Early Paleozoic felsic magmatic evolution of the western Central Qilian belt,Northwestern China,and constraints on convergent margin processes |
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| Institution: | 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;2. MLR Key Laboratory of Genesis and Exploration of Magmatic Ore Deposits, Xi''an Center of Geological Survey, China Geological Survey, Xi''an 710054, China;3. Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia;1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an, 710069, China;2. Department of Geology and Geophysics, School of Physical Sciences, The University of Adelaide, South Australia 5005, Australia;1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2. School of Earth Sciences and Resources, Chang''an University, Xi''an 710054, China;3. Tianjin Institute of Geology and Mineral Resources, Tianjing 300170, China;4. School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China;1. MOE key Laboratory of Orogenic Belt and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China;2. Institute of Earth Science and State Key Laboratory of Geological Processes and Mineral Resources, Chinese University of Geosciences, Beijing 100083, China;3. Department of Earth Sciences, Durham University, Durham DH1 3LE, UK;4. Institute of Oceanology, Chinese Academy of Science, Qingdao 266071, China;1. Key Laboratory of Deep-Earth Dynamics of MNR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;3. Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA |
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| Abstract: | The western part of the central belt of the Qilian orogenic belt, northeastern Tibetan Plateau, includes a compositionally diverse range of Cambrian to late Silurian felsic intrusions that reflect the changing tectonic process that molded this Paleozoic convergent margin. The Hf-isotopic compositional range of zircon from these rocks shows a significant role for Proterozoic crust – likely as microcontinents – rather than simply a history through oceanic arc accretion. Felsic magmatism includes shoshonitic magmas, and rarer shoshonite-OIB associations, dated from c. 465 to 445 Ma, which together form at least 30% of the presently exposed Paleozoic granitic crust of this region. Accepting a typically shoshonite petrogenesis for these magmas, involving asthenospheric upwelling and consequent remobilization of subduction-modified lithosphere, would infer a post-collisional setting at this stage. This could, perhaps, reflect slab-detachment, convective thinning of the lithosphere or orogenic collapse, resulting from collision and subduction between the Quanji block and the Central Qilian block. However, this requires the onset of a post-collisional setting at least 25 Ma before previously thought, and at the same time as intra-oceanic subduction is thought to have been active in other parts of the Qilian belt. These findings either require a reappraisal of the evidence for c. 490–440 Ma intra-oceanic subduction, or the formation of voluminous high-K and shoshonite magmatism in a pre-collisional setting, perhaps related to a period of unusually strong syn-arc rifting. |
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