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

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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^(40)Ar-^(39)Ar geochronology of Cenozoic Linzizong volcanic rocks from Linzhou Basin, Tibet, China, and their geological implications

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Chinese Science Bulletin 2004年第18期49卷 1970-1979页

作者： ZHOUSu MOXuanxue dongguochen ZHAOZhidan QIURuizhao GUOTieying WANGLiangliang KeyLaboratoryofLithosphericTectonicsDeep-levelProcessandExplorationMinistryofEducation ChinaUniversityofGeosciencesBeijing100083China ChinaUniversityofGeosciences Beijing100083China InstituteofGeology ChineseAcademyofGeologicalSciencesBeijing100037China

Whole-rock and mineral separate Ar-Ar dating was carried out for the Linzizong volcanic rocks at Linzhou Basin in Tibet to constrain the time span of volcanism and the corresponding stratigraphic sequence. Sampling was based on detailed geologic mapping and stratigraphic se-quence of Dianzhong, Nianbo, Pana Formations, systemati-cally from the bottom to near the top. The results indicate that the Linzizong volcanic rocks erupted from Paleocene to middle of Eocene (64.43—43.93 Ma). Among them, the Pana Formation formed from ca. 48.73 to 43.9 Ma, the Nianbo Formation around 54 Ma and the Dianzhong Formation from 64.4 to 60.6 Ma. In combination with evidence from the geochemical characteristics of the volcanic rocks, and from stratigraphy in southern Tibet, it is postulated that the age of the lowest member in the Dianzhong Formation of the Lin-zizong volcanic rock, which overlies unconformably the Late Cretaceous Shexing Formation, likely corresponds to the inception of the collision between Indian and Asian conti-nents in southern Tibet.

关键词： ^40Ar-^39Ar地球年代学新生界火山岩林州盆地西藏地质学

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