The carbonated source region of Cenozoic mafic and ultra-mafic lavas from western Qinling: Implications for eastern mantle extrusion in the northeastern margin of the Tibetan Plateau |
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| Institution: | 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;2. Geologie, Technische Universität Bergakademie Freiberg, 09599 Freiberg, Germany;3. Institute of Geology, University of Azad Jammu and Kashmir Muzaffarabad, AJK 13100, Pakistan;1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;3. U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025, USA;4. Physics of the Earth, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain;5. University of Chinese Academy of Sciences, Beijing 100049, China;1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;3. Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China;4. Department of Geophysics, Faculty of Science, Cairo University, Giza, Egypt;5. Physics of the Earth, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain;1. Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Rd, Beijing 100037, China;2. Laboratoire de géologie de Lyon, CNRS UMR 5570, Université de Lyon 1, Villeurbanne, France;3. ISTerre, Université Grenoble Alpes, Grenoble, France;4. CNRS, ISTerre, Grenoble, France;5. ETH — Zurich, Geological Institute, Earth Surface Dynamics, Sonneggstrasse 5, 8092 Zurich, Switzerland;1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China;2. State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences and Resources, Research Center for Tibetan Plateau Geology, China University of Geosciences (Beijing), Beijing 100083, China;3. State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China;4. Center for Study of Imaging and Dynamics of the Earth, Institute of Geophysics and Planetary Physics, University of California, Santa Cruz, CA 95064, USA;5. Graduate University of Chinese Academy of Sciences, Beijing 100049, China;6. Liaoning Non–Ferrous Geological Exploration Institute, Shenyang 110013, China;7. University of Houston, 4800 Calhoun Rd, Houston, TX 77004, USA;1. Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;3. ARC Center of Excellent for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2019, Australia |
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| Abstract: | Cenozoic Indo-Asian collision caused significant crustal shortening in central Tibet. The strike-slip faults around the Tibetan Plateau (TP) are generally attributed to extrusion tectonics, resulting from lower crust flow. Therefore, the mantle extrusion site corresponding to the Cenozoic elevation of the TP needs to be identified. This paper reports the petrology and geochemistry of Cenozoic mafic and ultra-mafic volcanic rocks in the Xiahe and Lixian areas, at the northeastern margin of the TP. Detailed analysis indicates a regular change in partial melting conditions and source regions of the volcanic rocks from west to east, revealing a Cenozoic eastward mantle extrusion in the eastern margin of the TP. The Xiahe volcanic rocks display ocean island basalt affinity with negative K anomalies and positive Nb and Ta anomalies. They are alkaline with extremely high Na2O/K2O ratios and relatively enriched Sr–Nd–Pb isotopic compositions, indicating that these basalts were derived from partial melting of carbonated pyroxenite. The Lixian picro-basalts are closely associated with igneous carbonatites. They have relatively high TiO2 (3.47–4.66%) and MgO (11.24–18.88%) contents and low SiO2 (41.14–44.82%) and Al2O3 (5.84–9.18%) contents. Based on the depleted Sr–Nd–Pb isotopic compositions, we propose that the Lixian picro-basalts may have originated from the partial melting of carbonated lithosphere mantle peridotites at relatively high pressure (> 3 GPa). Minor hornblendite in their source region can account for the high TiO2 and Na2O + K2O contents. Thus, we argue that these volcanic rocks were formed by episodic decompression melting of the carbonated mantle lithosphere during the eastward extrusion of the Tibetan lithosphere, in contrast to the conventional view that they were formed in a continental rift setting. Their partial melting and eruption processes may be closely related to the Cenozoic strike-slip fault activities in the northeastern margin of the TP. |
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