The Study Of Paleozoic Tectono-magmatism And Geodynamic Evolution In Eastern Tianshan, Northwest China

Located in the central of Asia, Tianshan Belt is an important component of the Central Asian orogenic belt (CAOB) in China. It is a crucial key for understanding the complex crustal evolution and collisional tectonics of the CAOB. The Tianshan Belt, which resulted from polyphase subduction-collision orogenies, is classically divided into three subunits:North Tianshan, Central TianshanSouth Tianshan. The North Tianshan experienced a complex tectonic evolution, including Paleozoic accretion and collision, Mesozoic thermal subsidence and Cenozoic thrusting and uplift. The post-collisional strike-slip and transtension movements reworked the original architecture of the orogen, making it difficult to identify the original geological features. This has triggered various controversies on the process and characters of the Paleozoic tectonic setting and tectonic evolution of the North Tianshan. This dissertation focuses on the Ophiolitic melange and ductile deformation of Northern boundary fault of Central Tianshan and two pahses of Late Paleozoic crust extension. Our study provides new insights into the Paleozoic tectonic evolution of East Tianshan.In the Bogda-Harlik area, we have identified two episodes of Late Paleozoic bimodal suites formed during Early Carboniferous and Early Permian in Hongshankou area and south Bogda area. Zircons from Hongshankou basalts yield crystallization ages of347.1±3.9Ma, whereas those from the rhyolites are dated as344.4±1.9and345.0±2.5Ma. The major and trace element signatures of basalts suggest high alumina basalt (HAB) as the source magma which was generated from metasomatised asthenosphere mantle. Broadly homogenous Rb-r and Sm-Nd isotopic compositions of mafic and felsic rocks are also characteristic. The basalts display a distinct enrichment in incompatible elements and positive εNd(345Ma)(from+5.87to+8.25). The rhyolites also display high εNd(345Ma)(from+6.35to+8.53). Consequently, we suggest a mixing process between a source similar to mid-ocean ridge basalt (depleted end-member mantle) and arc-like magmas for the petrogenesis of the mafic rocks, whereas the rhyolites are best interpreted to have formed by fractional crystallization from the parental magma of the basalts. We propose that the essentially bimodal character of this complex reflects the features of back-arc extensional magmatism. The post-collisional event generated an E-W trending bimodal volcanic zone that extends for ca.500km in the southern Bogda Mt. to the easternmost Tianshan Belt. The rhyolites show a close affinity to aluminous A-type granites, indicates a bimodal high-K calc-alkaline affinity. The basalts are likely derived from a metasomatized lithospheric mantle. The LA-ICP-MS analysis on zircons from four rhyolites and two basalts yielded similar ages ranging from295.8±2.8to293.3±1.7Ma, suggesting an Early Permian event. Zircons from the basalts show a wide sHf(t) range, suggesting a mixing of crust-mantle material. The magmatic zircons from the bimodal volcanic rocks yielded two peaks of TDM2between600-820Ma and1160-1360Ma, probably representing two significant episodes of continental crust growth. The bimodal volcanic rocks are correlated with the early stage of post-collision.The post-collisional tectonic-magmatic activity in Xinjiang region is very intensive, and the isotopic age of the activity ranges mainly from270to290Ma. Except the bimodal volcanic, there are also some typical I-type high-K calc-alkaline granite. Zircons from Xiaopu granite these plutons are dated by U-Pb method and yield weighted mean age of297±2and295±2Ma respectively, which is considered representing early period of post-collisional magmatism. The triplet of basic dyke swarms and alkaline granites as well as the underwater olistostrome in the Baiyanggou could be regarded as a prominent indication of the initial stage of post-collision, and they occurred synchronously and coexisted in Harlik region. All above mentioned indicates could cease by the end of carboniferous.Northern boundary fault of Central Tianshan experienced at least two stages of ductile shear tectonic evolution:Early Paleozoic north vergent thrusting ductile shear and Early Permian strike-slip deformation. Nearly vertical mylonitic foliation and sub-horizonal stretching lineation define its strike-slip feature; various kinematic indicators, such as asymmetric folds, non-coxial asymmetric macro-to micro-structures and C-axis fabrics of quartz grains of mylonites, suggest that it is a dextral strike-slip shear zone oriented in a nearly E-W direction. Geodynamical analysis suggests that Early Paleozoic north vergent thrusting ductile shear large-scale and dextral strike-slip ductile shearing are likely the result of intracontinental adjustment deformation after the collision of North Tiashan Arc and Middle Tianshan block and Late Carboniferous oblique collision caused by the final closure of Tianshan ocean.The ophiolitic melange and mafic granulites are outcropped in the Weiya segment which is near to the northern boundary fault of the Central Tianshan belt. The petrological and geochemical data suggest that the geochemical features of the diabase and gabbro of the ophiolitic melange are similar to ones of the typical EMORB-type basalt. All the mafic granulites on the TAS diagram are located in the subalkline gabbro-diabase area, similar to island-arc or active continental margin basalt, the mafic granulite displays geochemical feature of immature island-arc tholeiite. Combining with previous results, we conclude that the mafic-ultramafic blocks in the eastern Weiya belongs to the dismembered EMORB-type ophiolite suit, which formed in mid-oceanic ridge of and represents fragments of ancient Tianshan oceanic basin. The protolith of mafic granulites were formed in an immature island-arc setting. The ophiolitic melange and mafic granulites may be related to the subduction of oceanic plate along the northern boundary fault of the Central Tianshan and represents the relics of the Tianshan Ocean.On the basis of the characteristics of the ductile deformation zone and Weiya ophiolitic melange and sedimentary sequence and magmatism in the Bogda, we conclude that, Tianshan Paleoocean southward subduction along the Northern boundary fault of Central Tianshan and Kelamaili suture zone during Early Paleozoic and Later Paleozoic, respectively. The evolution progress of the Eastern Tianshan can be divided into5stages during Paleozoic.1, active continental margin along the Central Tianshan during the early paleozoic;2, collision peroid during the early paleozoic;3, Late Devonian-Early carboniferous back-arc extension;4, later carboniferous arc-continental collision;5, Early Permian post-collision.