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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.

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 TiO2, P2O5, Sr, Zn, Ga and low concentrations of Al2O3, 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 TiO2, Al2O3, P2O5, 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.

—We have estimated the P–T conditions of formation of basaltoid melts: P = 1.15–1.06 GPa and T = 1379–1293 ºC. Olivine pyroxenites (Ol + Cpx + Grt) are assumed to be a mantle source for nepheline-normative basanitic melts. During ascent, the melt trapped mantle xenoliths, which were disintegrated into olivine and augite xenocrysts. A decrease in pressure and temperature led to the crystallization of highly magnesian (Mg# = 86) olivine and diopside and Timgt + Ilm ± Pl phenocrysts. Then, the Ol + Cpx + Timgt + Pl microlite paragenesis formed. Alkaline aluminosilicates (acid plagioclase + nepheline + leucite) were the last to crystallize in the rock interstices. The presence of residual glass indicates that the last stage of crystallization took place under subsurface conditions. The volcanic area of southern Transbaikalia (Khentei Ridge) resulted, most likely, from the mantle plume impact on the lithosphere. The age of this area is estimated at 3.51 Ma. PREMA was the main mantle source for these volcanics, and the contribution of HIMU was strongly subordinate. In geochemical features the studied volcanics correspond to mafic rocks of oceanic islands. They are similar in composition to alkali basalts of the South Baikal volcanic area.

The isotope-geochemical characteristics of the Eocene-Oligocene magmatic rocks of Western Kamchatka were studied. It is shown that the igneous rocks of the Eocene (45-53 Ma) Kinkil complex of Western Kamchatka are characterized by geochemical signs of super-subduction volcanism. Their isotopic composition of Sr, Nd and Pb, low concentrations of HFSE and HREE relative to the composition of MORB, suggest the formation of primary melts from depleted or poorly enriched in isotopic composition of the mantle wedge sources in different contaminated quartz-feldspath sialic sediments. From the end of the Middle Eocene on the territory of Western Kamchatka, K-Na alkaline-basalt magmatism (46-31 million years), whose geochemical characteristics are similar to E-MORB, as well as ultrapotassic alkaline-basalt magmatism, which continued to develop to the early Miocene (35-17 million years). The geodynamic nature of Late Paleogene alkaline magmatism involves the implementation of processes of diffuse rifting.

Based on the analysis of modern genetic models, an algorithm for extracting the derivatives of potentially ore-bearing magmas among Precambrian rocks of the tonalite-trondhjemite-granodiorite associations (TTGA) is developed and tested. The algorithm is implemented as a sequence of geochemical criteria to identify the genetic nature of the sources of substance and on this basis to distinguish the products of melting of geochemical reservoirs enriched in Au and other ore elements. The proposed algorithm (a set of potential geochemical criteria of ore content) can be used for metallogenic zoning of TTGA rocks, within ancient cratons.

An algorithm for identifying derivatives of potentially ore-bearing magmas among Precambrian rocks of tonalite–trondhjemite–granodiorite associations (TTGAs) is elaborated and tested using current genetic models. The algorithm is a succession of geochemical criteria for identifying the genesis of source of matters and, as a result, the melt products of geochemical reservoirs enriched in Au and other ore elements. This algorithm (a set of geochemical criteria of the ore potential) can be used for metallogenic zoning of the territories composed of TTGA rocks within ancient cratons.

Based on a study of pyrite from the Uzelga Cu-Zn volcanogenic massive sulfide (VMS) deposit (South Urals) the age of ore mineralization was first determined with the direct age-dating method, based on the fraction of radiogenic helium, incorporated into the pyrite crystal lattice from submicron inclusions of U and Th minerals. Taking into account the measurement errors, the obtained age of 377 ± 8 Ma (MSWD = 1.2) is quite consistent with the independent age dates available for the ore mineralization (Late Eifelian-Early Givetian, 385-390 Ma).

Based on a study of pyrite from the Uzelga Cu–Zn volcanogenic massive sulfide (VMS) deposit (South Urals) the age of ore mineralization was first determined with the direct age dating method, based on the fraction of radiogenic helium, incorporated into the pyrite crystal lattice from submicron inclusions of U and Th minerals. Taking into account the measurement errors, the obtained age of 377 ± 8 Ma (MSWD = 1.2) is quite consistent with the independent age dates available for the ore mineralization (Late Eifelian–Early Givetian, 385–390 Ma).

We present the detailed mineralogical, geochemical (major and trace element), bulk Nd and Sr and mineral O isotope data for eclogites, associated orthogneisses and metasedimentary rocks from the recently discovered eclogite-bearing complex of the Zamtyn-Nuruu range (SW Mongolia). Trace element studies reveal the enrichment of eclogites with LILE and LREE relative to typical mid-ocean ridge basalts at similar levels of HREE and HFSE but without a clear arc-derived Nb minimum. The eclogites have relatively narrow range of mostly radiogenic eNd(T) values (+2.3 to +3.7 as back-calculated for 550 Ma) and model age TDM of 1.47–1.77 Ga at a wide range of initial 87Sr/86Sr ratio due to variable LILE mobilization in pre- or synmetamorphic processes. The geochemical and isotope data indicate the variably enriched MORB-like protolith for eclogites that have their variable composition through differentiation of precursor melts, slightly enriched mantle source and/or variable degree of crustal contamination. The mostly siliciclastic eclogite-hosting metasediments are likely derived from the intra-continental riftogenic basin, whereas the medium-pressure metapelites of the Maykhan Tsakhir formation originate from deeper passive margin sedimentation. Their Mesoproterozoic two-stage model age TDM−2ST of 1.49–1.63 Ga and crustal eNd(T) values (−3.5 to −5.3) significantly differ from the Zamtyn Nuruu complex rocks and indicate a mixed Nd source likely resembled from the juvenile Neoproteorozoic magmatism with depleted Nd signatures and the older Meso- or Paleoproterozoic crustal substrate. Contrastingly, eclogite-hosting gneisses exhibit the eNd(T) of the ancient crust (−12.9) and Paleoproterozoic model age TDM−2ST of 2.22 Ga. Both eclogites and metasediments evidently exhibit an input from the ancient crustal source that may be represented by orthogneisses of the Alag Khadny complex. Oxygen isotope data (δ18O of garnets mostly within +5.5 to +6.6) at high Fe3+/ΣFe ratio of bulk rocks (0.15–0.21) indicate variable but mostly limited interaction of precursor rocks with oceanic water, degassing at shallow crustal level or at the surface or relatively more oxidized (back-arc or subarc) mantle source. Considering the limited trace element evidence for the input from continental crust, generally elevated Fe3+/ΣFe for samples showing different retrograde alteration degrees, and no or limited δ18O evidence of pre-metamorphic interaction with oceanic water, we may suggest that primitive low-K tholeitic basalts close to T-MORB tholeites derived from a heterogeneously enriched mantle source of a rifted continental margin as precursor rocks for the studied eclogites. As a consequence of the geological structure and geochemical evidence, connection of rifting to the Rodinia break-up is proposed.

In the present paper, the results of our isotope–geochemical studies on eclogites of the ultrahighpressure metamorphic complex of the Kokchetav massif are reported. The fact that the distribution of nonmobile elements in most of the samples was close to that of E-type MORB basalts is shown by using geochemical multielement diagrams normalized to N-MORB. Six samples were found to have a negative anomaly over niobium that may have resulted from contamination with crustal material. For eclogites of the Kokchetav massif, the 147Sm/144Nd ratio was found to range widely from 0.143 to 0.367. The eNd-values calculated for the age of the highly barometric stage of metamorphism (530 million years) varied from–10.3 to +8.1. Eclogites show a dispersion of model ages from 1.95 billion years to 670 million years. On the graphs in the eNd(T)–87Sr/86Sr and eNd(T)–T coordinates, eclogites were shown to form trends that can be interpreted as a result of contamination of the eclogite protolith by the host rocks. Based on the data obtained, it is proposed that the basalts of rift zones that may have geochemical characteristics of N-MORB basalts and at the same time may be contaminated by the continental crust may have served as proxies for eclogite protoliths of the Kokchetav massif.

New Sr isotope data from well-preserved aragonite ammonoid shell material from the Mesozoic are compared with that from a living Nautilus shell. The prominent negative Sr isotope excursions known from the Middle Permian, Jurassic and Cretaceous probably have their origins in intensive plate tectonic activity, followed by enhanced hydrothermal activity at the mid-ocean ridges (mantle volcanism) which supplied low radiogenic Sr to seawater. The maximum positive (radiogenic) shift in the lower Mesozoic Sr isotope curve (Lower Triassic peak) was likely caused by a significant expansion of dry land surfaces (Dabie-Sulu Triassic orogeny) and their intensive silicate weathering in conditions of extreme warming and aridity in the very end of the Smithian, followed by warm and humid conditions in the late Spathian, which apparently resulted in a significant oceanic input of radiogenic Sr through riverine flux. The comparatively high 87Sr/86Sr ratio obtained from the living Nautilus shell is probably a function of both the Alpine orogeny, which was accompanied by significant continental weathering and input of radiogenic Sr to the oceans, and the weakening of mantle volcanism.

This study provides 87 Sr/ 86 Sr, δ 13 C and δ 18 O data from the best-preserved limestone and dolomite of the Ediacaran carbonate-dominated Khorbusuonka Group of the Olenek Uplift, NE Siberian Craton, as well as detrital zircon geochronological data from both underlying and overlying sandstones. The Maastakh Formation is characterized by 87 Sr/ 86 Sr ratios of ca. 0.70822 and δ 13 C values between + 4.8 and + 6.0‰. 87 Sr/ 86 Sr ratios in limestones of the Khatyspyt Formation are fairly uniform, ranging from 0.70783 to 0.70806. The carbon isotopic composition slowly decreases from bottom (+ 3.7‰) to top (− 0.2‰) of section. The Sr isotopic composition of the Turkut Formation varies from 0.70824 to 0.70914, value of δ 13 C is about zero: − 0.7…+0.7 ‰. The youngest population of detrital zircons from Maastakh Formation indicates that these rocks were formed not later than 630 Ma. U–Pb detrital zircons data of Kessyusa Group has a single peak at about 543 Ma, which is almost identical to the earlier dating. Based on biostratigraphy and isotopic data, the Sr isotopic compositions from the Khatyspyt Formation ( 87 Sr/ 86 Sr = 0.70783–0.70806) represent the composition of seawater at 560–550 Ma. Such low values of 87 Sr/ 86 Sr ratio in Ediacaran water were probably caused by the quick opening of Iapetus Ocean.

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.

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 ± 27 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.

The Central Asian orogenic belt (CAOB) is one of the largest orogens on Earth characterized by complex and long-term 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 melange 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°С, 2.2–9.5 kbar and 760–1030°С, 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 (eNd(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.

The results of geochronological (U–Pb, Ar–Ar), geochemical, and isotopic (Sr, Nd) studies of the Ordovician and Devonian mafic volcanic–subvolcanic rock associations of the Minusinsk Depression are presented. The obtained ages of magmatic associations and the basite composition, considering previous studies, witness to the impact of two mantle plumes different in age (Late Cambrian–Ordovician and Devonian) on suprasubduction rock complexes in active continental margin settings.

The Eastern Sayan ophiolites (1020 Ma) of the Tuva-Mongolian microcontinent are believed to be the most ancient ophiolite of the Central Asian Orogenic Belt [Khain et al., 2002].

The Neoproterozoic to Cenozoic collage of the Central Asian Orogenic Belt is well-known to include Precambrian continental blocks and microcontinents traditionally attributed to rifting of Siberia or Gondwana prior to CAOB assembly that significantly contributed into the geochemical and isotopic composition of younger subduction- and accretion-related crustal lithologies via processes of crust-mantle interaction and crustal recycling.

Data are presented relating to volcanic series in the Belogolovskii Massif, Sredinnyi Range, Kamchatka. We discuss new geochronologic data, the distributions of rare elements and platinum elements in the rocks, and list the isotope characteristics of volcanic series with normal and moderate alkalinities. We show that the Late Pliocene to Early Pleistocene rocks that belong to the moderate alkaline series of the Belogolovskii volcanic massif are different from rocks in the normally alkaline series of the Late Miocene to Middle Pliocene volcanogenic basement in having higher concentrations of the HFSE and LILE components. We propose a model for the generation of moderate alkaline magmas involving a heterogeneous depleted and a heterogeneous enriched source of material. According to the isotope data, one of these sources may be the subducted oceanic lithosphere of the Pacific and the Commander-Islands type, while the other source was recycled material of the Indian MORB type.

The classical models of adakite formation by melting of basaltic layer of oceanic lithosphere in the subduction zone were verified using geochemical and Sr–Nd isotope data on the Early Paleozoic granitoids of Eastern Sayan. The presence of adakites in fold belts is usually regarded as geochemical proxy for paleogeodynamic reconstruction. The formation of felsic derivatives with adakitic signatures in the collisional orogens is inconsistent with these models and requires their revision. It is shown that the composition of the granitoids and their evolution cannot be described with these models. In order to solve this problem, two hypotheses of granitoid formation by mixing of two geochemically contrasting reservoirs were proposed and verified. According to the first hypothesis, the granitoids represent the mixing products between alkaline olivine basalts and partial melts of the gray gneiss basement of this region. The second model relates the formation of the granitoids with melting of geochemically 2700 Ma-old enriched source in the subcontinental lithospheric mantle. In spite of differences, both these hypotheses are based on the remobilization of sources formed at the previous stages of the geological evolution of the region. In both cases, adakitic geochemical characteristics of forming felsic magmas are determined by the composition of protolith rather than by their geodynamic position. Obtained preliminary results place constraints on genetic models and geochemical reservoirs participating in the formation of the granitoids.

The Pb isotope composition of polyformational Mesozoic igneous rocks of the Ketkap–Yuna igneous province (KYIP) and lower crustal metamorphic rocks of the Batomga granite–greenstone area (the complex of the KYIP basement) of the Aldan Shield was studied for the first time. Based on the data obtained, several types of material sources participating in petrogenetic processes were distinguished. The mantle source identified as PREMA is registered in most of the igneous formations and predominates in mafic alkaline rocks. According to the isotope characteristics, the upper crustal source corresponds to a source of the “Orogen” type by the model of “plumbotectonics” or to the average composition of the continental crust by the Stacey–Kramers model. The lower crust is the third material source; however, the type of lower crustal protolith involved in the igneous process is still not defined, which makes difficult to estimate its role in the petrogenetic processes.

The paper presents new data on age, geochemistry, and Sr and Nd isotope composition of rocks from the Akatui massif and comagmatic rocks from the lower unit of the Kailas Formation (Akatui volcano-plutonic association), localized within the Aleksandrovskii Zavod depression. The amphibole 40Ar/39Ar age date the monzogabbro of the early phase of the Akatui massif at 154.8 ± 4.4 Ma; the monzonite of the main phase yields a 40Ar/39Ar age of 160.7 ± 3.9 Ma, and the shoshonite basalt of the lower unit of the Kailas Formation yields a 40Ar/39Ar age of 161.5 ± 1.7 Ma. The leading petrogenetic mechanism for the Akatui volcano-plutonic association is crystal fractional differentiation of melts with minor crustal contamination, which can be suggested from the mineralogical and petrographic features and geochemical and isotope characteristics of rocks. The geochemical data for the Akatui volcano-plutonic association show LILE, LREE, U, Th, and Pb enrichment with a characteristic depletion in high-field strength elements (HFSE), such as Nb and Ti. They are also depleted in P. Sr–Nd isotope data (87Sr/86Sr(160 Ma) = 0.70642–0.70688 and εNd(160 Ma) = – 0.6 to – 2.2) suggest an EMII-type mantle source and could also indicate a negligible degree of crustal contamination in the evolved melts.

New palaeotemperature reconstructions have been obtained on the basis of oxygen isotopic analysis of 178 aragonitic shell samples taken from specimens of three ammonoid orders (and some corresponding families): Phylloceratida (Phylloceratidae), Lytoceratida (Tetragonitidae) and Ammonitida (Oppeliidae, Desmoceratidae, Silesitidae, Cleoniceratidae and Douvilleiceratidae). Those obtained from aragonite shells, secreted in the lower epipelagic and in the middle mesopelagic zones during coolest season (winter), range from 15.4 to 16.8 °C, and from 11.8 to 12.0 °C, respectively. Presumed spring/autumn palaeotemperatures obtained from aragonite shells, secreted apparently in the upper and lower epipelagic, upper and middle mesopelagic zones, are somewhat higher. Presumed summer palaeotemperatures, calculated apparently for the upper and lower epipelagic, and upper mesopelagic zones range from 19.4 to 21.7 °C, from 17.7 to 19.4 °C, and from 14.4 to 16.1 °C, respectively. The predominant part of investigated ammonoids from Madagascar inhabited the epipelagic zone, but some phylloceratid, tetragonitid and silesitid ammonoids preferred deeper, cooler conditions (upper-middle mesopelagic zone). The study supports the hypothesis that Madagascar was located in middle latitudes within the tropical-subtropical climatic zone during the early Albian. Available carbon and strontium isotope data allow us to assume a more or less expressed carbon and strontium isotope stratification of the water column in this region in the early Albian. On the basis of the stable isotope data, following partly Lukeneder (2015), two large ethological groups can be recognised mainly in mid-aged and adult ammonoids. Some ammonoids (group 1) preferred apparently mesopelagic conditions, and to a lesser degree the epipelagic zone, being mainly cool-requiring animals. However, a significant part of the isotopically investigated ammonoids (group 2) preferred, on the contrary, only epipelagic conditions, being mainly thermophilic dwellers.

The Chiron Basin extends along the southern periphery of the Siberian Craton and the western margin of the Mongol&ndash, Okhotsk Belt. Here, we present whole-rock geochemical data (major and trace elements and Sm&ndash, Nd isotopes) along with zircon U&ndash, Pb geochronology and Lu&ndash, Hf isotopic data from Paleozoic sedimentary rocks within the Chiron Basin to investigate their provenance and tectonic history. &epsilon, Nd(t) values of the siliciclastics rocks of the Khara&ndash, Shibir, Shazagaitui, and Zhipkhoshi formations vary from &minus, 17.8 to &minus, 6.6, with corresponding two-stage Nd model ages (tNd(C)) ranging from 2.56 to 1.65 Ga. Detrital zircon grains from these rocks are predominantly Archean, Paleoproterozoic, and Carboniferous&ndash, Devonian in age. The data suggest that the southern flank of the Siberian Craton is the only viable source area for Archean and Paleoproterozoic zircon grains with Hf model ages (tHf(C)) of &gt, 2.20 Ga. The majority of zircon grains from sandstones from the Khara&ndash, Shibir, Shazagaitui, and Zhipkhoshi formations are Devonian&ndash, Carboniferous in age. With respect to their Hf model ages, the zircon grains can be subdivided into two groups. The first group of Devonian&ndash, Carboniferous zircon grains is characterized by relatively old (mainly Paleoproterozoic) tHf(C) model ages of 2.25&ndash, 1.70 Ga and the source was the southern margin of the Siberian Craton. The second group of Devonian&ndash, Carboniferous zircon grains is characterized by significantly younger (mainly Neoproterozoic) tHf(C) model ages of 1.35&ndash, 0.36 Ga, which are consistent with a juvenile source, most likely eroded island arcs. Our data, show that sedimentary rocks of the Chiron Basin likely formed in a back-arc basin on the southern periphery of the Siberian Craton facing the Paleozoic Mongol&ndash, Okhotsk Ocean.

This article is focused on dacitic pumices, which are the felsic members of the basalt–andesite–dacite series. The phenoscrysts of all of the rocks from this series are the same: plagioclase, olivine, clino- and orthopyroxenes, and titanomagnetite. The groundmass of dacitic pumices that contain microlites of the same minerals and felsic glass has been studied in detail. Quartz and K–Na feldspar are absent. The study of that microlite zoning that formed in the upper parts of the channels or at the surface under the most nonequilibrium conditions was one of the most important tasks; it revealed several interesting features. As an example, anorthite plagioclases were found as microlites. The resorption zones are absent in both plagioclase phenocrysts and microlites, which implies the major role of fractionation rather than magma mixing.

The problem of the geochemical classification of granitoid magmatism in the zone of interaction of oceanic and continental plates is considered in this paper by the example of Mesozoic granitoids of the Krutogorova and Kol’ intrusive complexes of the Sredinny Range, Kamchatka. Based on new geological, petrological, and geochemical data (including the Sr, Nd, and Pb isotope systematics of rocks), it was shown that the protoliths of the granitoids were volcanic-terrigenous sequences accumulated within a Cretaceous marginal basin in the eastern Asian continent. The granitoids crystallized at ∼80 Ma (SHRIMP U-Pb age) under the conditions of the andalusite-sillimanite depth facies corresponding to a pressure of approximately 2 kbar and induced contact metamorphism in the host sequences, which are made up of sediments with sheetlike bodies of mafic and ultramafic volcanics (Kikhchik Group and its metamorphic analogues of the Kolpakova, Kamchatka, and Malki groups). The lower age boundary of sedimentation of the host sequences and the time of basic volcanism coincide with the beginning of the formation of the Okhotsk-Chukotka volcanic belt. Such a correlation is not accidental and reflects a genetic connection between the processes of magmatic activation in the continental-margin sedimentary basin and the formation of the continental margin volcanic belt in eastern Asia. The development of basic volcanism in the sedimentary basin accompanied by the ascent of deep fluids resulted in the entrainment of crustal materials into magmatic processes and the formation of crustal magma chambers, the activity of which was manifested by the eruption of intermediate and silicic lavas and emplacement of shallow granitoid intrusions of considerable areal extent. These intrusions induced contact metamorphism in the enclosing volcanosedimentary complexes. The subsequent Eocene (60-50 Ma) collision processes related to the obduction of the oceanic segment of the crust of the transitional zone onto the Asian continental margin resulted in the tectonic piling of the rocks of Central Kamchatka and strong crustal thickening, which was favorable for its metamorphic alteration reaching the kyanite-sillimanite depth level of the amphibolite facies under the influence of a thermal front and deep fluids affecting lower crustal zones. The Eocene regional metamorphism caused not only metamorphic transformations, migmatization, and granitization in the sequences of the Sredinny Range, which underwent only contact hornfels formation during the first stage, but also metamorphism, migmatization, and extensive foliation in the igneous rocks of the Kol’ and Krutogorova complexes, which were transformed into gneissic metagranites.

The results of isotope-geochemical studies of eclogits and host rocks of the North-Muya block are presented. The studies showed broad variations both in the character of distribution of incompatible elements and in the Nd and Sr isotope composition of eclogits from the North-Muya block. The Nd isotope composition of eclogits is characterized by broad variations, which is reflected in the value of ɛNd(T), which has both positive (from +0.3 to +6.9) and negative values (from −0.5 to −16.8). The isotope characteristics for the both samples of eclogits (Mu 12-11, 12-12) with the lowest values of ɛNd(T) clearly indicate protolith contamination by an ancient source of Meso- or Paleoarchean age. Consequently, the melts of the protoliths of the eclogites intruded into the continental crust, and the eclogite-gneiss complex of the North-Muya block may be considered as a paleozone of the continental subduction.

Geological and geochemical data indicate that the formation of the granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Range, Kamchatka, was caused by the contact metamorphism, metasomatism, and local melting of the initial volcanosedimentary rocks of the Vakhtalka Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused the transformation of the basic volcanic rocks of the sequence into two pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase hornfelses, while sedimentary rocks were converted into garnet-biotite ± cordierite hornfelses. The hornfelsed basic volcanic rocks were locally subjected to metasomatic alteration and magmatic replacement with formation of biotite-orthopyroxene-plagioclase metasomatic bodies containing biotite-orthopyroxene-plagioclase ± garnet veinlets and aggregates. During these processes, sedimentary interlayers were converted into garnet enderbites at 700–800°C and 3.2–4.8 kbar. The comparison of the chemical composition of basic volcanic rocks of the Vakhtalka Sequence and their transformation products indicates that the metasomatic alteration and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) and cause subsequent and uneven influx of SiO2, Al2O3, Na2O, K2O, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, Cr, Co, Ti, Y, and S. REE data on basic metavolcanic rocks, hornfelses, and metasomatites suggest that the processes of hornfelsation, metasomatism, and magmatic replacement of the initial volcanic rocks were accompanied by significant increase in LREE and slight decrease in HREE. The Sr and Nd isotope study of the rocks in the aureole showed that the initial basic volcanic rocks of the Vakhtalka Sequence are isotopically close to both mature island arc tholeiites and mid-ocean ridge basalts. The metasomatic alteration and magmatic replacement of volcanic rocks in the aureole lead to the decrease of 143Nd/144Nd and increase of 87Sr/86Sr approximately parallel to mantle array. Pb isotopic ratios in the studied rocks become more radiogenic from initial metavolcanic rocks to metasomatites.

It has been found that the origin of the Patom Crater is related to endogenous processes with the main role played by deep flow of fluid components, which determine formation of the ejecta cone at about 500 years ago or more. This is evidenced by the zonal structure of the crater and geochemical peculiarities of rocks, caused by the long formation time for particular zones. Sandstone and schist blocks that were included into eruptive breccia within the crater were affected by gaseous or fluid components and intensively carbonized. During carbonatization, these rocks within the crater were being enriched in Ca and Sr, but the shares of the 87Sr and, consequently, 87Sr/86Sr ratio in them abruptly decrease. This is explained by the influence of deep fluids on terrigenous rocks, which were initially depleted in the radiogenic strontium isotope and might flow from a magmatic source with a low 87Sr/86Sr ratio. However, these fluids were enriched in CO2 and transported significant quantities of Sr, which led to enrichment of all terrigenous rocks in the crater in this element. The discovery of individual sandstone blocks with high concentrations of summarized rare earth elements (up to 557 g/t) and higher Sr and Ba contents among the fragments of host stratum within the Patom Crater allows us to suppose that there is a magmatic source enriched in fluid components at depths. The effect of the active fluid phase with low strontium isotopic ratios on rocks during the Patom Crater formation might lead to an abrupt decrease in values of the initial 87Sr/86Sr ratio in carbonized sandstones and schists.

Metamorphic units of the Khavyven Highland that crop out in the northern portion of the Khavyven Uplift of the basement structures of the Central Kamchatka Trough are formed by rocks of the Khavyven Formation, which are metamorphosed in the green-schist facies. The formation comprises two strata: the lower part that consists of amphibole-micaceous ± garnet, epidote-micaceous ± garnet crystalline schists, and micaceous ± garnet quartzite schists has a total thickness of some 500 m, and the upper part, which is formed by epidote-amphibole and phengite-amphibole green schists and overlying epidote-amphibole-micaceous quartzites, with a visible thickness of some 750 m. The isotopic ratios of Sr, Nd, and Pb were determined in the examined rocks of the Khavyven Formation for the first time. The high 87Sr/86Sr and low 143Nd/144Nd ratios and the high K/La, Ba/Th, Th/Ta, and La/Nb ratios in combination with a deep Ta-Nb minimum indicate that the original volcanites of the crystalline schists of the lower rock mass had a subduction nature. The green schist of the upper rock mass, whose composition corresponds to that of spilitic basalts, have elevated 87Sr/86Sr and 143Nd/144Nd ratios, thus combining indications of depleted melts of the N-MORB and E-MORB types and those of subduction melts, which explains the deep Ta-Nb minimum and the low (La/Yb)N ratios. The isotopic signatures of lead in rocks of the lower and the upper strata are similar. The composition points of the crystalline schists and the green schists are located near the trend of isotopic evolution of lead in the depleted mantle, which indicates that the rocks are closely related to this mantle source.

Isotope dating of ultrapotassic magmatism in the central Chukchi region was carried out. Based on the Rb-Sr isochron age of monzonites from the Linkor stock (107±2.2. Ma) and using earlier obtained geochronological isotope data, we have estimated the time of one of the stages of ultrapotassic magmatism, 107–113 Ma. Igneous rocks of this age occur throughout the central Chikchi region, which suggests their genetic relationship with some great geodynamic event there, likely, the accretion of the Mainit island arc to the Chukchi microcontinent. The results obtained permit the reconstruction of the geologic history of the region and can be used to construct more detailed geodynamic models.

The metamorphic rocks of the Khavyven Highland in eastern Kamchatka were determined to comprise two complexes of metavolcanic rocks that have different ages and are associated with subordinate amounts of metasediments. The complex composing the lower part of the visible vertical section of the highland is dominated by leucocratic amphibole-mica (±garnet) and epidote-mica (±garnet) crystalline schists, whose protoliths were andesites and dacites and their high-K varieties of the island-arc calc-alkaline series. The other complex, composing the upper part of the vertical section, consists of spilitized basaltoids transformed into epidote-amphibole and phengite-epidote-amphibole green schists, which form (together with quartzites, serpentinized peridotites, serpentinites, and gabbroids) a sea-margin ophiolitic association. The high LILE concentrations, high K/La, Ba/Th, Th/Ta, and La/Nb ratios, deep Ta-Nb minima, and low (La/Yb)N and high 87Sr/86Sr ratios of the crystalline schists of the lower unit are demonstrated to testify to their subduction nature and suggest that their protolithic volcanics were produced in the suprasubduction environment of the Ozernoi-Valaginskii (Achaivayam-Valaginskii) island volcanic arc of Campanian-Paleogene age. The green schists of the upper unit show features of depleted MOR tholeiitic melts and subduction melts, which cause the deep Ta-Nb minima, and low K/La and 87Sr/86Sr ratios suggesting that the green schists were formed in a marginal basin in front of the Ozernoi-Valaginskaya island arc. Recently obtained K-Ar ages in the Khavyven Highland vary from 32.4 to 39.3 Ma and indicate that the metamorphism of the protolithic rocks occurred in the Eocene under the effect of collision and accretion processes of the arc complexes of the Ozernoi-Valaginskii and Kronotskii island arcs with the Asian continent and the closure of forearc oceanic basins in front of them. The modern position of the collision suture that marks the fossil subduction zone of the Ozernoi-Valaginskii arc and is spatially restricted to the buried Khavyven uplift in the Central Kamchatka Depression, which is characterized by well-pronounced linear gravity anomalies.

The Northern Mongolia-Western Transbaikalia rift zone is the largest Mesozoic riftogenic structure in eastern Asia and extends for a distance of more than 1200 km. The zone consists of depressions and grabens, which were formed between 233 and 188 Ma and are filled with basaltic and basalt-comendite (bimodal) volcanic associations accompanied by numerous peralkaline granite massifs. Geochemical and isotope (Sr, Nd, and Pb) studies showed that mantle and crustal sources contributed to the formation of the magmatic rocks of the rift zone. The basalts were formed from incompatible element-enriched mantle sources. Geochemical and isotope-geochemical data suggest that the peralkaline salic rocks (comendites and peralkaline grantoids) and basalts are genetically related and were formed by the fractionation of a common parental magma. In addition, the magmatic associations contain peralkaline granites and comendites whose isotope signatures indicate their formation through the crustal contamination of derivatives of basaltic melts. The rift zone has arisen during the formation of the Mongolia-Transbaikalia zoned magmatic area in a complex geodynamic setting, combining collision in the Mongolia-Okhotsk suture with a mantle plume impact. The rift zone occupies the northern periphery of the area, being controlled by the Northern Mongolia-Transbaikalia fault system, which marks the boundaries (sutures) of large terranes in the lithosphere. Asthenospheric traps beneath suture boundaries served as pathways for the penetration of a mantle plume into the upper lithosphere, thus playing an important role in the localization of the riftogenic processes.

The western Transbaikal region was repeatedly subjected to rifting during the Mesozoic. The Early Mesozoic was marked by the formation of a system of grabens filled in with Late Triassic‐Early Jurassic bimodal volcanic sequences of the Tsagan-Khurtei Group and Kunalei Complex, which is traced by massifs of alkalic granites [1‐5]. In the Late Mesozoic, a new rift system, generally conformable with the previous one, appeared [6‐9] and began to develop until the Late Cenozoic. Therefore, the grabens and horsts of the rift system are readily traceable in the present-day topography. The Late Mesozoic epoch commenced with the formation of the bimodal basalt‐trachybasaltic andesite‐trachydacite‐trachyrhyolite‐comendite volcanic association of the Ichetui Formation. Its compositional and structural similarity to the Tsagan-Khurtei Group served as the basis for determining the age of bimodal volcanic associations in some areas of the region, resulting in misleading interpretation of the structure, scale, and geodynamic settings of different-age rifting events. This problem can be exemplified by the Khambin volcanic field (Fig. 1), one of the largest in the region. The volcanic field is outlined as a ridge in geological maps, because it resembles the majority of outcrops of the Tsagan-Khuntei Formation in the topography [10, 11]. In such an interpretation, the field occupied the westernmost part of the Early Mesozoic rift system and, thus, governed its dimensions and structural peculiarities in the pinchout area. At the same time, the Khambin field, located between the western (Dzhida) and central (Khilok‐Tugnui) segments of the Late Mesozoic rift system (Fig. 1, inset), occupies a relatively large fragment of the rift system. In available maps, the fragment is shown as an anomalous zone because of the absence of Late Mesozoic magmatism. In this communication, we present systematic geological and geochronological (Rb‐Sr, K‐Ar) data, which point to the formation of Khambin field lava sequences as a result of Late Mesozoic rifting in several stages of volcanic activity. This conclusion is consistent with the multistage character of magmatic processes in other areas of the rift system. The materials obtained allow us to scrutinize specific features of the manifestation of early stages of Late Mesozoic rifting. The Khambin volcanic field extends in the NNW

We present a reconnaissance paleomagnetic study of rocks from several formations of the Chita region of south-central Russia (representative location λ=51°N, φ=116°E), within the Mongol–Okhotsk collision zone. Sections of siltstone and fine-grained sandstone, dated using fossils as Bashkirian to Moscovian epochs (USSR Carboniferous system, Middle Carboniferous), Early Permian, Late Permian and Triassic in age were sampled. The resulting collection from five stratigraphic sections is a total of 319 oriented samples from 51 sites. Generally, 5 to 10 samples per site were collected. Sample orientation was determined using both magnetic and sun-shadow azimuths. All paleomagnetic measurements were completed in the Paleomagnetic Laboratory at the University of Pittsburgh using a 3-component 2G Superconducting Rock Magnetometer (SRM) in a magnetically shielded room. Thermal demagnetization was completed using between 12 to 20 heating steps up to temperatures of 685°C. Principal component analysis of the demagnetization data was successful in isolating two characteristic remanent magnetizations. The lower unblocking temperature component, component A, fails the fold test, is always of downward directed magnetic inclination, and may record the present-day Earth's (PDF) magnetic field [PDF I=69.4°, D=351.8°; component A, Ig=65.1°, Dg=356.5°, α95=6.2°, N (sites)=50]. The higher unblocking temperature magnetic component (B), was observed in the Triassic (BT, N=3), Late Permian (BLP, N=14), Early Permian (BEP, N=5) and Bashkirian to Moscovian epochs of the Late Carboniferous (BMC, N=8) sections. The B component differs significantly from component A, and is recorded by sites of both downward and upward directed magnetic inclinations in the Late Permian and Bashkirian to Moscovian epoch sections. Component BMC and BLP may represent primary remanent magnetizations. The Bashkirian to Moscovian epochs of the Late Carboniferous mean paleolatitude is λMC=19.9°±14.8° and the Late Permian mean paleolatitude is λLP=19.6°±14.5°. Both are similar to that expected from reference paleomagnetic poles from the North China block, but significantly different from paleolatitudes calculated using reference poles from the Siberian or European plates. We interpret the results of this study to suggest that the sampled sections were located near, or associated with, the North China Block during their deposition.