Lithium isotopic evidence for subduction of the Indian lower crust beneath southern Tibet |
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| Institution: | 1. State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China;2. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;3. Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA;4. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;5. School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 10083, China;6. Roosevelt High School, Seattle, WA 98115, USA;7. Jiangxi Provincial Institute of Geological Survey, Nanchang 330030, China;1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China;2. School of Earth Sciences, Gansu Key Laboratory of Mineral Resources in Western China, Lanzhou University, Lanzhou 730000, China;1. Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;1. Chengdu Institute of Geology and Mineral Resources, Chengdu 610081, China;2. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;3. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China;4. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 10049, China;5. School of Geosciences, The University of Sydney, NSW 2006, Australia;6. Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029, China |
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| Abstract: | Post-collisional K-rich volcanic rocks (KVRs) can provide an opportunity to constrain the architecture of the lithosphere and the mechanisms of plateau uplift. However, their petrogenesis and geodynamic setting remain in dispute. Lithium concentrations and isotopic compositions of 87 potassic, ultrapotassic and Mg-rich potassic volcanic rocks (PVRs, UPVs, and MPRs, respectively) in SW Tibet, along with new Pb–Sr–Nd isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. These rocks are characterized by very similar δ7Li values: PVRs vary from ?4.9‰ to +3.2‰, UPVs from ?3.9‰ to +1.7‰, and MPRs from ?1.2‰ to +3.5‰. They can be classified into two groups: Group I (19 out of 87 samples) with heavier δ7Li values (+1.0‰ to +3.5‰) similar to those reported for mid-ocean-ridge and ocean-island basalts (MORBs and OIBs, respectively), and Group II (68 out of 87 samples) with lighter values (?4.9‰ to +1.0‰) similar to those of Indian lower crust. These variable isotopic compositions may record the isotopic signature of the early-middle Miocene subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of isotopically light mantle domains beneath the Lhasa terrane, which were ascribed to the interaction with fluids/melts derived from the subducted Indian lower crust. The modeling curves of Indian lower crust with a metasomatized mantle composition fully account for compositional variations in the PVRs, UPVs, and MPRs. They were generated by the partial melting of SCLM, which was metasomatized by fluids/melts derived from the subducted Indian lower crust (ca. 4–14%, ca. 4–10%, and ca. 6–10% for the PVRs, UPVs, and MPRs, respectively). The Li isotopic data indicate that the Indian lower crust was subducted beneath the central Lhasa subterrane, and this sheds new light on the formation of the Tibet Plateau. |
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