HR: 14:55h
AN: T23G-05    [Abstracts]
TI: A Magmatic Perspective and Zircon Hf Isotope Constraints on Tibetan Orogenesis
AU: * Chung, S
EM: sunlin@ntu.edu.tw
AF: Dept Geosciences, National Taiwan University, Taipei, 106, Taiwan
AU: Chu, M
EM: meifei@ntu.edu.tw
AF: Dept Geosciences, National Taiwan University, Taipei, 106, Taiwan
AU: Chu, M
EM: meifei@ntu.edu.tw
AF: Institute of Earth Sciences, Academia SInica, Taipei, 116, Taiwan
AU: Chu, M
EM: meifei@ntu.edu.tw
AF: GEMOC, Dept Earth& Planet. Sci., Macquarie University, Sydney, NSW2109, Australia
AU: O'Reilly, S
EM: soreilly@els.mq.edu.au
AF: GEMOC, Dept Earth& Planet. Sci., Macquarie University, Sydney, NSW2109, Australia
AU: Pearson, N
EM: npearson@els.mq.edu.au
AF: GEMOC, Dept Earth& Planet. Sci., Macquarie University, Sydney, NSW2109, Australia
AU: Liu, D
EM: liudunyi@public.bta.net.cn
AF: Institue of Geology, Chinese Academy of Geological Science, Beijing, 100037, China
AU: Song, B
EM: songbiao@cags.net.cn
AF: Institue of Geology, Chinese Academy of Geological Science, Beijing, 100037, China
AU: Ji, J
EM: grsange@pku.edu.cn
AF: School of Earth & Space Sci., Peking University, Beijing, 100871, China
AU: Lo, C
EM: loch@ntu.edu.tw
AF: Dept Geosciences, National Taiwan University, Taipei, 106, Taiwan
AU: Lee, T
EM: t44001@ntnu.edu.tw
AF: Dept Earth Sci., National Taiwan Normal University, Taipei, 106, Taiwan
AB: The continental collision between India and Asia represents the latest albeit ongoing phase of sequential geologic processes that, evolving repeatedly from the southern to northern hemispheres, involved Gondwana dispersion, Tethyan subduction and terrane amalgamation. The immense plateau, in particular its southern part, is now underlain with the thickest continental crust on Earth. When and how was such thick crust formed, which would have exerted pivotal controls to the plateau?|s formation, however, remains an issue with little consensus owing to the scarcity of reliable constraints. Here we review the pre- to postcollisional magmatic evolution in southern Tibet, and report in-situ Hf isotopes of zircons from the Gangdese batholith and associated igneous rocks, which enable us to unravel the sequential geologic processes related to Neo-Tethyan subduction and Tibetan orogeny. These zircons, crystallizing from ca. 190 to 15 Ma, possess juvenile mantle type Hf isotopes comparable to those of certain granitoids in eastern Australia derived from east Gondwana and suggesting basaltic magma underplating as a key crustal formation process in the subduction zone. Our data further indicate that in southern Tibet the crust underwent significant tectonic thickening during ca. 45 and 30 Ma, thus shedding new insights about when, and how, the Earth?|s thickest continental crust was formed. The lower part of the thickened crust consisted prevailingly of mafic lithologies, which we infer to have resulted from intense basaltic underplating and remelting that occurred during Cretaceous and mid-Eocene time owing to the Neo-Tethyan subduction, a process responsible for not only the juvenile crust production but also creation of a thermally softened lithosphere. Subsequent collision of India with Asia, therefore, caused distributed lithospheric thickening and formation of an orogenic root underneath southern Tibet. Root foundering during the Oligocene gave rise to the postcollisional magmatism, regional uplift, and onset of northward underthrusting of the Indian plate that has since played a role in forming the entire Tibetan plateau.
DE: 8100 TECTONOPHYSICS
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8178 Tectonics and magmatism
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting