S. Z. Yakovleva

Composition, sources, and mechanism of continental crust growth in the Lake zone of the Central Asian Caledonides: I. Geological and geochronological data

Data on the composition, inner structure, and age of volcanic and siliceous-terrigenous complexes and granitoids occurring in association with them in the Caledonian Lake zone in Central Asia are discussed in the context of major relations and trends in the growth of the Caledonian continental crust in the Central Asian Foldbelt (CAFB). The folded structures of the Lake zone host basalt, basalt-andesite, and andesite complexes of volcanic rocks that were formed in distinct geodynamic environments. The volcanic rocks of the basalt complex are noted for high concentrations of TiO2 and alkalis, occur in association with fine-grained siliceous siltstone and siliceous-carbonate rocks, are thus close to oceanic-island complexes, and were likely formed in relation to a mantle hotspot activity far away from erosion regions supplying terrigenous material. The rocks of the basalt-andesite and andesite complexes have lower TiO2 concentrations and moderate concentrations of alkalis and contain rock-forming amphibole. These rocks are accompanied by rudaceous terrigenous sediments, which suggests their origin in island-arc environments, including arcs with a significantly dissected topography. These complexes are accompanied by siliceous-terrigenous sedimentary sequences whose inner structure is close to those of sediments in accretionary wedges. The folded Caledonides of the Lake zone passed through the following evolutionary phases. The island arcs started to develop at 570 Ma, their evolution was associated with the emplacement of layered gabbroids and tonalitetrondhjemite massifs, and continued until the onset of accretion at 515–480 Ma. The accretion was accompanied by the emplacement of large massifs of the tonalite-granodiorite-plagiogranite series. The postaccretionary evolutionary phase at 470–440 Ma of the Caledonides was marked by intrusive subalkaline and alkaline magmatism. The Caledonides are characterized by within-plate magmatic activity throughout their whole evolutionary history, a fact explained by the accretion of Vendian-Cambrian oceanic structures (island arcs, oceanic islands, and back-arc basins) above a mantle hotspot. Indicators of within-plate magmatic activity are subalkaline high-Ti basalts, alkaline-ultrabasic complexes with carbonatites and massifs of subalkaline and alkaline gabbroids, nepheline syenites, alkaline granites, subalkaline granites, and granosyenites. The mantle hotspot likely continued to affect the character of the lithospheric magmatism even after the Caledonian folded terrane was formed.

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

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.

The Kalar Compex, Aldan-Stanovoi Shield, an Ancient Anorthosite-Mangerite-Charnockite-Granite Association: Geochronologic, Geochemical, and Isotopic-Geochemical Characteristics

The autonomous (massif-type) anorthosite massifs of the Kalar Complex (2623 ± 23 Ma) intrude high-grade metamorphic rocks of the Kurulta tectonic block at the junction of the Aldan and Dzhugdzhur-Stanovoi fold area. These rocks belong to the most ancient anorthosite-mangerite-charnockite-granite (AMCG) magmatic association, whose origin was constrained to the Mesoproterozoic (1.8–1.1 Ga). The charnockites are typical high-potassium reduced granites like rapakivi, which affiliate with the A type. The Nd and Pb isotopic composition of these rocks suggests their predominantly crustal genesis, whereas the anorthosites were most probably produced by a mantle magma that was significantly contaminated by crustal material at various depth levels. The intrusions of the Kalar Complex were emplaced in a postcollision environment, with the time gap between the collisional event and the emplacement of these massifs no longer than 30 m.y. The southern Siberian Platform includes two definitely distinguished and spatially separated AMCG associations, which have different ages and tectonic settings: (i) the Late Archean (2.62 Ga) postcollision Kalar plutonic complex and (ii) the Early Proterozoic (1.74–1.70 Ga) anorogenic Ulkan-Dzhugdzhur volcano-plutonic complex.

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.