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Discovery of Early Triassic conodonts in western gangdisê and the establishment of the Tangnale Formation

Science China Earth Sciences 2007年第12期50卷 1767-1772页

作者： ZHENG YouYe1, XU RongKe1,2, WANG ChengYuan3, MA GuoTao1, LAI XuLong1, YE DeJin2, CAO Liang1 & LIANG JiWei4 1 Faculty of Earth Resourse, China University of Geoscience, Wuhan 430074, China 2 Research Institute of Geological Survey of Gansu Province, Lanzhou 730050, China 3 Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China 4 Department of Geology, Northwest University, Xi’an 710069, China Faculty of Earth Resourse China University of Geoscience Wuhan China Research Institute of Geological Survey of Gansu Province Lanzhou China Nanjing Institute of Geology and Palaeontology Chinese Academy of Sciences Nanjing China Department of Geology Northwest University Xi’an China

Most geologists believe that there are no Early and Middle Triassic strata in the W. Gandisê stratigraphic subregion, but the present authors have found Early Triassic conodonts for the first time in the Shiquanhe area, including five conodonts genera (Form genera): Pachycladina, Neohindeodella, Cornudina, Hadrodontina and Hibbardella sp. etc. Then we affirm that Early Triassic deposits exist in the Gandisê stratigraphic subregion, and establish the Tangnale Formation. The conclusion is new important complementary basal data for Triassic stratigraphy division of gangdisê, reconstructing palaogeography and studying gangdisê from Paleozoic to Mesozoic island-arc evolution and transi-tion.

关键词： Conodonts, Early Triassic, western gangdisê

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Timing of Magma Mixing in the gangdisê Magmatic Belt during the India-Asia Collision: Zircon SHRIMP U-Pb Dating

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Acta Geologica Sinica(English Edition) 2005年第1期79卷 66-76页

作者： MOXuanxue DONGGuochen ZHAOZhidan GUOTieying WANGLiangliang CHENTao StateKeyLaboratoryofGeo-ProcessesandMineralResources ChinaUniversityofGeosciencesBeijing100083 StateKeyLaboratoryofGeo-ProcessesandMineralResources ChinaUniversityofGeosciencesBeijing100083／／SchoolofEarthScienceandResourcesChinaUniversityofGeosciencesBeijing100083

Abundant mafic microgranular enclaves (MMEs) extensively distribute in granitoids in the gangdise giant magmatic belt, within which the Qüxü batholith is the most typical MME-bearing pluton. Systematic sampling for granodioritic host rock, mafic microgranular enclaves and gabbro nearby at two locations in the Qüxü batholith, and subsequent zircon SHRIMP II U-Pb dating have been conducted. Two sets of isotopic ages for granodioritic host rock, mafic microgranular enclaves and gabbro are 50.4±1.3 Ma, 51.2±1.1 Ma, 47.0±l Ma and 49.3±1.7 Ma, 48.9±1.1 Ma, 49.9±1.7 Ma, respectively. It thus rules out the possibilities of mafic microgranular enclaves being refractory residues after partial melting of magma source region, or being xenoliths of country rocks or later ***, it is believed that the three types of rocks mentioned above likely formed in the same magmatic event, i.e., they formed by magma mixing in the Eocene (c. 50 Ma). Compositionally, granitoid host rocks incline towards acidic end member involved in magma mixing, gabbros are akin to basic end member and mafic microgranular enclaves are the incompletely mixed basic magma clots trapped in acidic magma. The isotopic dating also suggested that huge-scale magma mixing in the gangdise belt took place 15-20 million years after the initiation of the India-Asia continental collision, genetically related to the underplating of subduction-collision-induced basic magma at the base of the continental crust. Underplating and magma mixing were likely the main process of mass-energy exchange between the mantle and the crust during the continental collision, and greatly contributed to the accretion of the continental crust, the evolution of the lithosphere and related mineralization beneath the portion of the Tibetan Plateau to the north of the collision zone.

关键词： SHRIMP isotopic dating granites magma mixing mafic microgranular enclaves Qüxü pluton gangdisê Tibet

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