Central Asian Orogenic Belt

Collision-related magmatism in accre-tionary-to-collisional orogens records a tectonic transition from early subduction-accretionary processes to collisional oro-genesis, and also plays a significant role in continental growth. Here, we present an integrated study of field observations, geochem-istry, whole-rock Rb-Sr and Sm-Nd isotopes, and zircon U-Pb ages and Lu-Hf isotopes for the Laohushan mafic to felsic magmatic rocks related to initial collision between the Alxa terrane and the Central Qilian block along the North Qilian orogenic belt, northeastern Tibet. The Laohushan magmatic rocks are dominated by quartz diorites (ca. 426 Ma), with minor tonalites enclosing dioritic enclaves (ca. 430 Ma) and horn-blendite xenoliths (ca. 448 Ma), and some coeval dolerite dikes (ca. 427 Ma) intruded into the accretionary complex. The quartz diorites are characterized by light rare earth element (LREE)-and large ion lithophile element (LILE)-enrichment but have high field strength element (HFSE)-depleted trace element patterns and negative initial ε Nd (-1.6 to-2.9) and positive zircon initial ε Hf (+3.0 to +6.2) values. The dioritic enclaves are also characterized by LREE-enriched and HFSE-depleted patterns and have mostly negative initial ε Nd (-9.2 to +0.03) but positive zircon initial ε Hf (+3.0 to +5.9) values. These geochemical and isotopic features, together with isotopic mixing calculations, suggest that the quartz diorites were likely derived from partial melting of the lower crust dominated by accreted mafic oceanic rocks with minor sediments, whereas the dioritic enclaves originated from underplated mantle-derived magmas mixed with crust-derived melts. The hornblendite xenoliths have high MgO, Cr, and Ni contents, positive Th, U, and Pb anomalies, and negative Nb, Ta, and Ti anomalies. They have negative initial ε Nd (-2.8), near chondritic zircon initial ε Hf (-0.4 to +1.4) values and an Archean Nd model age (T DM = 2.74 Ga), suggesting that the hornblendites were likely produced by partial melting of subcontinental lithospheric mantle peridotite that was metasomatized by subduction-related melts beneath the Archean-Proterozoic Alxa terrane. We propose that partial melting of the lower crust of the early Paleozoic North Qilian orogenic belt was in response to slab breakoff and astheno-spheric upwelling during the initial stage of collisional orogenesis. This study demonstrates that heterogeneous magma sources, involving accretionary materials (i.e., ac-creted oceanic crust and sediments) and various mantle-derived components, were mixed to form the collision-related magmatic rocks. It also highlights the significance of collision-related magmatism in continental growth and stabilization of newly-assembled crust in accretionary-to-collisional orogens.

Popov, L.E. and Cocks, L.R.M. 2017. Late Ordovician palaeogeography and the positions of the Kazakh ter-ranes through analysis of their brachiopod faunas. Acta Geologica Polonica, 67 (3), 323–380. Warszawa. Detailed biogeographical and biofacies analyses of the Late Ordovician brachiopod faunas with 160 genera, grouped into 94 faunas from individual lithotectonic units within the Kazakh Orogen strongly support an archipelago model for that time in that area. The Kazakh island arcs and microcontinents within several separate clusters were located in the tropics on both sides of the Equator. Key units, from which the Late Ordovician faunas are now well known, include the Boshchekul, Chingiz-Tarbagatai, and Chu-Ili terranes. The development of brachiopod biogeography within the nearly ten million year time span of the Late Ordovician from about 458 to 443 Ma (Sandbian, Katian, and Hirnantian), is supported by much new data, including our revised identifications from the Kazakh Orogen and elsewhere. The Kazakh archipelago was west of the Australasian segment of the Gondwana Supercontinent, and relatively near the Tarim, South China and North China continents, apart from the Atashu-Zhamshi Microcontinent, which probably occupied a relatively isolated position on the southwestern margin of the archipelago. Distinct faunal signatures indicate that the Kazakh terranes were far away from Baltica and Siberia throughout the Ordovician. Although some earlier terranes had joined each other before the Middle Ordovician, the amalgamation of Kazakh terranes into the single continent of Kazakhstania by the end of the Ordovician is very unlikely. The Late Ordovician brachiopods from the other continents are also compared with the Kazakh faunas and global provincialisation statistically determined.

The South Tianshan is located to the north of the Tarim block and defines the southern margin of the Paleozoic Central Asian Orogenic Belt (CAOB). This study presents new structural data, geochronological and geochemical results for the Wuwamen ophiolite mélange in the Chinese segment of the South Tianshan. In the south, the Wuwamen ophiolite mélange shows typical block-in-matrix fabrics and occurs in the foot-wall of a south-dipping thrust fault, hanging wall of which is composed of weakly metamorphosed and de-formed Lower Paleozoic marine to deep marine se-quences from the South Tianshan. In the north, a south-dipping thrust fault juxtaposes the Wuwamen ophiolite mélange in its hanging wall against the high-grade and strongly deformed metasedimentary rocks from the Central Tianshan in its footwall. Three stages of ductile deformation are distinguished on the basis of structur-al and kinematic analyses on different litho-tectonic units across the region. They are, from older to young-er: (1) regional top-to-the-north ductile shearing linked with subduction and accretionary tectonics; (2) wide-spread refolding of the earlier foliation, which likely resulted from a subsequent collision (or amalgamati-on) event; and (3) localized ductile right lateral strike-slip faulting attributed to late-orogenic extrusion tec-tonics. Geochemical data indicate that the igneous rocks in the Wuwamen ophiolite mélange include MORB and OIB-type volcanic rocks with arc-like fea-tures. Sr and Nd isotopic data further indicate that the-se igneous rocks were formed in a back-arc oceanic basin. Our zircon U-Pb ages combined with published data indicate that the igneous blocks in the mélange were formed during 334–309 Ma. An undeformed granite dike crosscutting the ophiolite mélange yielded an age of ~300 Ma and provides a minimum age of mélange formation. Meta-sandstones, which were previously interpreted as Devonian or Proterozoic in age, were deposited during the Late Carboniferous (~325–310 Ma) and yielded U-Pb detrital zircon ages consistent with a single Central Tianshan provenance. We propose an updated geodynamic model for the Paleozoic tectonic and paleogeographic evolution of the South Tianshan. We suggest that the Central Tianshan was locally separated from the Tarim block by back-arc oceanic basins during Devonian to Carbon-iferous. This study shows that the Paleozoic tectonics of the Central and South Tianshan was characterized by the opening and subsequent closing of back-arc basins prior to its final amalgamation to the Tarim block, simi-lar to the Cenozoic tectonic evolution of the western Pacific region, and such processes may be a major characteristic of orogenic belts during the transition from accretionary to collisional systems.

Acknowledgements:
This study was co-sponsored by the National Nature Science Foundation of China (41390445, 41222019, 41172197, and 41311120069). This study is a contribution to the IGCP-592 Project.

Proceedings of the Second Russia–China International Meeting on the Central Asian Orogenic Belt, Irkutsk (Russia), 6–12 September 2017. Geodynamics & Tectonophysics 8 (3), 581–582. doi:10.5800/GT-2017-8-3-0297

In order to better understand the tectonic role of the Yili Block on the Paleozoic evolution of the Chinese Tianshan Belt, we
performed a primary paleomagnetic study on Carboniferous and Permian rocks from different areas in the Yili Block, NWof China. More than 320 sedimentary and volcanic samples were collected from 39 sites. Except for the Ordovician samples showing a weak and unstable magnetic remanence, the majority of this collection presents characteristic remanent magnetization carried by magnetite and hematite. In the study area, though positive fold test has been observed on the Early Carboniferous rocks, a general re-magnetization of these rocks has been identified and attributed to the Late Carboniferous magmatism. Moreover, all Early and Late Carboniferous samples from the interior of the Yili Block yield stable and coherent magnetic directions with exhaustively
reverse magnetic polarity. The Late Carboniferous (C2) is considered as the magnetic remanence age since these rocks are covered or intruded by synchronous magmatic rocks of the Yili arc, which lasted until to ∼310 Ma. The C2 paleomagnetic pole is therefore calculated at 68.6°N, 290.6°E with A95=6.4° and n=15. The Late Carboniferous rocks located close to a deformation zone present a consistent magnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation. Although no fold test can be performed due to the monoclinal bedding, stable magnetic components are isolated from Late Permian (P2) red beds in the interior of the Yili Block with also a solo reverse magnetic polarity, the P2
paleomagnetic pole of the Yili Block has been, therefore, calculated from the characteristic remanent magnetization: 79.7°N, 172.0°E with A95=11.3° and n=5. Keeping important uncertainties in mind, comparisons of the C2 and P2 paleomagnetic poles of the Yili Block with available coeval poles of Junggar, Tarim and Siberia indicate (1) no significant relative motion between the Yili and Junggar blocks since the Late Carboniferous, (2) no significant or weak latitudinal relative motion occurred since the Late Carboniferous among these blocks, but (3) the 46.2°±15.1° and the 31.6°±15.1° counterclockwise rotations of the Yili–Junggar blocks with respect to Tarim and Siberia took place during C2 to P2. These rotations are accommodated by the Permian dextralstrike–slip faults along the northern and southern sides of Tianshan Belt and sinistral strike–slip faulting along the Erqishi Fault of Altay Belt, resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts, respectively.

The Tianshan belt (northwestern China) is a major tectonic element of the southern Central Asian Orogenic Belt that contains a number of ophiolitic mélanges and (ultra)high-pressure metamorphic belts formed after closure of oceanic and back-arc basins that resulted in terrane collisions. Deciphering its tectonic evolution is thus crucial for understanding the amalgamation of Central Asia. We produce robust 40Ar/39Ar laser-probe evidence that the Tianshan is a Late Palaeozoic (ultra)high-pressure metamorphic collision belt, not a Triassic one, as suggested by some SHRIMP zircon ages in recent literature. Instead of trying to date the peak pressure conditions we focused on 40Ar/39Ar analysis of white mica formed during retrograde recrystallisation when the (ultra)high-pressure metamorphic rocks of the Changawuzi-Kekesu complex were exhumed. Exhumation was coeval with their northward thrusting over the southern margin of the Yili terrane, the easternmost element of the Kazakhstan compo...