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Quanren Yan, Institute of Geology, Chinese Academy of Geological Sciences (China)
Zhen Yan, Institute of Geology, Chinese Academy of Geological Sciences (China)
Zhongqi Wang, Institute of Mineral and Resources, Chinese Academy of Geological Sciences (China)
Tao Wang, Institute of Geology, Chinese Academy of Geological Sciences (China)
Zhongjing Xiang, Institute of Geology, Chinese Academy of Geological Sciences (China)
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The eastern margin of the Tibetan plateau (EMTP, from the Ganzi-Litang méange to the Longmengshan thrust-fold belt) has been traditionally considering as an adjustment for absorbing the deformation caused by India-Asia collision. However, our systematic U-Pb analyses for zircons from magmatic rocks used the SHRIMP-II at the Beijing SHRIMP Center showed that the EMTP has a complex geologic history. The oldest Precambrian basement in the EMTP formed in the Paleoproterozoic (2401±39-1912±55 Ma), was affected first by Mesoproterozoic tectonothermal events (1361±42-1040±19 Ma) in agreement with the Grenville, and then by the strongly arc-related magmatic events in the Neoporterozoic (791±10-817±13 Ma). In Particular, the basement of the Songpan-Ganzi block (SGB) is of affinity with oceanic crust derived from the Neoproterozoic rifting (681±20-655±10 Ma), and by which the Precambrian blocks in the EMTP were split off from the basement of the Yangtze block and/or the Qiangtang block within the Tibetan plateau most likely.
The ages of 551±14-534±21 Ma yielded lines of evidence for the Pan-African event in the EMTP. But no lines of evidence for the tectonic events in the Caledonian has been discovered in the EMTP up to now. Paleotethys in the EMTP opened in the early Permian (292±4-277±6 Ma) followed with breakup of the east Gondwanaland, and then closed in the early Mesozoic by arc-continent collision (224±3-213±3 Ma). Significant magmatism occurred in the middle Jurassic (175±4 Ma) in the EMTP, but its dynamics remains unclear. The timing of magmatism at the beginning of the late Cretaceous (97±2 Ma) might represent the initial subduction of the Indian plate downward to the Eurasian continent. Cenozoic magmatism in the EMTP (50±1 Ma), as popular consideration, marks the collision timing of India ad Asian, while magmatism with the age of 18±0.3 Ma was related to syntexis caused by large-scale strike-slip faulting. It is problematic, however, whether the Neoproterozoic rift/ocean closed completely in the Pan-African time and then became the basement upon which the Ganzi-Litang paleotethys developed, or had been keeping to be an ocean basin until the late Paleozoic and then rejuvenated by the Ganzi-Litang Paleotethys.
The financial support from the National Natural Science Foundation of China (grants 40472119 and 40172071), the Programme of Excellent Young Scientist of the Ministry of Land and Resources, P. R. China (grant 2004239), the Ministry of Science and Technology, P. R. China (grants 2002CB412608 and 2006BAB01A11).
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