I. K. Kozakov

Age of Palaeozoic granites and metamorphism in the Tuvino-Mongolian Massif of the Central Asian Mobile Belt: loss of a Precambrian microcontinent

Abstract The Tuvino-Mongolian Massif (TMM) was previously interpreted as a Precambrian block within the Central Asian Mobile Belt. According to this idea, it consists of tectonic slices composed of metamorphic rocks of pre-Mesoproterozoic basement that experienced two episodes of regional metamorphism, and Mesoproterozoic ‘cover rocks’ that were reworked together with the basement during high-grade metamorphism. Zircon U–Pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536±6 Ma, significantly later than the deposition of the cover rocks. Regional upper amphibolite-facies metamorphism, which affected all metasedimentary units of the TMM, occurred in the short time interval of 497±4 to 489±3 Ma. We propose that there is no simple basement-cover relationship in the Tuvino-Mongolian Massif. Instead, the massif consists of tectonic domains, composed of heterogeneous metasedimentary successions with distinct pre-metamorphic and pre-tectonic histories that were juxtaposed by thrusting prior to 497 Ma and then metamorphosed under upper amphibolite facies conditions. We suggest that this thrusting event was associated with early Palaeozoic collisional processes related to amalgamation of the Central Asian Mobile Belt. Our new model for the evolution of the TMM implies derivation of Neoproterozoic terrigeneous metasediments from ∼800–900 Ma granitoids formed in an Andean-type active continental margin setting.

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.

Mechanisms of continental crust formation in the Central Asian Foldbelt

Geological and isotopic study of rocks occurring in the Early and Late Baikalian, Caledonian, Hercynian, and Indosinian fold regions of Central Asia is carried out. The juvenile crust formation occurred in these fold regions have determined the systematic differences in isotopic compositions of the crust. In the course of the subsequent (postaccretion) evolution, the crust of these domains underwent multiple reworking. These processes were accompanied by variations in the Nd isotopic compositions of the crust, which, in turn, are recorded in the isotopic compositions of granites and felsic volcanics as products of crust melting. Three types of crust differing in Nd isotopic composition and structure and, as a consequence, in formation mechanisms, are distinguished. The isotopically homogeneous crust is a source of igneous rocks with constant model Nd isotopic age (TNd(DM2st)) irrespective of the age of the crustal igneous rocks. These are the isotopic provinces, the crust of which remained isolated from addition of alien materials during postaccretion evolution. The axial zone of the Hercynides in the Central Asian Foldbelt is an example. The isotopically heterogeneous layered crust consists of fragments differing in isotopic composition. The products of its melting are characterized by widely scattered ɛNd(T) and (TNd(DM2st). The appearance of alien sources of melt is considered in terms of underplating. This mechanism develops either due to subduction of the juvenile oceanic lithosphere beneath the mature continental lithosphere at convergent boundaries or as a result of plume-lithosphere interaction. The first mechanism operated during the formation of granitoids pertaining to the Tuva-Mongolia microcontinent. The second mechanism was responsible for the formation of batholiths in the zonal Hangay, Barguzin, and Mongolia-Transbaikalia magmatic fields. The isotopically heterogeneous mixed crust is composed of fragments differing in isotopic composition, which are tectonically mixed, resulting in the formation of an isotopically uniform reservoir in the domain of magma generation. As a result, the products of melting acquire isotopic parameters substantially distinct from the juvenile rocks of the corresponding structural zone. The formation of such a crust is related to the tectonic delamination, which provides for juxtaposition and a high degree of tectonic mingling of heterogeneous fragments at deep levels. The Caledonides of the Central Asian Foldbelt are characterized by such a mechanism of crust formation.

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.

Age and sources of granitoids in the junction zone of the Caledonides and Hercynides in southwestern Mongolia: Geodynamic implications

The paper reports newly obtained geological, geochronological (U-Pb zircon method), Nd isotopic, and geochemical data on Middle and Late Paleozoic granitoids and metamorphic rocks from the southern slope of the Mongolian Altai and Gobi Altai and on granitoids from the Trans-Altai Gobi. Tectonically, the former rocks are hosted in the margin of a Caledonian paleocontinent, and the latter are localized among island-arc and oceanic complexes related to the development of the Hercynian Southern Mongolian Ocean. According to their geological setting, the intrusive complexes are subdivided into two major groups: (i) related to processes of regional metamorphism and (ii) separated from these processes. Geochemical data suggest that the source of most of the granitoids and metamorphic rocks contained island-arc rocks and their erosion products. Nd isotopic evidence indicates that practically all of the allochthonous granitoids, regardless of their composition, age, and structural setting, have positive ɛNd(T) values [i.e., belong to the ɛ(+) type] and could not be formed by the melting of metaterrigenous rocks widespread at the modern erosion level. These granitoids in both the Caledonian and the Hercynian structures have practically identical Late Riphean Nd model ages [TNd(DM) = 0.97–0.60 Ga], which become slightly younger in the granitoids of the Hercynides. The exception are ultrametamorphic subautochthonous ɛ(−) granites of the first group localized in the peripheral part of migmatite fields. The sources of these granitoids could be the host metaterrigenous rocks. The results obtained in the course of this research suggest, with regard for preexisting data on granitoids in the isotopic provinces in Central Asia, that the sources of the Paleozoic granitoids were the rocks of the “juvenile” Caledonian and Hercynian island-arc crust and of the older crust of cratonic blocks with a Early Precambrian and Late Riphean basement, respectively. The Late Riphean crustal material in Caledonian and Hercynian structures related to the development of the corresponding oceanic basins most probably consisted of clastic sediments or relatively small fragments of the Late Riphean crust. The occurrence of this crustal material in the sources of the granitoids can be explained by the involvement of sediments in subduction zones and the participation of these sediments and fragments of Late Riphean complexes in the accretionary-collision processes during the closure of the paleoceanic basins. Simultaneously, the subduction zones received juvenile material that could be later involved in the melting processes together with older rocks.

Crystalline complexes of the Tarbagatai block of the Early Caledonian superterrane of Central Asia

The oldest crystalline complexes of the Early Caledonian superterrane of Central Asia were formed in the Early Precambrian. They are exposed in the basement of microcontinents, which represent old cratonic fragments. Among the latters are the crystalline complexes of the Tarbagatai block previously ascribed to the Dzabkhan microcontinent. It was shown that the crystalline complexes of the Tarbagatai block have a heterogeneous structure, consisting of the Early Precambrian and later Riphean lithotectonic complexes. Structurally, the Early Precambrian complexes are made up of tectonic sheets of gneisses, migmatites, and gneiss granites of the Ider Complex that are cut by gabbroanorthosite massif. The Riphean Jargalant Complex comprises alternating hornblende crystalline schists and biotite (sometimes sillimanite-bearing) gneisses with marble horizons. The upper age boundary of the Riphean Complex is determined by the subautochthonous granitoids with age about 810 Ma. The presence of the Riphean high-grade rocks indicates that structures with newly formed crust were formed in the paleooceanic framing of the Early Precambrian blocks of the Rodinia supercontinent by the Mid-Late Riphean. Divergence that began at that time within old Rodinian cratons and caused rifting and subsequent break-up of the supercontinent was presumably changed by convergence in the paleooceanic area.

The Early Baikalian Crystalline Complex in the Basement of the Dzabkhan Microcontinent of the Early Caledonian Orogenic Area, Central Asia

Fragments of continental blocks or microcontinents are represented in the Early Caledonian orogenic area of Central Asia (or Early Caledonian superterrane); the largest of these are the Dzabkhan and Tuva-Mongolian microcontinents, with Early and Late Precambrian crystalline basements, respectively. In the linkage zone of these microcontinents, crystalline rocks of the Tarbagatai and Songino blocks that are considered as units of the Early Precambrian ensialic basement of the superterrane are also known. They are composed of strongly metamorphosed rocks formed during the Early Baikalian orogeny about 790 to 820 Ma. U-Pb zircon dating and Nd isotope studies revealed, within the northwestern Dzabkhan microcontinent, the Dzabkhan-Mandal zone of crystalline rocks associated with the Riphean crust-forming process. The age of the gneiss substrate of this zone is estimated as 1.3 to 0.86 Ga. An early episode of metamorphism is dated at about 856 ± 2 Ma. The data available so far indicate a heterogeneous structure of the Dzabkhan microcontinent basement represented by Early Precambrian and Early and Late Baikalian crystalline formations.

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.

Late Riphean episode in the formation of crystalline rock complexes in the Dzabkhan microcontinent: Geological, geochronologic, and Nd isotopic-geochemical data

The Early Caledonian Central Asian Orogenic Belt hosts fragments of continental blocks with Early and Late Precambrian crystalline basement. One of the structures with an Early Precambrian basement was thought to be the Dzabkhan microcontinent, which was viewed as an Early Precambrian “cratonal terrane”. The first geochronologic data suggest that the basement of the Dzabkhan microcontinent includes a zone of crystalline rocks related to Late Riphean tectonism. Geological, geochronological (U-Pb zircon dates), and Nd isotopic-geochemical data were later obtained on the northwestern part of the Dzabkhan microcontinent. The territory hosts the most diverse metamorphic complexes thought to be typical of the Early Precambrian basement. The complexes were determined to comprise the Dzabkhan-Mandal and Urgamal zones of high-grade metamorphic rocks. Gabbrodiorites related to the early metamorphic episode and dated at 860 ± 3 Ma were found in the Dzabkhan-Mandal zone, and the gneiss-granites marking the termination of this episode were dated at 856 ± 2 Ma. The granitoids of the Dzabkhan batholith, whose emplacement was coeval with the termination of the late high-grade metamorphic episode in rocks of both zones, have an age of 786 ± 6 Ma. Similar age values were determined for the granitoids cutting across the Late Precambrian rocks of the Songino and Tarbagatai blocks, which mark the stage when the mature Late Riphean continental crust was formed. The Late Riphean magmatic and metamorphic rocks of the Dzabkhan microcontinent were found out to have Nd model ages mostly within the range of 1.1–1.4 Ga at ɛNd(T) from +1.9 to +5.5. The Nd model age of the metaterrigenous rocks is 2.2−1.3 Ga at ɛNd(T) from −7.2 to +3.1. The results of our studies provide evidence of convergence processes, which resulted in the Late Riphean (880−780 Ma) continental crust in Central Asia. Simultaneously with these processes, divergence processes that were responsible for the breakup of Rodinia occurred in the structures of the ancient cratons. It is reasonable to suggest that divergence processes within ancient continental blocks and Rodinia shelf were counterbalanced by the development of the Late Riphean continental crust in the convergence zones of its surrounding within established interval.