E. B. Sal’nikova

Early Precambrian crystalline complexes of the Central Asian microcontinent: Age, sources, tectonic position

In the Early Caledonian superterrane of Central Asia, an accretionary orogen of mosaic structure, pre-Riphean Baidaragin and Bumbuger complexes are exposed in the Baidarik block of the Dzabkhan microcontinent. Zircon dating on ion-ion SHRIMP II microprobe and Nd isotopic-geochemical systematics are used to establish protolith age for Neoarchean orthogneisses of the Baidaragin complex, age constraints for accumulation of Lower Proterozoic metasediments of the Bumbuger Complex and provenance of sedimentary material. The results of isotopic dating facilitate correlation of the Baidarik block crystalline complexes with basement formations of North Eurasian ancient cratons. Possible position and migration path of the Dzabkhan microcontinent during the Early Proterozoic transformation of supercontinents Columbia-Rodinia-Pangea are considered based on interpretation of paleomagnetic data.

Geochronology of igneous rocks and formation of the Late Paleozoic south Mongolian active margin of the Siberian continent

The succession of magmatic events associated with development of the Early Carboniferous-Early Permian marginal continental magmatic belt of southern Mongolia is studied. In the belt structure there are defined the successive rock complexes: the older one represented by differentiated basalt-andesite-rhyodacite series and younger bimodal complex of basalt-comendite-trachyrhyolite composition. The granodiorite-plagiogranite and banatite (diorite-monzonite-granodiorite) plutonic massifs are associated with the former, while peralkaline granite massifs are characteristic of the latter. First systematic geochronological study of igneous rock associations is performed to establish time succession and structural position of both complexes. Geochronological results and geological relations between rocks of the bimodal and differentiated complexes showed first that rocks of the differentiated complex originated 350 to 330 Ma ago at the initial stage of development of the marginal continental belt. This is evident from geochronological dates obtained for the Adzh-Bogd and Edrengiyn-Nuruu massifs and for volcanic associations of the complex. The dates are consistent with paleontological data. The bimodal association was formed later, 320 to 290 Ma ago. The time span separating formation of two igneous complexes ranges from several to 20–30 m.y. in different areas of the marginal belt. The bimodal magmatism was interrelated with rifting responsible for development of the Gobi-Tien Shan rift zone in the belt axial part and the Main Mongolian lineament along the belt northern boundary. Loci of bimodal rift magmatism likely migrated with time: the respective magmatic activity first initiated on the west of the rift system and then advanced gradually eastward with development of rift structures. Normal granitoids untypical but occurring nevertheless among the products of rift magmatism in addition to peralkaline massifs are assumed to have been formed, when the basic magmatism associated with rifting stimulated crustal anatexis and generation of crustal granitoid magmas under specific conditions of rifting within the active continental margin.

Late Paleozoic subductional and collisional igneous complexes in the Naryn segment of the Middle Tien Shan (Kyrgyzstan)

The Late Paleozoic Tien Shan fold belt was formed in the course of subduction of the crust underlying the past Turkestan ocean under the Kazakh continent and subsequent collision of the latter with the Alai and Tarim massifs. The onset of subduction is evidenced by development of flysch sequences and olistostromes in the accretionary complex of the South Tien Shan: in the terminal Visean‐Serpukhovian (~330‐325 Ma ago) in the west and in the second half of the Bashkirian Age (~315 Ma ago) in the east of the Kyrgyz Tien Shan [1]. 1 The onset of collision between the Kazakh continent and Tarim Massif is dated back to the terminal Late Carboniferous based on the initiation of a foredeep along the northern margin of the latter [6]. The mature collision stage began in the mid-Asselian, when the last sea basins disappeared in the Tien Shan and granitoids intruded in its southern segment [1, 9]. The position and age of the volcanic arc that was forming in the course of convergence between the Kazakh continent and the Tarim Massif remain unclear. Recently, this problem acquired particular significance, since the data available for the territory of China imply subduction under the Tarim Massif and an Early Carboniferous age of collision, i.e., substantially older as compared with that assumed for the Kyrgyz region [7]. 1 Ages are given after [13].

Geology, Geochronology, and Geodynamics of the Khan Bogd Alkali Granite Pluton in Southern Mongolia

The Khan Bogd alkali granite pluton, one of the world’s largest, is situated in the southern Gobi Desert, being localized in the core of the Late Paleozoic Syncline, where island-arc calc-alkaline differentiated volcanics (of variable alkalinity) give way to the rift-related bimodal basalt-comendite-alkali granite association. The tectonic setting of the Khan Bogd pluton is controlled by intersection of the near-latitudinal Gobi-Tien Shan Rift Zone with an oblique transverse fault, which, as the rift zone, controls bimodal magmatism. The pluton consists of the eastern and the western ring bodies and comes into sharp intrusive contact with rocks of the island-arc complex and tectonic contact with rocks of the bimodal complex. The inner ring structure is particularly typical of the western body and accentuated by ring dikes and roof pendants of the country island-arc complex. According to preliminary gravity measurements, the pluton is a flattened intrusive body (laccolith) with its base subsiding in stepwise manner northwestward. Reliable geochronologic data have been obtained for both plutonic and country rocks: the U-Pb zircon age of alkali granite belonging to the main intrusive phase is 290 ± 1 Ma, the 40Ar/39Ar ages of amphibole and polylithionite are 283 ± 4 and 285 ± 7 Ma, and the Rb-Sr isochron yields 287 ± 3 Ma; i.e., all these estimates are close to 290 Ma. Furthermore, the U-Pb zircon age of red normal biotite granite (290 ± 1 Ma) and the Rb-Sr age of the bimodal complex in the southern framework of the pluton are the same. The older igneous rocks of the island-arc complex in the framework and roof pendants of the pluton are dated at 330 Ma. The geodynamic model of the Khan Bogd pluton formation suggests collision of the Hercynian continent with a hot spot in the paleoocean; two variants of this model are proposed. According to the first variant, the mantle plume, after collision with the margin of the North Asian paleocontinent, reworked the subducted lithosphere and formed a structure similar to an asthenospheric window, which served as a source of rift-related magmatism and the Khan Bogd pluton proper. In compliance with the second variant, the emergence of hot mantle plume resulted in flattening of the subducted plate; cessation of the island-arc magmatism; and probably in origin of a local convective system in the asthenosphere of the mantle wedge, which gave rise to the formation of a magma source. The huge body of the Khan Bogd alkali granite pluton and related volcanic rocks, as well as its ring structure, resulted from the caldera mechanism of the emplacement and evolution of magmatic melts.

Late Riphean alkali granites of the Zabhan microcontinent: Evidence for the timing of Rodinia breakup and formation of microcontinents in the Central Asian Fold belt

The estimation of chronological boundaries in the geological history of the Rodinia supercontinent, in particular, the age of its breakup is far from a final solution. The enormous size of the supercontinent rules out synchronization of geological events throughout its territory. In addition, the estimation is complicated by unreliable reconstructions of positions of particular cratons within the supercontinent and a shortage of geochronological data on substantiation of the timing of breakup in separate parts of Rodinia. Most likely, this was a long-term process similar to that of the breakup of Pangea, which lasted for almost 150 Ma from the Early Jurassic to the Early Cenozoic [1]. The long-term character of these events is evidenced by the available geochronological data on the processes of rifting that initiated the breakup in various parts of Rodinia. For example, according to the reconstruction [2], two age levels of rifting are established beyond the Laurasian part of the supercontinent. The older event occurred from 830 to 795 Ma ago. The younger event (780‐ 745 Ma ago) completed the breakup of the continental lithosphere. The Laurasian part of Rodinia was broken into the Siberian and Laurentian continents 720‐ 630 Ma ago [3]. The breakup of Rodinia promoted the origin of the Paleoasian ocean, the evolution of which produced the Central Asian Fold belt (CAFB). The terranes (microcontinents) of the Precambrian crust within the fold belt are regarded as fragments of supercontinent margins [3]. Such an interpretation is supported by structural and historical similarities of the terranes with some continental massifs in Rodinia and by wide occurrence of shelf complexes therein. However, the timing of separation of these terranes from the supercontinent and their initial location remain uncertain. In this communication, new data on the isotopic age and composition of the Late Riphean alkali granites of the Zabhan Terrane established in the CAFB are reported for the first time and the timing of the breakup and approximate position of this microcontinent in Rodinia is outlined. Geological characteristics. The Zabhan microcontinent (Fig. 1) represents terranes with an Early Precambrian basement, which are rare in the CAFB. The oldest metamorphic rocks of the Baidarik Block are subdivided into the Upper Archean Baidarag and the Lower Proterozoic Bumbuger crystalline complexes [4]. The stages of the microcontinent evolution are broadly correlated with those of the North Chinese and Siberian cratons [4]. The collisional processes responsible for the formation of the main tectonic units of these cratons and the microcontinent occurred almost synchronously 1.90‐1.85 Ga ago. In the northeast, the basement rocks are unconformably overlapped by primarily greenschist-facies rocks of the Ul’dzit-Gol Complex (metasandstones, black shales, and marmorized dolomites) of presumably Middle‐lower Upper Riphean age. Based on the K‐Ar actinolite dating, the age of greenschist-facies metamorphism of rocks of this complex is estimated at ~840 Ma [5]. In the western part of the microcontinent, the basement rocks are overlain by gently dipping subaerial volcanics of the Zabhan Group [6]. They are composed of virtually unmetamorphosed violet, black, and redbrown subaerial volcanic glasses, vitreous rhyodacites and trachyrhyolites, as well as ignimbrites with rare small feldspar and quartz phenocrysts. The subordinate basic and intermediate volcanic rocks are usually confined to the base of the group and to its roof in some places [6]. Their share increases toward the western

Crust-forming processes in the Hercynides of the Central Asian Foldbelt

The paper reports data on the evolutionary history of magmatism, its conditions, and sources in the process of the development of the Southern Mongolian Hercynides during the pre-accretion, continental-margin, and rifting stages within the time span from the Silurian to Early Permian. The Hercynian continental crust in the southern Mongolian segment of the Central Asian Foldbelt (CAFB) was determined to have grown in the environment of ensimatic island arcs, backarc basins, spreading centers, and oceanic islands or plateaus, with material coming from the depleted and, perhaps, also enriched mantle sources in the open ocean that surrounded the Siberian paleocontinent on the side of the Caledonian margin. This made it possible to recognize the Early-Middle Paleozoic epoch of juvenile crustal growth in CAFB and the corresponding isotopic crustal province with a total area of more than 200 thousand km2. The principal differences between the composition and structure of the blocks surrounding the Hercynian regions (Caledonides in the Gobi Altai and Grenwillides in the South Gobi microcontinent) testify that the southern margin of the Caledonian Siberian continent and the Grenvillides of the South Gobi microcontinent had different geological histories and were spatially separated. The structural complex of the Paleoasian ocean, including the terranes of the South Gobi microcontinent, were transformed into a continental block in the latest Devonian-earliest Carboniferous, in relation with accretion processes, folding, metamorphism, and tectonic delamination along the boundaries of structurally heterogeneous domains. The subsequent recycling of the crust by magmatic processes was related to the development of an active continental margin (ACM). The development of an ACM in the Hercynides resulted from and was a continuation of the motions of the continental and oceanic lithospheric plates, i.e., processes that brought about the Hercynian accretion. The evolution history of the ACM was subdivided into two stages: early (a continental-margin stage proper) and late (rifting stage). The rocks of the early stage were produced at 350–330 Ma and compose a differentiated basalt-andesite-rhyodacite complex and related massifs of the granodiorite-plagiogranite and banatite (diorite-monzonite-granodiorite) associations. During the rifting stage at 320–290 Ma, a bimodal basalt-comendite-trachyrhyolite association was formed, along with accompanying alkali granite massifs. In the southern Mongolian segment of the Hercynides, the rocks of the rifting stage compose two subparallel rift zones: Gobi-Tien Shan, which extends along the boundaries of the South Gobi microcontinent, and the Main Mongolian lineament, which marks the boundaries between the Hercynides and Caledonides in the CAFB. The rift structures are made up of alkali granitoids and normal-alkalinity granitoids, which are atypical of rift zones. Their genesis is thought to have been related to crustal anatexis, a process that was triggered by rift-related magmas at an unusual combination of rifting and ACM tectonic setting. The basic rocks of the rift associations have geochemical signatures atypical of continental rifting. They show Ta and Nb minima and K and Pb maxima, as is typical of rocks generated at convergent plate boundaries. Nevertheless, the broad variations in the concentrations and ratios of some major and incompatible trace elements and in the Sr, Nd, and O isotopic composition of the rift basaltoids allowed us to distinguish their high-and low-Ti varieties, which were produced with the participation of three mantle sources: depleted mantle similar to the source of basalts in midoceanic ridges, enriched mantle like the source of basalts in oceanic islands, and the mantle material of the metasomatized mantle wedge. The origin of andesites in the rift zones is explained by the contamination of mantle basaltoid melts with sialic (predominantly sedimentary) material of the continental crust or the assimilation of anatectic partial granite melts.

Age constraints of high-temperature metamorphic events in crystalline complexes of the Irkut block, the Sharyzhalgai ledge of the Siberian platform basement: Results of the U-Pb single zircon dating

Geochronological data obtained in this work and previously known results of U-Pb geochronology suggest that principal metamorphic events, which took place in eastern part of the Irkut block (the Sharyzhalgai marginal ledge of the Siberian platform basement), correspond in age to (1) about 2.8 Ga, (2) 2649 ± 6 to 2562 ± 20 Ma, and (3) 1865 ± 4 to 1855 ± 5 Ma. Structural and metamorphic reworking of the earliest event originated under conditions of the granulite facies, whereas conditions of granulite and amphibolite facies were characteristic of the second and third events. Metasomatites after carbonate rocks originated in eastern part of the Sharyzhalgai ledge during the Early Proterozoic metamorphic event that lasted approximately 20 m.y. Being combined with age data, which are known at present for the reference syn-and post-collision granitoids in the Siberian platform basement and flanking foldbelts, new geochronological results show that accretion of basement blocks to the Siberian craton progressed from the east to the west between 1900 and 1840 Ma. To a first approximation, this geochronological interval characterizes time span of the Paleoproterozoic ocean closure and ultimate time, when the craton and supercontinent Columbia became amalgamated.

Early paleozoic granitoids in the Lesser Khingan terrane, Central Asian Foldbelt: Age, geochemistry, and geodynamic interpretations

The U-Pb zircon dates obtained for the Sutara (480 ± 4 Ma), Kabalinskii (471 ± 10 Ma), and Durilovskii (461 ± 5 Ma) massifs reliably confirm an Early Proterozoic orogenic event, which took place after granulite metamorphism at approximately 500 Ma (Wilde et al., 2003) in the Lesser Khingan (Jiamusi) terrane. The rocks emplaced most shortly after the main metamorphic event are the granites of the Sutara Massif and leucogranites of the Kabalinskii Massif, whose geochemistry is close to that of collision granites. The quartz diorites and subalkaline granites of the Durilovskii Massif, whose geochemistry suggests their origin in a postcollision environment with the participation of an enriched mantle source, were emplaced longer after metamorphic event and after the aforementioned massifs.

Vendian stage in formation of the Early Caledonian superterrane in Central Asia

Granitoids and metamorphic rocks of the Baidarik basement block of the Dzabkhan microcontinent are studied in terms of geology, geochronology (U-Pb dating of zircon microfractions and individual grains) and Nd isotopic-geochemical systematics. As is established, the formation history of metamorphic belt (disthene-sillimanite facies) in junction zone of the Baidarik block and Bayankhongor zone of the Late Riphean (∼665 Ma) ophiolite association characterizes development of the Vendian (∼560–570 Ma) active continental margin. The high-P metamorphic rocks of that time span evidence formation of structures with the Earth’s crust of considerable thickness. In Central Asia, events of the Vendian low-gradient metamorphism are established also in the Tuva-Mongolian massif, Kan block of the East Sayan Mountains, and South Chuya inlier of the Caledonides in the Altai Mountains. Based on these data, it is possible to distinguish the Late Baikalian stage in development of the Early Caledonian superterrane of Central Asia, which antedated the subsequent evolution of this structure during the Late Cambrian-Ordovician. The high-gradient metamorphism that affected most intensively the southeastern part of the Baidarik block can be correlated with the Early Paleozoic (525–540 Ma) evolution of active continental margin and associated development of the Vendian oceanic basins and island arcs of the Ozernaya zone.

The time length of formation of the Angara-Vitim batholite: Results of U-Pb geochronological studies

This paper describes the results of geochronological studies (U-Pb method over micro lots and single grains of zircon) of autochtonous and allochtonous granitoids of the Barguzinskii complex of the Angara-Vitim batolite of the petrotypical area in the basin of the Dzhirga and Kovyli rivers (tributaries of the Barguzin River). The age of crystallization of gneissose granitoids is 297 ± 5 Ma, and that of intrusive leucocratic biotite granites is 291 ± 1 Ma. The estimates of the age finalize the discussion on the age of granitoids of the Barguzin complex and cannot be considered as “rejuvenated.” The analyses of the geochronological data that have been obtained up to the present for granitoids of the Angara-Vitim batolite with the SHRIMP and U-Pb methods for large samples of zircons show that in the majority of cases they cannot be used for precise estimation of the age of their crystallization. The geochronological data obtained with use of the U-Pb method over micro samples and single grains of zircon allow one to make a conclusion on the formation of granitoids of the described complexes of the Angara-Vitim batholite that occurred within 303 ± 7–281 ± 1 Ma. Thus, the time length of formation of the largest in the eastern segment of the Central Asian belt of the Angara-Vitim batholite is not more than 22 Ma (minimum 6 Ma), which allows us to consider it as a large granitic province and is a boundary condition for development of the geodynamic models of its formation.