite. Unaltered carbonatites are enriched in 18 О and 13 С relative to mantle values, but in general fall within the compositional range of carbonatites around the world. Hydrothermal and supergene processes modified the mineral composition of carbonatites, which was accompanied by a change of the initial Sr, O, and C isotope composition. The Sr and Nd isotope composition of rocks of carbonatite complexes of the province in general depends on the age of the basement of a definite volcanic area. Carbonatites and associated silicate rocks have close isotope characteristics, but carbonatites usually show relative enrichment in ( 87 Sr) and depletion in radiogenic neodymium ( 143 Nd). The formation of the Late Mesozoic carbonatite province is related to the activity of mantle plumes, which controlled the Late Mesozoic magmatism in Central Asia. The plumes obviously were accompanied by fluid flows enriched in СО 2 , F, and S. This caused the enrichment of lithospheric mantle in volatile components, as well as REE, Sr, Ba, and K, which were extracted by a fluid en route to the surface. Subsequent melting of metasomatized mantle produced parental melts of carbonate-bearing rock complexes.

The study of the O and C isotope composition of calcite from nepheline syenites, ijolites and carbonatites of the Chik intrusion and the intrusions of the Erzin–Tarbagatay group of Sangilen (Eastern Siberia, Russia) showed derivation from alkaline melts enriched with a carbonate component from the host marbleized sedimentary rocks. The calculations showed that about 40% of the initial mass of carbonates involved in the interaction with silicate melts have remained after decarbonation. During the assimilation of the carbonate, an oxygen isotope exchange took place between the residual carbonate material and the silicate phase. Crystallization products of such hybrid magmas are carbonatite veins, calcite-rich nepheline rocks and their pegmatites with a calcite core.

The age and geochemical characteristics of alkaline rocks of the Dugda massif (Eastern Tuva), attributed to the East Sayan Late Paleozoic rare-metal magmatic zone, have been determined. The massif is composed of nepheline syenites, which dominate over alkaline quartz syenites and alkaline granites. The age of the rocks has been estimated by the 40Ar/39Ar method for amphibole and the U–Pb SIMS method for zircon. The age of the rocks was 291 ± 5 Ma for nepheline syenites, 284 ± 2 Ma for alkaline quartz syenites, and 285 ± 4 Ma and 287 ± 1 Ma for alkaline granites. These estimations make it possible to determine the age of the massif with an average value of ~287 Ma. Rocks with different compositions are characterized by close geochemical characteristics. They are enriched in the majority of incompatible elements in comparison with the average composition of the Earth’s crust and are characterized by multi-element spectra similar to A-type granitoids. Variations in the contents of the incompatible elements in the rocks of the massif exceed two orders of magnitude and reach ore concentrations (Zr up to 2.5 wt %, Nb up to 0.2 wt %, Ta up to 250 ppm, and Y up to 0.2 wt %). The geological position of the massif and specific features of the composition of its rocks, allowed us, firstly, to clarify the relationship between the East Sayan Late Paleozoic rare-metal zone and the fault system, which defines the western boundary of the Tuva–Mongolian microcontinent; secondly, to limit the time of alkaline magmatism to the interval of 309–285 Ma; and thirdly, to conclude that the spectrum of rare-metal igneous rocks of the zone is not limited to alkaline granitoids, but also includes alkaline nepheline syenites.

In this paper, we provide insight into the evolution of syenite magmas based on geological data and petrographic, geochemical, and O-Nd isotope parameters of rocks of the Saibar intrusion located within the Minusinsk Trough, Altay-Sayan area. The intrusive suite includes predominant syenites, few bodies of melanocratic and leucocratic nepheline syenites (foyaites), and granites. In addition, dykes of granites and mafic rocks are present. The U-Pb zircon age from the melanocratic foyaites was determined to be 457 ± 10 Ma? Examined rocks show fractionated light rare earth element patterns, normalized to chondrite, with (La/Sm)n varying from 4 to 9, and a weakly fractionated distribution of medium and heavy rare elements, with (Dy/Yb)n from 0.35 to 1.23 and (Sm/Yb)n from 0.63 to 2.62. The spidergram normalized to the primitive mantle shows negative Ba, Sr, Nb, Ta, Ti, and Eu anomalies (Eu* = 0.48–0.60) and positive Rb, Th, and U anomalies. The δ18O values vary within 6.3 to 10.2‰, and εNd(t) from +4.1 to +5.0. We observe gradual transitions from syenites to foyaites. Assimilation by syenite magma of the host carbonate rocks was followed to transition from silica-saturated to silica-undersaturated conditions and removal of anorthite from the melt, which then led to nepheline. Granites of the main phase show depleted lithophile incompatible elements in comparison with syenites and foyaites. They originate via interaction of magmas at the marginal part (endocontact zone) of the intrusion, corresponding to north contact of the granites with the host felsic rocks. In comparison, the rock composition of granite dykes is enriched in lithophile incompatible elements, except for Zr, Hf, and Ti. These rocks are formed due to the differentiation of syenite magma without a significant effect of host rock assimilation. Mantle magmas must be used as parent magmas for syenites based on analysis of the formation model of other alkaline intrusions, which are similar in age to the Saibar intrusion. In the line of syenite intrusions of the Altai-Sayan province, the Saibar intrusion is no exception, and its origin is related to the evolution of mafic magmas that arose during the melting of the mantle under the influence of a mantle plume.

: Bastnaesite, a major economic rare earth element (REE) mineral, has been considered to serve as a U–Th–Pb geochronometer to record REE mineralization time. As previously noted, most bastnaesite contains more Th than U, with Th/U ratios larger than 1000 in some cases, indicating Th–Pb dating is more suitable than U-Pb geochronometry. In-situ bastnaesite Th-Pb dating by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method and Secondary Ion Mass Spectrometry (SIMS) has been performed. However, there is no independently determined, matrix matched Th-Pb bastnaesite reference material available. In this study, the bastnaesite “K-9” from Karasug siderite carbonatites (Tuva Republic) are evaluated as U–Pb and Th–Pb dating reference material by multiple methods. Major elements, including La, Ce, Pr, Nd, and F are homogenous at < 5 wt% level (1 relative standard deviation, RSD) as verified by electron microprobe analyses (EMPA). SIMS U-Th-Pb dating results prove its homogeneity and concordance of both U-Pb and Th-Pb system at < 2% level (1 RSD). Based on six ID-IRMS (isotope dilution isotope ratio mass spectrometry) analyses, 206 Pb/ 238 U age of 116.80 ± 0.13 Ma and 208 Pb/ 232 Th age of 116.59 ± 0.11 Ma are provided as recommended age data. In summary, K-9 bastnaesite is qualified to serve as an in-situ U–Pb and Th– Pb dating reference material and is expected to contribute significantly to the rapidly advancing bastnaesite geochronology in the coming future.

Identification of the Late Mesozoic carbonatite province in Central Asia is herein discussed. Its regional extent and distribution is investigated, and the areas with manifestations of carbonatite magmatism are described. It is shown that they were developed in terranes with heterogeneous and heterochronous basements: Siberian (Aldan Shield) and North China cratons; Early Paleozoic (Caledonian) and Middle-Late Paleozoic (Hercynian) structures of the Central Asian fold belt (Transbaikal and Tuva zones in Russia; Mongolia). Irrespective of the structural position, the carbonatites were generated within a relatively narrow time interval (150-118 Ma). The geochemical (Sr, LREE, Ba, F and P) specialization of carbonatites of the province is reflected in their mineral composition. Some rocks of the carbonatite complexes always include one or more distinctive minerals: fluorite, Ba-Sr sulfates, Ba-Sr-Ca carbonates, LREE fluorocarbonates, or apatite. Compared to counterparts from other age groups (for example, Maimecha-Kotui group in North Asia), these carbonatites are depleted in Ti, Nb, Ta, Zr and HE It is shown that the Sr and Nd isotope composition of carbonatites correlates with the geological age of the host crust. Rocks of carbonatite complexes associated with cratons are characterized by the lowest epsilon Nd(T) and highest ISr(T) values, indicating that their formation involved an ancient lithospheric material. Carbonatite magmatism occurred simultaneously with the largest plateau basalts 130-120 Ma ago in rift zones in the Late Mesozoic intraplate volcanic province of Central Asia. This interval corresponds to timing of global activation of intraplate magmatism processes, suggesting a link of the carbonatite province with these processes. It is shown that fields with the carbonatite magmatism were controlled by small mantle plumes ("hot fingers") responsible for the Central Asian mantle plume events. (C) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The Cambrian Kharly alkaline plutonic complex composed mainly of foidolite and nepheline syenite makes up a small intrusive field in the Sangilen Plateau in Tuva (southern Siberia). The rocks show large ranges of major oxides (38-58 wt% SiO2 ; 1-18 wt% Na2O + K2O; 11-28 wt% Al2O3; 1.5-20 wt% CaO; 0.1-8 wt% MgO; 2-12 wt % Fe2O3) controlled by variable percentages of minerals: clinopyroxenes, calcic amphiboles, micas, nepheline and feldspars. Alkaline rocks are cut by carbonatite veins composed of predominant calcite coexisting with femic minerals (10-15% of aegirine-ferrosalite-hedenbergite, sodic-calcic amphiboles, ferrobiotite, Ti-garnet), Na-K feldspar and nepheline (up to 15-20%), fluorapatite (up to 20-25%), Sr-apatite, and accessory carbocernaite, titanite, Ti-magnetite and ilmenite. Carbonatites (4057-8859 ppm Sr, 426-1901 ppm Ba (Sr/Ba >= 2), 290-980 ppm REE + Y, 2 to 100 ppm Zr, and 0.5 to 15 ppm Nb) possibly originated at high (>= 500-650 degrees C) temperatures as a result of liquid immiscibility. The isotope systematics of rocks and minerals (epsilon(Nd)(t) from similar to 2.9 to 6.5; Pb-20(7)/Pb-20(6)in = 0.89; (208)pb/Pb-206(in) = 2.15; Sr-87/Sr-86(t) = 0.70567-0.70733, delta O-18(v-smow) approximate to 7.2-19.5 parts per thousand, and delta C-13(v-PDB) from -6.0 to -1.4 parts per thousand) suggest mixing of PREMA and EM 1 material during magma generation and crustal contamination of the evolving melts. The rocks bear signatures of interaction with "magmatic-equilibrated" fluids or heated meteoric waters. LILE/HFSE ratios indicate mixed magma sources that involved the material of IAB and OIB, as well as a crustal component, possibly, due to interaction of a mantle plume with rock complexes on the active continental margin.

Обосновано выделение позднемезозойской магматической провинции Востока Азии и рассмотрена ее связь с глобальными геологическими событиями. Показано, что основные структурообразующие процессы и максимальная продуктивность магматизма в ее пределах совпали с пиком раннемеловой плюмовой активности Земли. Предложена модель формирования магматической провинции, связывающая ее развитие со сложной геодинамической обстановкой взаимодействия конвергентной границы с горячим полем мантии. Во фронтальной части зоны конвергенции, где происходила аккреция террейнов со стороны Тихого океана и формировались краевые магматические пояса, преобладали субдукционные механизмы магмообразования. В западной части магматической провинции вне зоны воздействия конвергентных процессов возникли внутриплитные вулканические области, связанные с активностью небольших мантийных плюмов.

Abstract—The Late Mesozoic volcanic province of East Asia is considered in relation to global geological events. The main structure-forming events and largest magmatic productivity of the province coincided with the peak of widely manifested plume activity in the Early Cretaceous. A geodynamic model of magmatic province formation of is proposed, relating the development of the province to the complex geodynamic setting for the interaction of the convergent boundary with the hot mantle field. The Pacific marginal magmatic belt formed in the front zone of convergence, where accretion of terranes occurred with prevalent supersubduction magma-forming mechanisms. In the western part of the province outside convergence zone an intraplate volcanic areas formed due to the activity of small mantle plumes.

—The Paleozoic foidolite–foyaite plutons of the Sangilen upland (Bayan-Kol, Dakhu-Nur, Chik, and Kharly ones) might have formed in the Late Cambrian–Early Ordovician (~490–500 Ma, Sm–Nd and U–Pb); they are the result of the oldest alkaline magmatism in southeastern Tuva. The intrusion was accompanied by the formation of high-temperature (up to ~600–900 ºC) endogenous carbonate rocks containing calcite, alkali pyroxene, Na–Ca amphibole, biotite, fluorapatite, microcline, and nepheline. Silicate and carbonate derivates were produced, most likely, from genetically related heterogeneous sources with εNd(T) varying from 3.0 to 6.3 and from –0.5 to 6.5, respectively, which might be due to the mixing of the depleted (PREMA) and enriched (EM) mantle materials. Initial ratios 207Pb/206Pb ≈ 0.89 and 208Pb/206Pb ≈ 2.15 in K-feldspar from calcitic rocks are close to those of EM 1. The correlation between the stable-isotope ratios (δ18O ~ 7.2–19.5, δ13C from –6.0 to –1.4‰) and the high 87Sr/86Sr(T) ratio (0.7057–0.7076) indicates a significant crustal contamination of magma in the upper horizons of the lithosphere and a minor impact of a meteoric fluid. The assumed synchronous formation of the studied plutons and other alkaline rock complexes of the Early Paleozoic Large Igneous Province in the west of the Central Asian Orogenic Belt as well as their isotope similarity do not rule out that the intrusion took place in the plume–lithosphere interaction setting.

This paper reports on geochronological U–Pb studies of baddeleyite from nepheline syenite of the Korgere–Daba alkaline massif, which is the largest massif within the Sangilen Highlands (Tyva). The established age of rocks, 295 ± 1 Ma, indicates that, in the Early Permian, undersaturation by silica magmatism occurred in the region in addition to the alkaline-granite magmatism (Ulug–Tanzek, etc.). This age furthermore points to the need to make corrections in the conceptions of a petrophysical type for the Devonian Sangilen complex, which is traditionally distinguished in this region. Until now, the Korgere–Daba massif has been considered in this regard.

This contribution presents U-Pb geochronological data for Ca-Fe-Ti garnets from compositionally diverse alkaline and carbonatitic intrusive rocks ranging from Neoarchean to Permian-Triassic (Cinder Lake and Eden Lake in Manitoba, Canada; Belaya Zima and Odikhincha in Siberia, Russia; Afrikanda in the Kola Peninsula, Russia) obtained using isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) and their trace-element compositions measured by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS). The studied garnets yield U-Pb concordant or subconcordant ages obtained with a precision of <0.5% owing to their relatively high content of U and negligible common Pb. The new ID-TIMS data are in excellent agreement with the previously reported zircon, baddeleyite and perovskite ages. The results of the present work demonstrate that postmagmatic alteration does not disturb or reset the U-Pb isotopic budget of these minerals, and that garnets representing the andradite-schorlomite-morimotoite system can serve as a robust reference material for micro-analytical geochronological studies of a wide spectrum of igneous and contact-metasomatic rocks. The new LA-ICPMS data demonstrate that the abundances of rare-earth, high-field-strength and other trace elements in calcic garnets from alkaline and carbonatitic rocks vary by at least two orders of magnitude and can therefore be used as reliable magma-evolution tracers. The rare-earth budget of these minerals is best described in terms of the chondrite-normalized ratios (Sm/La)cn and (Sm/Yb)cn, which are sensitive to lanthanide fractionation, and Y/Ho, which is interpreted to respond to garnet re-equilibration with a fluid.