Composition And Tectonic Evolution Of West Kunlun Orogenic Belt And Its Periphery In The Early Paleozoic-Early Mesozoic

West Kunlun Orogen, situated in the northwest margin of Qinghai-Tibet Plateau and in the westernmost of China‘s Central Orogenic System, is the transitional position of Paleo-Asian tectonic domain and Tethys tectonic domain. It is a compound orogenic belt having a long complicated geological history, overprinted by mutiple orogenic phases and different types of orogenies. Since Phanerozoic, West Kunlun orogenic belt had undergone the Proto-Tethys and Paleo-Tethys tectonic cycles, with the magmation, sedimentation and metamorphism at different stages of the formation and evolution, which record abundant evolution information about the two tectonic cycle. Thus, West Kunlun Mountain and its periphery is a unique area in which we can detailedly study the compound orogeny and formation evolution of West Kunlun, even the whole Qinghai-Tibet Plataeu. Combing the magmatism and sedimentary, this thesis mainly focus on the composition related to each evolution stage of West Kunlun Orogen in the Early Paleozoic- Early Mesozoic, with comprehensive research of geotectonics, structural geology, sedimentary, petrology, geochemistry, geochronology and high spatial remote sensing, on basis of which we analyze the orogeny and evolution of West Kunlun Orogen. Until now, some new points have been achieved as follows:1. The basin prototypes represented by the Ordovician Dongguashan Group(O2-3D) and the Lower Silurian Wenquangou Group(S1W) in Tianshuihai area belong to passive continental margin basin and remnant ocean basin at convergent plate margin, respectively. It suggest that the Proto-Tethys had developed along the Mazha- Kangxiwa-Subashi tectonic zone in early Paleozoic, and the ocean basin began to subduct in or before the Middle-Late Ordovician, and the polarity of subduction is northward. The micro-terranes collided and collaged together in the Early Silurian.2. Kudi pluton mainly is biotite monzonitic granite. Major element and rare element characteristics show that it is alkaline and metaluminous-weakly peralumious rocks, and have the A(A1)-type granite characteristics, and formed within plate tectonic setting. Kudi pluton was the product formed by low degree partial melting of mantle-derived basic rocks under the condition of high temperature, low pressure and non-H2 O.Zircon LA-MC-ICPMS U-Pb age of 404.3±2.8Ma indicates the pluton was emplaced during the Early Devonian, suggesting an end of the Proto-Tethys cycle.3. The newly discovered Qushenman ophiolite, emplaced along the Waqia fault, mainly consists of meta-basalt, meta-gabbro and meta-plagiogranite. The meta-basic rocks show N-type MORB features, and the meta-plagiogranites are similar to ocean ridge granite. Taking the tectonic setting and regional tectonic background into account, the geochemistry characteristics indicate that the Qushenma ophiolite is MOR-type ophiolite related to oceanic crust extension setting.Zircon LA-MC-ICPMS U-Pb age of 329~324Ma, indicates it was emplaced during the late Early Carboniferous. Qushenman ophiolite is the west-extending of Mazha-Kangxiwa- Subashi tectonic zone, represents the early oceanic crust of Paleo-Tethys, which opened up along the convergence zone of Proto-Tethys in the Early Carboniferous.4. The basin prototypes represented by the Permian Huangyangling Group(PH) and Late Triassic Bayan Har Moutain Group(TB) belong to fore-arc basin and remnant ocean basin at convergent plate margin, respectively. It suggest that the Mazha-Kangxiwa Paleo-Tethys ocean had begun to subduct in the Permian, and the micro-terranes collided in the Late Triassic.5. Jajiwaxi I-type pluton and Serikedaban volcanic rocks(mostly andesite with dacite in it), mafic microgranular enclaves are well developed in both of them. The two different type rocks belong to high-K calc-alkaline series in the same. Zircon LA-MC-ICPMS U-Pb ages of the pluton and encalve in it, and the volcanic rocks are 227.4±1.1Ma, 229.3±1.5Ma, and 223.2±2.2Ma, respectively, belonging to Late Triassic. Petrological and geochemical characteristics show both the pluton and volcanic rocks have the crust-mantle mixing and mingling features, and both formed in continental margin arc, suggest the subduction of Mazha-Kangxiwa Paleo-Tethys ocean had lasted to the early-middle Late Triassic, and the polarity of subduction is northward.6. Mazha pluton, intruded into the Lower Silurian Wenquangou Group(S1W), is strongly peraluminous S-type granite, and belongs to high-K calc-alkaline—shoshonite series. Zircon LA-MC-ICPMS U-Pb age is 221.8±2.3Ma, belonging to the middle Late Triassic. Mazha pluton was the product formed by dehydration melting of low-mature middle-upper crust. Mazha pluton has the features of typical continent-continent syn-collisional granites, indicating that the Mazha-Kangxiwa Paleo-Tethys ocean had closed up with the collision of micro-terranes in the middle Late Triassic.7. Muztage I-type pluton, intruded into the pre-Devonian strata, consisted of granodiorite, monzogranite, porphyritic biotite monzogranite and a little quantity of tonalite. There are contemporary vein-like mafic instrusive and gabbro dioritic-dioritic enclaves developed in the Muztage pluton. North Quanshuigou S-type pluton, with many enclaves in it, was intruded into the Permian Huangyangling Group. Geochemical features suggest that both of them are high-K calc-alkaline rocks. Zircon LA-MC-ICPMS U-Pb ages of Muztage monzogranite, North Quanshuigou granodiorite and enclaves developed in it are 208.6±1.6Ma, 210±1Ma, 217±2Ma, respectively, belonging to the Late Triassic. Petrological and geochemical characteristics show strong magma mixing and mingling during the magmatism. Different tectonic setting discrimination methods suggest that the two plutons were formed in the post-collisional stage. Early-Middle Jurassic continental facies coal bearing sediments are widely layered in West Kunlun and its peripheral, indicating an intra-continental stage, making West Kunlun Orogen had ended its ocean-existing history.8. West Kunlun Orogen had experienced the Proto-Tethys tectonic cycle in the Neoproterozoic-Early Devonian, and the Paleo-Tethys tectonic cycle in the Early DevonianLate Triassic. Both the Proto-and Paleo-Tethys systems independently formed under environment of multiple micro-terranes, ocean basins subductions and collisions. Totally, West Kunlun Orogen is the compound of Caledon and Indosinian orogenies. In each system, there exists subduction-accretion orogeny, collision orogeny and post-collision orogeny between ribben terranes, and followed by intra-continental orogeny. In short, West Kunlun Orogen is a continental compound orogenic belt having a long geological history, overprinted by different orogenic phases and different types of orogenies.