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U-Pb geochronological studies have shown at least two stages of Neoproterozoic magmatism, 940-933 and 804-789 Ma, throughout the geologic evolution of the Bureya continental massif in the east of the Central Asian Fold Belt. The first stage (940-933 Ma) was marked by the formation of the protoliths of metagabbroids and amphibole-biotite gneiss-granites of the Bureya River basin. The geochemical features of these rocks suggest that they formed, most probably, in an island-arc setting on the continental basement or on an active continental margin. The second stage (804-789 Ma) of magmatism was manifested as the formation of biotite leucogranites and hastingsite-lepidomelane granites (A-type granites) in the Chepkan River basin. The latter rocks are similar in geochemical features to within-plate granitoids. The established stages of magmatism are the most ancient for the continental massifs in the east of the Central Asian Fold Belt. It is not ruled out that these magmatism stages were related to the stages of formation and breakup of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

The performed 40 Ar/ 39 Ar geochronological studies yielded the first reliable age of trachyandesites of the Mogot volcanic field (115 ± 3 Ma), which, together with the age of trachyandesites of the Bomnak volcanic field (117 ± 1 Ma), gives grounds to recognize a new stage (117–115 Ma) of evolution of the Stanovoi volcanoplutonic belt superposed on the igneous and metamorphic complexes of the Dzhugdzhur–Stanovoi superterrane of the Central Asian Orogenic Belt and Stanovoi structural suture. The trachyandesites of the Mogot volcanic field are similar in geochemical features to adakites. Their parental melts resulted from the transformation of Precambrian continental crust under postcollisional extension after the formation of the Mongolo–Okhotsk orogen or under sliding along the boundary between the North Asian craton and the Amurian microcontinent. [ABSTRACT FROM AUTHOR]

There are four main types of boninites in ophiolite suites, which either spatially coexist with ophiolites though belong to other tectonic units (1), or are present as later constituents of ophiolite sequences (crosscutting dikes or lavas on top) (2), or build ophiolite sequences together with island-arc tholeiites and basaltic andesites, followed by younger volcanics of MORB or BABB affinites (3), or occupy the whole mafic portion of ophiolite sequences, together with island-arc tholeiites and basaltic andesites (4). The latter type, considered in more detail for the case of ophiolites from the southeastern Sayan Mountains (Siberia, Russia), presents an example of inconsistency between the model of ophiolite formation in mid-ocean ridge settings and subduction-related island-arc fingerprints in ophiolitic mafic rocks. The patterns of boninites record several evolution models of oceanic systems, with melting and intrusion of boninites in forearc, arc, and back-arc settings. The existing models are controversial, possibly, because there is no single mechanism to account for all types of boninites. [ABSTRACT FROM AUTHOR]

Abstract: We present new data on the geologic position, composition, and isotope characteristics of the Early Cretaceous granitoids of the Samarka terrane, Sikhote-Alin’, formed on a transform continental margin. Geological and geochronological data show that these granitoids were generated at two stages of magmatism: in the first half (Hauterivian–Barremian, 130–123 Ma) and second half (Albian–Cenomanian, 110–98 Ma) of the Early Cretaceous. Granitoids of the first stage form an autonomous (free of basic precursors) unimodal melanogranite–granite association and are characterized by normal alkalinity with domination of K over Na, low contents of Ca, and elevated contents of Al2O3. By composition, these are S-granites with a model Nd age of ∼1.3 Ga. Granitoids of the second stage are of more diverse petrogeochemical types. They show wider variations in K/Na and A/CNK, are richer in Ca and, sometimes, Sr, and are poorer in P than the granitoids of the first stage. Their compositions form a continuous trend from S- to I-granites, and their model Nd age is ≤1.2 Ga. Comparison of the petrochemical, trace-element, and isotope characteristics of the Early Cretaceous granitoids and upper-crustal rocks (sandstones and siltstones of the turbidite matrix of a Jurassic accretionary prism and basalts from the inclusions in it) of the Samarka terrane and the coeval garrboids has shown that the potassic S-granitoids formed at the early (Hauterivian–Barremian) stage of magmatism as a result of the anatexis of upper-crustal sedimentary rocks. At the late (Albian–Early Cenomanian) stage, the intrusion of mantle magmas led to a temperature increase in the lower crust, which favored more active anatexis, involvement of high-melting substrates (oceanic basalts) in the granite formation, and interaction of mantle and crustal magmas. This resulted in a great diversity of granitoids (from S- to I-type). [Copyright &y& Elsevier]

Abstract: We generalize results of geological, geochronological, geochemical, and isotope-geochemical studies of the Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia. Based on these data, we analyzed the scales of development of plagiogranitoid magmatism, studied the petrologic composition and isotope characteristics of granitoids, and established the main sources of plagiogranitoid-generating melts and the leading mechanisms of formation of Early Caledonian juvenile crust. [Copyright &y& Elsevier]

Abstract: The structure, composition, and age of Vendian–Early Cambrian plagiogranitoid associations composing the Kshta and Taraskyr massifs of the Yenisei pluton in the Altai–North Sayan island-arc belt are considered. We have established that these associations formed within 550–520 Ma and differ in petrographic composition and sources. Two stages of island-arc plagiogranitoid magmatism are recognized: early (550–540 Ma, formation of plagiogranitoids of the Kshta (545 ± 8 Ma) and Taraskyr (545 ± 7 Ma) massifs) and late (525–520 Ma, formation of plagiogranitoids of the Maina complex of the Yenisei (524 ± 2 Ma) and Tabat plutons). By petrochemical composition and geochemical characteristics, the rocks of the Kshta massif are high-alumina plagiogranitoids similar to adakites. They might have been produced through the melting of metabasites compositionally similar to N-MORB in equilibrium with garnet-containing restite during the subduction of oceanic slab at ≥15 kbar. The rocks of the Taraskyr massif are low-alumina plagiogranites. They formed through the melting of metabasites located in the lower layers and(or) the basement of the island-arc system in equilibrium with plagioclase-containing restite at 3–8 kbar. The low-alumina plagiogranitoids of the Yenisei pluton melted out under the same conditions. Isotope-geochemical studies showed that the Vendian–Early Cambrian plagiogranitoids formed at the early stage are characterized by high positive ∑Nd(T) values (7.5–4.9), Late Riphean model Nd-age (TNd(DM) = 0.64–0.98 Ga), and Sr isotope ratio varying from 0.7040 to 0.7053. These data point to the juvenile parental melts of the rocks and the varying content of ancient crustal material in the magma generation zone. [Copyright &y& Elsevier]

Abstract: In the Tannu-Ola zone, there are island-arc (Late Vendian) and subduction-accretionary (first half of the Early Cambrian) complexes whose mantle sources were generated from melts produced in suprasubduction conditions as a result of the melting of mantle wedge peridotites. The island arc, whose fragment is present in the Tannu-Ola zone, formed tentatively at 570 Ma and ceased as an individual structure at the Vendian–Cambrian boundary, though the subduction continued till the middle Early Cambrian. From the Vendian–Cambrian boundary to middle Early Cambrian, the accretion of island-arc, back-arc, oceanic, and metamorphic structure-lithologic complexes took place at this segment of the Central Asian Fold Belt. From middle to late Early Cambrian, the Tannu-Ola zone was an active continental margin, where basalts with island-arc characteristics, quartz microdiorites of adakite composition, basalts, dolerites, and quartz microdiorites of intraplate type, and diorite-tonalite-plagiogranite association of island-arc type formed. To explain the spatial and temporal conjugation of different magmatism manifestations on the active margin, we applied the slab–window model. [Copyright &y& Elsevier]

Abstract: The history of the Vendian–Early Paleozoic formation of protoliths of continental crust in the Gorny Altai segment of the Central Asian fold belt is considered, and their composition, isotopic characteristics, and formation mechanisms are estimated. We have established two stages of crust-forming processes in Gorny Altai: Early and Late Caledonian, with the different structures of formed geoblocks and nature and compositions of crustal protoliths. At the Early Caledonian stage, fragments of oceanic lithosphere of basic composition (MORB, OIT, OIB) (T Nd(DM-2st) = 0.65–1.1 Ga) formed, as well as island arcs with andesite-basaltic and andesitic protoliths with low contents of incompatible elements (T Nd(DM-2st) = 0.7–0.9 Ga). At the Late Caledonian stage, the redistribution of the substance of these blocks and the external supply of material led to the formation of heterogeneous crust of turbidite basins with an oceanic basement and andesite-dacitic upper-crustal protoliths (T Nd(DM-2st) varies from 0.8–0.9 Ga in the framing of the volcanic arc of Altaids to 1.4–1.6 Ga at the boundary of the Altai–Mongolian microcontinent). [Copyright &y& Elsevier]

Abstract: We discuss problems of the origin, settings, and age of Neoproterozoic A-type leucogranites widespread in the Yenisei Ridge. Combined analysis of geological, petrological, and geochemical (including isotope) data shows that some granitoids (Glushikha complex) were formed at the postcollisional stage (750–720 Ma), and others (Tatarka complex), in an anorogenic environment (680–630 Ma). The anorogenic complex contains diverse igneous rocks, including alkaline varieties and carbonatites. Leucogranites form separate plutons within different igneous complexes. They have high contents of potassium (up to ultrapotassic composition in the Glushikha complex), iron, and fluorine and are depleted in europium. Postcollisional granitoids show the highest concentrations of Rb, Th, and U, extremely low concentrations of Ba and Sr, whereas anorogenic granitoids are rich in Ta, Nb, Y, Sm, and HREE. The obtained data point to the augmented mantle contribution to the formation of continental crust of the Yenisei Ridge between 750 and 630 Ma. We also report new results of U-Pb zircon dating, including SHRIMP and Ar-Ar data. [Copyright &y& Elsevier]