Paleozoic geodynamic evolution of Yili Block<br />in West Chinese Tianshan

The Yili Block is a triangular area bordered by the northern and southern fault zones of the<br />Western Chinese Tianshan Belt, it is considered as a micro-continental block with Proterozoic<br />basement extending westward into Kazakhstan with unclear boundaries. The Yili Block is<br />important for understanding the Paleozoic geodynamic evolution of the Tianshan Belt, which<br />resulted from polyphase subduction-collision orogenies. The Tianshan Belt is classically divided<br />into three subunits: North Tianshan, Central Tianshan and South Tianshan. But our structural,<br />geochemical and paleomagnetic studies suggest that these subunits and their boundaries should be<br />redefined.<br />The Yili Block, previously considered as a part of the Central Tianshan, is in fact a Late<br />Paleozoic magmatic arc situated upon a Precambrian basement and an Early Paleozoic platform.<br />Carboniferous volcanic rocks are widespread along the margins of Yili Basin. Petrological and<br />geochemical features of Carboniferous volcanic rocks and granitoids show that: (1) volcanic rocks<br />are mainly composed of andesite, rhyolite and minor basaltic rocks, granitoids consist of gabbro,<br />diorite, granodiorite and granite, both volcanic and granitic rocks are belong to the calc-alkaline<br />series, (2) magmatic rocks display prominent Nb-Ta negative anomalies consistent with<br />subduction - related magmas, and (3) HFSE-based discriminations place volcanic rocks in the<br />field of continental arcs, and granitoids in the field of syn-collisional granites or arc-type granites.<br />Rb-Sr and Sm-Nd isotopic studies indicate that these magmatic rocks derived from a depleted<br />magma reservoir of mantle source, and suffered an important contamination of upper crust or<br />terrigenous sediments. Taking into account the shallow water sedimentation associated with the<br />magmatism, the Carboniferous magmatic rocks are suggested to form in an active continental<br />margin setting. Systematic zircon U-Pb ICPMS dating on the volcanic and granitic rocks from the<br />whole Yili Block indicates that their ages range from 389~301 Ma, i.e. Late Middle Devonian to<br />Carboniferous time, but the peak magmatism took place during 360~301 Ma.<br />The northern boundary of the Yili Block is represented by the North Tianshan Late<br />Carboniferous turbidite and Bayingou-Motuogou ophiolitic mélange. Stratigraphical and<br />petrological studies suggest that the turbidite and ophiolitic mélange form a subduction complex.<br />The ophiolitic mélange was a result of intra-oceanic tectonism and seafloor redeposition and<br />subsequent deformation during the subduction of the North Tianshan oceanic basin, which existed<br />at least during Late Devonian to Early Carboniferous on the basis of the microfossils found in<br />chert of ophiolite. Structural, kinematic, chronological and geochemical evidences from ophiolitic<br />mélange and turbidite argue that a southward subduction of the North Tianshan oceanic basin is<br />responsible for the formation of the Late Paleozoic Yili magmatic arc. Since this subduction<br />complex was redeformed and thrust northward upon the Junggar Cenozoic basin, the real North<br />Tianshan Suture is probably hidden by the Cenozoic thrust. New geochemical studies from<br />Houxia area indicate that the dolerite, andesite and dacitic rhyolite show similar features with<br />those of Yili arc, and therefore correspond to the North Tianshan arc. Temporal consistency and<br />spatial correlation between the North Tianshan subduction complex and the North Tianshan Arc suggest that the North Tianshan Suture extends probably eastwards until to the north of Bogdashan.<br />More detailed studies are needed to test this conclusion.<br />The southern boundary of the Yili Block is a complicated deformation zone including<br />high-pressure (HP) metamorphic complex, ophiolitic mélange, Proterozoic basement and platform<br />sedimentary rocks, affected by strike-slip faulting. Detail geological survey along the Kekesu<br />River revealed a top-to-the-North ductile shearing that is developed both in ocean-derived HP<br />rocks and in the southern margin of the Yili Block. Evidences for an extensional event, formed in<br />retrograde greenschist facies conditions, are recognized in the HP metapelites. Ar-Ar laser probe<br />dating of white mica in retrogressed blueschist facies metapelites and in greenschist facies<br />quartzite provides 330-315 Ma ages that are interpreted as the date of the exhumation of the HP<br />rocks. Taking into account the previous results, HP/UHP metamorphism should have taken place<br />earlier than 350Ma. Since the final magmatism of the Yili Arc is significantly younger than both<br />peak metamorphism and retrograde metamorphism, and the top-to-the-North kinematics of the HP<br />rocks, our results do not support the previous interpretation of the north directed subduction of the<br />“South Tianshan Ocean” producing the Yili magmatic arc. Near to the HP metamorphic belt,<br />Ophiolitic mélange of Changawuzi-Qiongkushitai areas extends eastward joining with Gangou -<br />Mishigou ophiolitic mélange, mafic rocks yield ages from Ordovician to Silurian, and the<br />kinematics of Gangou ophiolitic mélange is consistent with that of Kekesu HP metamorphic rocks.<br />To the south of Kekesu HP metamorphic complex, Qiongkushitai and Gangou mélange zone,<br />Ordovician-Silurian arc-type volcano-sedimentary rocks were recognized to be unconformably<br />covered by Carboniferous sandstone and carbonate. Proterozoic amphibolite facies metamorphic<br />rocks are distinguished as the basement of this volcanic arc, which is defined as the Central<br />Tianshan block. Therefore, the metamorphic and mélange zone is interpreted as a southward<br />subduction of Tianshan Ocean beneath the Central Tianshan block generating the<br />Ordovician-Silurian volcanic arc.<br />In Aheqi, Wushi and Heiyingshan areas, along the southern slope of the Central Tianshan, a<br />Late Devonian-Early Carboniferous ophiolitic mélange includes ca 390 Ma gabbro blocks with a<br />back-arc-basin geochemical affinity. Moreover, the evolution from platform carbonate<br />sedimentation to banded cherts deposition argues for a progressive deepening of the southern<br />margin of the Central Tianshan Block that is responsible for the opening of a marginal oceanic<br />basin between the Central Tianshan and Tarim during the Early to Middle Devonian. Field<br />observation and kinematics suggest that the mélange has been thrust to the North above the<br />deformed Silurian-Devonian volcaniclastic rocks and marble series. This ductile shearing occurred<br />before the deposition of the Lower Carboniferous sandstone and limestone that unconformably<br />cover the mélange and its tectonic substratum. These observations are quite consistent with those<br />from Kulehu and Kumux-Yushugou areas. Therefore, this ophiolitic mélange zone that stretches<br />from Aheqi to Kumux is defined as the South Tianshan Mélange Belt. This mélange has<br />previously often been considered as south-directed tectonic klippes, but our study indicates that<br />they are actually thrust from South to North, and rooted in the southernmost suture of the Chinese<br />Tianshan Belt.<br />The two boundaries of the Yili Block have been reworked by the Permian strike-slip faulting.<br />North Tianshan Fault redeformed the southern part of North Tianshan Carboniferous turbidite (the<br />northern boundary of the Yili Block). First-hand kinematic results suggest that the North Tianshan<br />Fault is a dextral ductile strike-slip fault, new Ar-Ar whole rock dating constraint this deformation from 285 to 245 Ma that is consistent with the age of Main Tianshan Shear Zone (280~250Ma).<br />The Nalati Fault that extends eastwards joining with the Qingbulak Fault cut the southern<br />boundary of the Yili Block, kinematic observations from Kekesu, Laerdun Pass and<br />Sangshuyuanzi coherently indicate a dextral shearing, previous chronological study also provide<br />~280 Ma age. The Permian right-lateral faulting is associated with alkaline magmatic rocks, such<br />as continental tholeiite, felsic volcanic rocks and alkaline granite. Late Permian redbeds and<br />conglomerate are widespread in whole Tianshan Belt and unconformably cover the pre-Permian<br />rocks. Therefore, Permian tectonics appears to be geodynamically distinct from the convergent<br />orogeny of the Tianshan Belt, and plays an important role on the final formation of its present<br />architecture.<br />In order to better constrain Permian transcurrent tectonics, primary paleomagnetic results on<br />Ordovician, Carboniferous and Permian rocks are obtained from the Yili Block and its adjacent<br />areas. More than 500 sedimentary and volcanic samples were collected from 61 sites. Laboratory<br />measurements on rock magnetism and magnetic remanence indicate that the magnetite and<br />hematite are principal remanent carriers for characteristic remanent magnetization. After careful<br />analysis of reliable characteristic remanences from Zhaosu, Xinyuan and Gongliu areas and<br />detailed discussion on remanence ages, two primary poles of Late Carboniferous and Late<br />Permian are calculated for the Yili Block. Comparison of these paleomagnetic poles of the Yili<br />Block with coeval poles of Junggar, Tarim and Siberia indicates (1) no significant relative motion<br />between the Yili and Junggar blocks since Late Carboniferous, (2) no significant or weak<br />latitudinal relative motion occurred since Late Carboniferous among these blocks, but (3) the<br />46.2°±15.1° and the 31.6°±15.1° counterclockwise rotations of the Yili-Junggar blocks with<br />respect to Tarim and Siberia took place during Late Carboniferous to Permian. These rotations are<br />accommodated by the Permian dextral strike-slip faults along the northern and southern sides of<br />the Tianshan Belt and sinistral strike-slip faulting along the Erqishi Fault of the Altay Belt,<br />resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts,<br />respectively.<br />Finally, on the basis of the removal of Permian lateral displacement and Cenozoic<br />reactivation during the Indo-Asian collision, a simplified evolutionary model involving the Yili<br />Block and its adjacent areas is proposed. During the Ordovician-Early Silurian, an oceanic basin<br />called the Tianshan Ocean existed between the Yili Block and the Central Tianshan Block. This<br />ocean began to close by southward subduction beneath the Central Tianshan during Late<br />Ordovician and Silurian generating the Central Tianshan volcanic arc. From Middle Silurian to<br />Middle Devonian time, the Tianshan Ocean was progressively closing. The oceanic subduction<br />was followed by the continental subduction of the southern part of the Yili Block and resulted in<br />the development of the HP metamorphic complex. Coevally with the closure of the Tianshan<br />Ocean, a marginal sea opened to the South of the Central Tianshan Block, as suggested by the<br />ophiolitic mélange and the progressive deepening of the southern margin of the Central Tianshan<br />Block during the Early to Middle Devonian. During the Early Paleozoic, the limestone and<br />sandstone deposits suggest that the northern margin of the Yili Block was a passive margin. From<br />the Late Devonian to Early Carboniferous, due to the closure of the Tianshan Ocean, the Yili and<br />Central Tianshan Blocks were welded together to form a single continental mass. At that time, the<br />HP metamorphic rocks were exhumed. The closure of the back arc basin was accommodated by a<br />south-directed subduction below the Tarim Block as suggested by the northward thrusting of the South Tianshan ophiolitic mélange. During Late Mid-Devonian to Early Carboniferous, southward<br />subduction of an oceanic basin, called the North Tianshan Ocean was responsible for the Yili<br />magmatic arc and the North Tianshan accretionary prism. The oceanic subduction ended in Late<br />Carboniferous when the continental collision occurred between Junggar Block and the Yili Block.<br />At the end of the Carboniferous, the N-S convergence finished when all the continental blocks<br />were amalgamated. Permian lateral large-scale displacement dragged the main blocks of Central<br />Asia close to their present position, and probably also disturbed original continuity (e.g. Yili and<br />Bogda magmatic arcs).