I. V. Anisimova

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].

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

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.

Early Paleozoic Age of Gabbroids of the Amur Complex (Bureya-Jiamusi Superterrane of the Central Asian Fold Belt)

The Bureya–Jiamusi superterrane (Fig. 1) [1] is one of the main tectonic elements in the eastern segment of the Central Asian fold belt. According to the existing concept, the basement of this terrane is made up of magmatic and metamorphic rocks of supposedly Early Precambrian age [2–4]. However, as yet there are no reliable geochronological and isotopic-geochemical data in support of this viewpoint. This is a serious obstacle in developing a common geodynamic scheme of the formation of both the Bureya–Jiamusi superterrane and the entire eastern segment of the Central Asian fold belt. This work reports the first geochronological data on the gabbroids of the Amur Complex, which, as the host Amur Group, appear to be the oldest rocks of the considered superterrane [2–5].

Mesozoic age of granitoids from the Beket complex (Gonzha block within the Argun terrane of the Central-Asian Fold Belt)

U-Pb geochronological results confirm the Mesozoic age (124 ± 1 Ma) of the Beket granitoid complex, previously interpreted as being one of the markers amongst the Early Proterozoic magmatic complexes within the Amur superterrane (microcontinent) of the Central Asian Fold Belt. This implies that the structural and metamorphic amphibolite facies overprints documented either in the Beket granitoids or Gonzha host rocks are evidently Mesozoic rather than Early Proterozoic in age.

Collisional granitoids of the Dzhida zone of the Central Asian Fold belt, Southwestern Transbaikalia: Age and conditions of the formation

AbstractaThis paper considers the geological structure, composition, and age of the Darkhintui, Barun-Gol, and Khuldat granitoid plutons of the Dzhida zone of Caledonides of the Central Asian Fold belts. These plutons were formed in the Late Cambrian-Early Ordovician in the range between 490 ± 2 and 477 ± 6 Ma, after tectonic juxtaposition of the oceanic and island-arc complexes of the Dzhida Zone and volcanogenic-carbonate-terrigenous rocks of the Khamardaban zone, i.e., at the collisional stage of the region evolution. Geological, geochronological, geochemical, and Nd isotope data indicate that the collisional granitoids of the Dzhida zone were derived by melting of continental crust thickened through accretion. The sources for parental melts of the granitoids were presumably Vendian-Early Cambrian juvenile igneous rocks of ophiolite and island-arc complexes, as well as the crustal material of the Lower Paleozoic flyschoid sediments of the back-arc basin of the Dzhida zone and metaterrigenous rocks of the Khamardaban zone.

Ophiolite sections of the Dzhalair-Nayman zone, South Kazakhstan: Their structure and age substantiation

The ophiolite complexes outcrop extending more than 700 km in the Dzhalair-Nayman zone located between the Chuya-Kendyk Tas (in the southwest) and Aktau-Dzhungar (in the northeast) Precambrian sialic massifs in South Kazakhstan. The most complete ophiolite sections are described in the central (Andassai massif) and southeastern (Dulankara massif) parts of the zone. Plagiogranites occur in a sheeted dyke complex in the upper part of ophiolite sections. A bimodal volcanic series occurs atop the section. The U/Pb zircons ages for the Andassai and Dulankara plagiogranites are 519 ± 4 and 521 ± 2 Ma, respectively. Ophiolites associated with bimodal volcanic series are also found in the north and northeast of Kazakhstan. They may form, together with the Lower Cambrian complexes of the Dzhalair-Nayman zone, a single paleostructure, whose fragments are well-traceable for over 2000 km

Crustal growth stages in the Songino block of the Early Caledonian superterrane in Central Asia: I. Geological and geochronological data

The Early Caledonian folded area in Central Asia (Early Caledonian superterrane) hosts micro-continent fragments with an Early and Late Precambrian crystalline basement, the largest of them being the Dzabkhan and Tuva-Mongolian fragments. Their junction zone hosts exposures of crystalline rocks that were previously thought to be part of the Early Precambrian Dzabkhan microcontinent. The Bayannur zone in the southern part of the Songino block hosts the Baynnur gneiss-migmatite and Kholbonur metavolcanic-terrigenous metamorphic complexes. The former is believed to be the Early Proterozoic crystalline basement, and the latter is thought to unconformably overly the Late Riphean cover complex of the Songino block. Various rocks of the tectono-stratigraphic complexes in the Bayannur zone were studied geologically and geochronologically (by the U-Pb technique of zircon). Regional metamorphism and folding in the Bayannur Complex were dated at 802 ± 6 Ma. The Nd model ages lie within the range of 1.5–2.0 Ga and thus preclude the correlation of these rocks with those in the Archean and Early Proterozoic basement of the Dzabkhan microcontinent. The upper age limit for folding and metamorphism in the Bayannur zone is marked by postkinematic granites dated at 790 ± 3 Ma, and the lower limit of the volcano-sedimentary complex is determined by the Nd model age of the sandstone (1.3 Ga). The upper age limit of the volcano-plutonic rocks in this zone is set by the gabbroids and anorthosites: 783 ± 2 and 784 ± 3 Ma, respectively. The complex of island-arc granitoids in the Bayannur zone is dated at 859 ± 3 Ma. The age constraints make it possible to correlate crystalline rocks in the Bayannur Complex of the Sangino block and the Dzhargalant Complex in the Tarbagatai block. Currently available data testify that the Precambrian Khangai group of blocks in the Early Caledonian Central Asian superterrane includes continental crustal blocks related to the processes of Early Precambrian, Late Riphean, and Vendian tectonism.

The Middle Riphean Volcanogenic Complex of Kokchetav Massif (Northern Kazakhstan): Structural Position and Age Substantiation

The volcanogenic Kuuspek Formation is a well-defined part of the succession of the Pre-Vendian complexes of the Kokchetav massif (Northern Kazakhstan). The formation is built up of mildly metamorphosed acid lavas, tuffs, and tuffaceous sandstones. At the reference site to the west of the Kokchetav Mountains, the rocks of the Kuuspek Formation compose hinges of small anticlinal folds with sericite-quartz schists of the Late Riphean Sharyk Formation forming the limbs. The Kuuspek Formation lavas are high-alumina rhyolites of high-potassium calc-alkaline series. The U-Pb zircon age of the rhyolites is 1136 ± 4 Ma, thus referring to the Middle Riphean. The Kuuspek rhyolites form the basal part of the Precambrian sedimentary cover of the Kokchetav massif. The cover also comprises schists, limestones, and dolomites of the Sharyk Formation, and quartzites and quartzitic schists of the Late Riphean Kokchetav Formation.