An Erratum to this paper has been published: https://doi.org/10.1134/S001670292522001X

As a result of petrographic and thermobaric studies, 4 mineral associations were identified in the blastolite: relict, main, association corresponding to biotitisation and association of retrograde changes. The relict association probably represented eclogites The main mineral association corresponds to apoeclogite zoisite amphibolites. Determination of the P-T parameters of the formation of this association showed an interval of P = 13.0–15.0 kbar and T = 580-670 °C. At the same time, if we take the most ferruginous outer rim of amphibole-1 for calculations, then the P-T parameters shift to a higher temperature region of P = 13.0–17.0 kbar and T = 660-780 °C. The association corresponding to the imposed biotitisation is well structurally expressed. The lines of monomineral equilibria of the biotite association constructed from various combinations of garnet, biotite and plagioclase compositions showed good convergence in the local region corresponding to the interval P = 12.5-13.2 kbar and T = 810-830 °C. The last association of retrograde changes includes low-alumina amphibole-2.The association corresponding to the imposed biotitisation.ше is well structurally expressed. The last association of retrograde changes includes low-alumina amphibole-2. Based on the obtained isotopic data, a mineral Sm-Nd isochron was constructed for the rock, garnet and two amphiboles. The age of the isochron is 171 ± 3 million years, the latter is interpreted as the time of formation of the main mineral association and corresponds to its crystallization at the peak of metamorphism.

Petrographic and thermobarometric data obtained on a blastolite lens hosted in sheared rocks of the Gonzhinskii block indicate that the lens contains four mineral assemblages: relict, main, an assemblage corresponding to biotitization, and that of retrograde alterations. The relict assemblage was likely eclogitic. The main mineral assemblage corresponds to zoisite amphibolite developing after eclogite. The P−T metamorphic parameters of this assemblage are P = 13.0−15.0 kbar and T = 580−670°C. However, if the calculations are done using the iron richest outer rim of amphibole 1, the P−T parameters are P = 13.0−17.0 kbar and T = 660−780°C. The mineral assemblage corresponding to the overprinted biotitization is clearly distinct texturally. The lines of the monomineral equilibria of the biotite assemblage plotted using various combinations of the composition of the garnet, biotite, and plagioclase well converge within a compact field of P = 12.5−13.2 kbar and T = 810−830°C. The latest assemblage of retrograde minerals includes low-Al amphibole 2. Our original Sm−Nd isotope date were used to calculate an isochron for a whole-rock sample, garnet, and two amphiboles. The isochron age of the rock is 171 ± 3 Ma and is interpreted as the age of the main mineral assemblage, i.e., its crystallization at the metamorphic peak.

Biotite–garnet–sillimanite–cordierite gneisses from the Leshchev zone of the Middle Volga megablock in the Volgo-Uralian segment of the East European craton have been studied. The Sm–Nd model age of the rocks was measured at 2.8 Ga. U–Pb isotope–geochronological studies of the cores of zircons from these rocks were carried out. Several generations of zircon cores were revealed on cathodoluminescence images. The age of the main sources of detrital zircons was measured at 2.58 Ga. Single zircon grains have core ages of 2.4, 2.7, and 3.1 Ga. The calculated εNd(2500) = –0.6 indicates the crustal or mixed mantle–crustal origin of the protolith of these gneisses. The high-alumina metaterrigenous rocks of the Bolshoi Cheremshan Group, the Leshchev zone, and the South Volga supracrustal complex are considered as rock complexes of Neoarchean and Paleoproterozoic basins with a proto-cratonic basement and a passive continental margin, which supersede each other in time and space.

The paper presents authors’ original detailed data on rocks of the Archean Pon’goma-Navolok charnockite−enderbite complex in northern Karelia. The rocks practically have not been modified and are preserved within a rigid block among Paleoproterozoic zones of ductile deformations and metamorphism. The geochemistry of the rocks and their isotope−geochemical features indicate that the protolith from which the enderbite melts of the main phase of the massif were derived may have been amphibolites. The enderbite melts were derived from these amphibolites under the effect of K2O-, Na2O-, and SiO2-bearing fluids; and the enderbites were subsequently charnockitized with the involvement of fluids enriched in K2O and SiO2. Physicochemical modeling indicates that the enderbite melt was derived from the amphibolite protolith at a depth of about 45 km (P = 14.8 kbar, T = 1030−1080°C) under the effect of saline H2O−CO2 fluid. Comparison of the P−T parameters of the granulite-facies metamorphism of the metabasites and the parameters under which the enderbite melts were derived indicates that Archean granulite-facies metamorphism in the Belomorian belt in northern Karelia was of contact but not regional nature and was induced by the high-temperature field of an emplaced enderbite massif. The orthogneisses hosting the Pan’goma-Navolok massif inherit geochemical features of the unsheared, ungneissose, and unmetamorphosed enderbites. This means that enderbites analogous to those of the Pan’goma-Navolok massif may have served as the protolith of some of the orthogneisses, and that enderbites may have been spread more widely in the Archean than the currently preserved single enderbite massifs.

— The paper presents authors’ original detailed data on rocks of the Archean Pon’goma-Navolok charnockite−enderbite complex in northern Karelia. The rocks practically have not been modified and are preserved within a rigid block among Paleoproterozoic zones of ductile deformations and metamorphism. The geochemistry of the rocks and their isotope−geochemical features indicate that the protolith from which the enderbite melts of the main phase of the massif were derived may have been amphibolites. The enderbite melts were derived from these amphibolites under the effect of K 2 O-, Na 2 O-, and SiO 2 -bearing fluids; and the enderbites were subsequently charnockitized with the involvement of fluids enriched in K 2 O and SiO 2 . Physicochemical modeling indicates that the enderbite melt was derived from the amphibolite protolith at a depth of about 45 km ( P = 14.8 kbar, T = 1030−1080°C) under the effect of saline H 2 O−CO 2 fluid. Comparison of the P − T parameters of the granulite-facies metamorphism of the metabasites and the parameters under which the enderbite melts were derived indicates that Archean granulite-facies metamorphism in the Belomorian belt in northern Karelia was of contact but not regional nature and was induced by the high-temperature field of an emplaced enderbite massif. The orthogneisses hosting the Pan’goma-Navolok massif inherit geochemical features of the unsheared, ungneissose, and unmetamorphosed enderbites. This means that enderbites analogous to those of the Pan’goma-Navolok massif may have served as the protolith of some of the orthogneisses, and that enderbites may have been spread more widely in the Archean than the currently preserved single enderbite massifs.

The structure of the Central Asian Orogenic Belt (CAOB) is determined by a combination of Neoproterozoic and Paleozoic paleoceanic and island-arc complexes and blocks with Precambrian basement. The Dzabkhan terrane, one of the largest blocks in the central segment of the CAOB, was initially considered as a microcontinent with an Early Precambrian basement. It included the Baidarik, Tarbagatai, Songino and proper Dzabkhan terranes, which are attributed to the Early Precambrian “cratonic” terranes. At present, the Early Precambrian complexes were established only in the northwestern Baidarik terrane and the Ider complex of the Tarbagatai terrane. Their folded structure was formed at ~ 1855 Ma, which is typical of the basement of ancient Northern Eurasia craton. The Early Proterozoic charnockitoids of the Baidarik terrane with an age of 1854 ± 5 Ma contain granulite-facies xenoliths. Previously, these xenoliths were considered as belonging to the Bumbuger Complex, which hosts the charnockitoid massif. However, zircons from two-pyroxene granulite xenoliths yielded an age of 1850 ± 4 Ma, which almost coincided with the age of 1854 ± 5 Ma determined for host charnockites. These data showed that charnockitoids generated at the lower crustal depth brought up the fragments of the Early Proterozoic lower crust to the amphibolite-facies metamorphic zone. The obtained new age data on zircon (2413 ± 12 Ma) extracted from two-pyroxene quartz diorite (enderbite) xenolith in these charnockitoids provided new insight into Early Proterozoic granulite facies metamorphism, which reflects the stages of the lower crustal growth in the geological evolution of the Early Precambrian complexes of the Baidarik terrane.

The article presents data on migmatites of the Taratash metamorphic complex, Southern Urals. We studied the Sm-Nd isotopic system in ultrametamorphic processes leading to partial melting of a protolith and migmatization. The Sm-Nd isotopic system of the rocks indicates the contribution of Paleoarchean protolith (T Nd2 = 3.2–3.6 Ga) to the Taratash metamorphic complex, with the predominance of the crustal component (ɛ Nd (T) from −7.6 to −10.5). Based on morphology, U-Pb isotopic age, and Th/U-ratio, zircon found in melanosome and leucosome of the migmatites is subdivided into two generations. The main stage of migmatization occurred within 2.1–1.8 Ga ago and was triggered by granulite metamorphism at 2.06 Ga. Zircon grains of this stage have the low uranium concentration (213–469 ppm) at typical thorium concentration (180–631 ppm), Th/U = 0.8–1.7.

The Dzabkhan terrane, represented by the basement of the Tsaganolom Formation carbonate sequences referring to the Ediacaran shelf cover, was considered as an ancient cratonic fragment in the structure of the Central Asian Fold Belt. It was assumed that its sequences marked the regional unconformity between the Early and Late Precambrian deposits. However, the occurrence of the Tsaganolom Formation rocks on high-temperature metamorphic rocks is established only in the Bogdoingol block (eastern part of the Dzabkhan terrane), where dolomites unconformably superimpose migmatized gneisses and intruding granitoids, for which the age of 717 ± 5 Ma was determined. Synmetamorphic quartz diorites with zircon age of 847 ± 3 Ma (U–Pb, ID-TIMS) are found in gneisses and migmatites of the Bogdoingol block. The tNd (DM-2st) values are 1.97 Ga in gneisses, while in quartz diorites tNd (DM-2st) = 2.0 Ga.

The Paleozoic granitoids of the Urgamal pluton in the crystalline basement of the northwestern part of the Dzabkhan Terrain are considered. The age of these granitoids, 463 ± 2 Ma, was determined by the U–Pb method (ID-TIMS). The Urgamal pluton is located on the border of the Dzabkhan Terrain with the Early Caledonian formations of the Ozernaya Zone. The pluton was emplaced after the the Dzabkhan Terrain had converged with the paleo-oceanic and island arc complexes of the Ozernaya Zone. The granitoids are therefore attributed to post-accretionary formations occurring in the Early Caledonian superterrain of Central Asia. Intrusive complexes similar in age and structural position are known in the Caledonian Ozernaya Zone, Sangilen Block, Tuva-Mongolian Terrain, and the Olkhon Terrain of the framing of the East Siberian Platform. Geological settings of these complexes allow the time of integration of the Early Caledonian superterrain of Central Asia and the Siberian Platform to be estimated.

In the Late Paleozoic and Early Mesozoic, during about 100 m.y., the world’s three largest batholiths (Angara-Vitim, Khangai, and Khentei, each up to 1 000 000 km3 in volume) had formed within the limits of the Central Asian orogenic belt. Considering the case of the Khangai batholith, the problem of how, when, and why such an extensive granite formation took place is analyzed. The geochronological data for granitoids of the batholith by U–Pb (ID-TIMS) and 40Ar/39Ar dating methods are systematized to distinguish three age groups of rocks. These rock groups are correlated to the geological events occurred in the region. The earliest group includes granitoids formed in the interval of 302–283 Ma. They tend to the western and southern framings of the batholith and correspond to the fragments of two igneous belts that crossed the region, where the batholith formed later, and reached the areas far beyond. The youngest group of igneous rocks (230–200 Ma) is developed in the eastern periphery of the batholith and corresponds to the marginal part of the large Early Mesozoic Mongol-Transbaikalian igneous zone, with the main part being located far away to the east of there. Igneous complexes that formed in the interval of 273–238 Ma correspond to the batholith proper. They are concentrated within the zone of 350 × 400 km in size and are represented by rocks of two associations: granite-granodiorite (Khangai complex) and granite-leucogranite (Sharaus Gol complex). The coeval analogs of these rocks are reported only in the framing of the batholith. The comparison between the Khangai batholith and two other giant ones (Angara-Vitim and Khentei) revealed their similarity in terms of structure and evolution. They are all composed of similar rock associations and are of comparable sizes and age intervals of formation. For example, the Angara-Vitim and Khentei batholiths formed mainly in the intervals of 305–275 and 229–195 Ma, respectively. The obtained estimates of formation time of ~30 m.y. should seemingly be considered as the time necessary for chambers of anatectic magmas, which to certain degree formed giant (~1 000 000 km3 in volume) batholiths, to cool down in the Earth’s interior. The formation of giant batholiths is attributed to the effect of mantle plumes on the lithosphere of a young fold zone that appeared as a result of accretionary-collisional events in the marginal part of the Siberian paleocontinent.

Представлены данные о возрасте метаморфизма пород Елабужской зоны и детритового циркона из парагнейсов Елабужской зоны, их корреляция с ранее датированными породами Средневолжского мегаблока. Геохимические особенности и степень метаморфизма пород Елабужской зоны и Средневолжского блока сходны. Различие заключается в явном проявлении палеопротерозойского метаморфизма изученных пород Елабужской зоны, что контрастирует с ситуацией в Средневолжском геоблоке, где метаморфизм такого возраста не проявлен. Изучение U-Pb-изотопной системы внешних зон циркона Елабужской зоны деформаций даёт основание считать, что метаморфическое преобразование пород происходило в два этапа, 1,99 и 1,95 млрд лет назад.

Tectonic sheets of various size along the southern slope of the Mongolian and Chinese Altai ranges and in eastern Kazakhstan include high-grade metamorphic rocks, which are collectively referred to as the Southern Altai Metamorphic Belt. Rocks of the sheets show traces of amphibolite-facies elevated-pressure metamorphism of the kyanite–sillimanite type M2. Some of the tectonic sheets display evidence of polymetamorphism: the rocks preserve textures and mineral assemblages of an earlier metamorphic episode (of elevated temperature and relatively low pressure) of the andalusite–sillimanite facies series M1. The earlier metamorphic episode occurred at 390–385 Ma, and the later one, at ~370–356 Ma. The protoliths of the high-grade metamorphic rocks were mostly Early Paleozoic terrigenous rocks and subordinate amounts of volcanic rocks analogous to the weakly metamorphosed or unmetamorphosed rocks in their northern surroundings. Typical rocks of the tectonic sheets are mafic dikes and massifs of the Gashun Nuur Complex, which were emplaced between metamorphic episodes M1 and M2. According to their geochemistry and Nd isotopic parameters, most of the metabasites are similar to enriched basalts of mid-oceanic ridges and oceans plateaus. The quantitatively subordinate group of the layered mafic bodies displays geochemical characteristics of subduction-related rocks. Correlations between the metamorphic events and magmatism in the continental (Mongolian and Chinese Altai) and paleoceanic (Trans-Altai Gobi and eastern Junggar) regions led us to suggest a geodynamic model for the development of the Southern Altai Metamorphic Belt. The volcano-terrigenous rocks, which were later metamorphosed, were accumulated mostly in the Early Paleozoic as an accretion wedge on an active continental margin. The earlier episode of high-temperature metamorphism M1 and coeval large-scale calc–alkaline magmatism occurred at the same active continental margin after the magmatic front shifted southward (in modern coordinates). The emplacement of the swarms of mafic bodies of the Gashun Nuur Complex and simultaneous rifting in the southern Chinese Altai were triggered by the subduction of an spreading ridge of an oceanic or backarc basin beneath the active margin. The second metamorphic episode (elevated-pressure metamorphism) M2 and overthrusting in the structures of the Altai are correlated with deformations at low angles and the transition from oceanic to continental volcanism in the Trans-Altai Gobi and Junggar. These tectonic processes were induced by the accretion of a system of mid-Paleozoic ensimatic island arcs of the Trans-Altai Gobi and Junggar to the Altai margin of the Siberian paleocontinent.

This data on the age of metamorphism of rocks of the Yelabuga zone and the geochronological data on detrital zircon from paragneisses of the same zone are presented, and their correlation with the ages of rocks from the Middle Volga megablock is made. The rocks of the Yelabuga zone and Middle Volga megablock have similar geochemical characteristics and degrees of metamorphism, but demonstrate the following differences: Paleoproterozoic metamorphism was clearly expressed in the studied rocks of the Yelabuga zone, which contrasts to rocks of the Middle Volga megablock, where no metamorphism of this age is manifested. The study of the U–Pb isotopic system in external zones of zircon crystals from the Yelabuga deformation zone suggests that rocks underwent two stages of metamorphism, 1.99 and 1.95 Ga ago.

Within the northern fringe of the western (Khangai) flank of the Mongol–Okhotsk fold belt, magmatic complexes of intermediate to moderately acidic rocks occur. They comprise widely distributed gabbro–diorites, diorites, tonalites, and granodiorites. Geochronological studies have demonstrated that these rocks were formed in the time span of 437 to 375 Ma. The geochemical affinities of the rocks suggest their formation in subduction tectonic settings; hence, their paleotectonic position corresponds to the continental margin of the Mongol–Okhotsk paleoocean. It has been concluded that this Middle Paleozoic igneous activity occurred in the active continental margin settings, formed by subduction of the paleooceanic plate under the Siberian continent.

Приводятся результаты изотопно-геохронологического изучения метаосадочных пород большечеремшанской серии Волго-Уральского сегмента Восточно-Европейского кратона с целью выявления их древнейшей составляющей. Для 16 образцов высокоглиноземистых гнейсов и кристаллосланцев, отобранных из керна скважин, было проведено Sm-Nd изотопное исследование и рассчитаны модельные возрасты TNd(DM). Из пород с максимальными значениями модельных возрастов (выше 3.2 млрд лет) были выделены акцессорные цирконы в трех скважинах: Миннибаевская 20 000, Ново-Елховская 20 009 и Зай-Каратайская 12 930. Изотопное U-Pb датирование 200 зерен циркона было выполнено на масс-спектрометре вторичных ионов Cameca 1280 NORDSIM в Музее естественной истории в Стокгольме. Наилучшие для анализа участки кристаллов циркона были предварительно выбраны по их катодолюминесцентным изображениям. Проведенный анализ продемонстрировал многообразие как возрастных (от 3.8 до 2.6 млрд лет), так и геохимических групп цирконов в метаосадках большечеремшанской серии, что свидетельствует о гетерогенном составе и возрасте питающих провинций в момент их денудации. Наличие эоархейских и палеоархейских цирконов в терригенной составляющей метаосадочного протолита большечеремшанской серии указывает на существование фрагментов древнейшей архейской коры в Волго-Уральском сегменте Восточно-Европейского кратона.

The Dzabkhan microcontinent was earlier considered as a fragment of an ancient craton in the structure of the Central Asian Orogenic Belt. Deposits of the Tsagaan Oloom Formation were included in the shelf zone, under the assumption that they were related to the regional unconformity between the Early-Late Precambrian crystal formations. The carbonate sequence of the Tsagaan Oloom Formation overlaps crystalline rocks only in the eastern part of the Dzabkhan microcontinent, where dolomites lie unconformably on high-grade metamorphic rocks intruded by granitoids of the Bogdyngol massif. The latter were included in the composition of both the Early Precambrian basement and the Middle Riphean intrusive complex. We have determined the U-Pb zircon age of these granitoids at 717 ± 5 Ma and the Nd model ages of granitoids and gneisses of the basement of the Tsagaan Oloom Formation at 2.0–1.9 Ga at εNd = −10.0...−6.6. Recent geochronological and Nd and Pb-Pb isotopic and geochemical data indicate that intrusive and high-grade metamorphic complexes are absent in the crystalline basement of the Dzabkhan microcontinent, similar to those in ancient cratons. One can assume that the Late Riphean carbonate cover (Tsagaan Oloom Formation) deposited on the Late Precambrian continental block.