Timing and mechanism of Bangong-Nujiang ophiolite emplacement in the Gerze area of central Tibet |
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| Institution: | 1. Guangdong Provincial Key Laboratory of Mineral Resources & Geological Processes, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou 510275, China;2. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, and Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China;3. Département de Géologie et Génie Géologique, Université Laval, Québec G1V 0A6, Canada;1. The College of Earth Sciences, Jilin University, Changchun 130061, PR China;2. No. 5 Geological Party, Tibet Bureau of Geology and Exploration, Golmud 816000, PR China;3. School of Earth Science and Resources, China University of Geosciences, Beijing 10083, PR China;1. Chengdu Institute of Geology and Mineral Resources, Chengdu 610081, China;2. Research Center for Tibetan Plateau Geology, China Geological Survey, Chengdu 610081, China;3. Department of Geosciences, National Taiwan University, Taipei 10617, China;4. Institute of Earth Sciences, Academia Sinica, Taipei 11529, China;5. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;6. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China;7. Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China;1. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China;2. Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China;3. Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, James Lee Science Building, Hong Kong |
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| Abstract: | The Bangong-Nujiang suture zone (BNSZ) separates the Lhasa terrane from the Qiangtang terrane and contains remnants of the Bangong-Nujiang oceanic lithosphere (ophiolites). Despite decades of research, when and how the Bangong-Nujiang ophiolites were emplaced remains enigmatic. In the Gerze area (western segment of the BNSZ), the geochemistry and provenance discrimination of chromian spinels (Cr-spinels) from the pre-collisional subduction complex (Mugagangri Group) and syn-collisional peripheral foreland basin succession (Wuga Formation) can help us solve this fundamental problem in the BNSZ evolution. This study compares the geochemistry of Cr-spinels from the Mugagangri Group and Wuga Formation with those from the Bangong-Nujiang ophiolites. Cr-spinels in the Bangong-Nujiang ophiolites have either low TiO2 (0.01–0.15%) and low Al2O3 (11.74–26.76%), indicating an SSZ peridotite origin, or high Al2O3 (45.28–49.15%), indicating a MORB peridotite origin. Cr-spinels from the ultramafic fragments within the Mugagangri Group have extremely low TiO2 (<0.06%) and geochemically overlap with those from the Dong Co ophiolite, suggesting that these ultramafic fragments were sourced from the Dong Co ophiolite above the subduction zone rather than off-scrapped remnants from the subducting oceanic lithosphere. Compositional fingerprints of detrital Cr-spinels from the Wuga Formation indicate provenance either derived from the Bangong-Nujiang ophiolites or recycled from the Mugagangri Group in the north, with minor input possibly from the Lhasa terrane in the south, consistent with the depositional pattern of a peripheral foreland basin. Provenance data reveals that the Bangong-Nujiang ophiolites in the Gerze area had been emplaced and exposed to erosion during northward oceanic subduction prior to the Lhasa-Qiangtang collision. Contrasting the Tethyan-type Yarlung-Zangbo ophiolites in southern Tibet, the Bangong-Nujiang ophiolites in central Tibet are Cordilleran-type in terms of emplacement mechanism, which were uplifted above sea-level by progressive growth of the subduction complex structurally beneath ophiolite. The emplacement of the Cordilleran-type ophiolites in the western segment of the BNSZ is divided into two stages: (1) intra-oceanic subduction initiation at ~177–179 Ma based mainly on zircon U-Pb dating of plagiogranite from the SSZ-type Laguo Co ophiolite; (2) accretionary emplacement of the ophiolites at ~151–168 Ma constrained by the depositional age of the Mugagangri subduction complex. Final closure of the Bangong-Nujiang Tethyan Ocean may convert the ophiolite emplacement mechanism from “accretionary” to “collisional” at ~150–152 Ma, evidenced by the first development of a peripheral foreland basin. |
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