Closing History of the Late Palaeozoic Oceanic Basins between Tarim and Junggar: Geodynamics and Stratigraphic Problems

The history of the Late Palaeozoic convergence and collisions in the South Tian Shan, Tarim, and South Junggar has been considered based on new geological and geochronological data. The Late Palaeozoic (Pennsylvanian and Permian) in this region represents a distinct phase in the regional tectonic history and, thus, corresponds to the original meaning of the concept of “geological period (system)”. This phase begins with several events, which occur in the Early and Middle Pennsylvanian as follows: 1) development of a continuous north-dipping subduction zone along the northern edge of the South Tian Shan at ca. 320–315 Ma and beginning of overthrusting in the South Tian Shan; 2) collision of the Bogdoshan arc with the Yili continental arc of Eastern Kazakhstan at ca. 315–310 Ma. This led to elimination of magmatism in these two arcs and was followed by deposition of carbonates and subsidence in the Bogdoshan and Junggar basins; 3) beginning of collisions between the Alai and Tarim microcontinents and the southern margin of the Kazakhstan continent at ca. 310 Ma. Subduction and collisional deformations in the South Tian Shan were accompanied by top-to-the south overthrusting and deposition of turbidites and olistostromes that range in age from the Bashkirian in the north to the Asselian in the south. Amalgamation of two continents in the South Tian Shan led to elimination of the last deeper marine basins with cherty deposits in the Late Pennsylvanian and to termination of turbidite sedimentation in the Asselian. Beginning of the late collisional phase in the South Tian Shan during the late Asselian and Sakmarian is expressed by: 1) overthrusting changed to folding and strike-slip faulting; 2) general uplift of the fold-and-thrust belt that led to elimination of the last marine basins and consequent deposition of coarse continental molasses; 3) initiation of post-collisional granitoid magmatism, which at least by part may also reflect melting of the continental crust due to heating by the Tarim mantle plume. Tectonic events that took place in the Tian Shan-Junggar region during the Triassic and Jurassic are conventionally considered as intraplate, and this allows to define the end of the Late Palaeozoic tectonic phase near the boundary of the Permian and Triassic

Early Palaeozoic Deep Subduction of Continental Crust in the Kyrgyz North Tianshan: Evidence from Field Relationships and Geochronology

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Late Palaeozoic to Mesozoic kinematic history of the Talas-Ferghana strike-slip Fault (Kyrgyz West Tianshan) as revealed by 40Ar/39Ar dating of syn-kinematic white mica

International audienceThe NW-trending Talas-Ferghana Fault (TFF) in Kyrgyzstan, Central Asia, is one of the largest intracontinental strike-slip faults in the world. It extends over a distance of more than 2000 km from southern Tourghai to western Tarim and exhibits a maximum dextral offset of ∼200 km during the late Palaeozoic to present. The history of the fault provides important insights for the understanding of the evolution of southern Central Asia but remains poorly constrained due to lack of reliable geochronological data. We present new Ar-Ar ages and structural data from the Kyrgyz West Tianshan, that elucidate the kinematic history of the TFF in the Palaeozoic and Mesozoic. 40Ar/39Ar ages on mylonitic white micas document a deformational history consisting of several episodes. A late Carboniferous age of 312 ± 4 Ma point to initiation of top-to-the-south and dextral transpressional deformation during a metamorphic overprint in Precambrian and Palaeozoic rocks along the northern compartment of the TFF. The main phase of dextral motion along the entire fault occurred in the Permian as suggested by minimum ages of 260-290 Ma obtained at two different locations in the NW and central parts of the TFF. Partial isotopic resetting occurred between 240 and 210 Ma and younger ages of <200-210 Ma are ascribed to late brittle reactivation and hydrothermal fluid flow in the Late Triassic to Early Jurassic times. The Jurassic trans-tensional phase is featured by emplacement of 195 ± 3 Ma pegmatitic dykes. The Ar-Ar mineral ages and structural data argue for a major phase of dextral shearing, affecting the entire region from the West Tianshan to Mongolia in the late Permian and leading to formation of almost equally spaced major NW-trending dextral strike-slip faults. The uniform character of this deformation indicates that the process of amalgamation in this part of the CAOB ended prior to the late Permian, and Central Asia evolved as a single coherent continental block since that time

Tectonic models for accretion of the Central Asian Orogenic Belt

Tectonic models for accretion of the Central Asian Orogenic Bel

Geochronological evidence for Late Carboniferous high-pressure metamorphism and provenance of mélange sediments in the South Tianshan of Kyrgyzstan

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Zircon and muscovite ages, geochemistry, and Nd-Hf isotopes for the Aktyuz metamorphic terrane: Evidence for an Early Ordovician collisional belt in the northern Tianshan of Kyrgyzstan

International audienceThe Aktyuz metamorphic terrane in the Kyrgyz northern Tianshan consists of granitoid gneisses and migmatites with subordinate paragneisses, greenschists, presumed meta-ophiolites, and garnet amphibolite dykes that contain HP eclogite relicts. The gneisses and migmatites were previously considered to be Archaean and Palaeoproterozoic in age on the basis of α-Pb and U-Pb multigrain zircon dating. Zircons from a post-tectonic granite were previously dated at 692 ± 15 Ma, constraining the time of main deformation and metamorphism in the Aktyuz terrane to the Precambrian. The chemical characteristics of most granitoid samples are consistent with melting of chemically evolved crustal material, which is supported by Nd and Hf isotopic data. Zircon U-Pb SHRIMP ages were obtained for the main varieties of metamorphic rocks, for a gabbro of a low-grade ophiolite complex and for several post-kinematic igneous rocks. In addition, metamorphic muscovite was dated by the 40Ar-39Ar method, and whole-rock Sm-Nd isotopic systematics were obtained on several granitoid rocks. Our magmatic zircon crystallization ages for granitoid gneisses in the Aktyuz and Kemin Complexes range from 778 ± 6 to 844 ± 9 Ma which we interpret to reflect the time of magmatic emplacement of the gneiss protoliths. These rocks reflect an episode of Neoproterozoic granitoid magmatism, which is also documented in southern Kazakhstan, the Kyrgyz Middle Tianshan, the Chinese Central Tianshan and the Tarim Craton. Nd and Hf isotopic systematics show these rocks to be derived from Mesoproterozoic to Archaean sources. The calc-alkaline composition of these rocks seems compatible with a subduction setting, but is most likely inherited from the source, therefore the tectonic scenario for emplacement of the gneiss protolith remains unknown. Two ages of 562 ± 7 and 541 ± 3 Ma and negative εNd(t)-values for granitoid gneisses document a later crustal melting episode. Muscovite 40Ar/39Ar ages of ca. 470 Ma for Aktyuz gneisses constrain the main fabric-forming metamorphism to the Early Palaeozoic. A migmatitic paragneiss, which was previously interpreted as Palaeoproterozoic, contains detrital zircons with an age spectrum from 503 to 1263 Ma; the youngest grain suggests a maximum Cambrian age of protolith deposition. An ophiolitic metagabbro of the Kemin Complex yielded an Early Cambrian age of 531 ± 4 Ma, which is close to the age of ophiolites in the adjacent Djalair-Naiman belt of Kazakhstan, suggesting a possible genetic link. Two samples of quartz diorite from the post-kinematic Dolpran pluton yielded Early Ordovician zircon ages of 471.9 ± 3.5 and 472.0 ± 3.1 Ma. The presence of a 783 ± 7 Ma xenocrystic zircon points at Precambrian crust at depth, which may explain an earlier, discordant apparent age obtained by multigrain zircon dating of this pluton. Undeformed rhyolite and basalt in the East Kyrgyz Range, previously classified as Neoproterozoic and Cambrian, yielded Late Ordovician ages of 451.9 ± 4.6 and 448.9 ± 5.6 Ma respectively. Our data imply that the Aktyuz terrane is not a single segment of continuous Precambrian continental crust but represents a complex amalgamation of Neoproterozoic continental and Early Palaeozoic ophiolitic slivers, which were stacked together in a subduction and collisional setting. Large fragments of continental crust were subducted to HP eclogite-facies conditions, which led to eclogite metamorphism in the mafic dykes. The main deformational event occurred during the latest Cambrian to earliest Ordovician between 503 and 472 M

Reassessment of continental growth during the accretionary history of the Central Asian Orogenic Belt

We argue that the production of mantle-derived or juvenile continental crust during the accretionary history of the Central Asian Orogenic Belt (CAOB) has been grossly overestimated. This is because previous assessments only considered the Palaeozoic evolution of the belt, whereas its accretionary history already began in the latest Mesoproterozoic. Furthermore, much of the juvenile growth in Central Asia occurred in late Permian and Mesozoic times, after completion of CAOB evolution, and perhaps related to major plume activity. We demonstrate from zircon ages and Nd–Hf isotopic systematics from selected terranes within the CAOB that many Neoproterozoic to Palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous Precambrian crust or through mixing of old continental crust with juvenile or short-lived material, most likely in continental arc settings.At the same time, juvenile growth in the CAOB occurred during the latest Neoproterozoic to Palaeozoic in oceanic island arc settings and during accretion of oceanic, island arc, and Precambrian terranes. However, taking together, our data do not support unusually high crust-production rates during evolution of the CAOB. Significant variations in zircon εHf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources. We reiterate that the chemical characteristics of crustally-derived granitoids are inherited from their source(s) and cannot be used to reconstruct tectonic settings, and thus many tectonic models solely based on chemical data may need re-evaluation. Crustal evolution in the CAOB involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history, and we see many similarities with the evolution of the SW Pacific and the Tasmanides of eastern Australia