西藏冈底斯过铝花岗岩的岩石成因及其对冈底斯地壳生长的约束

In general, peraluminous granite relate to the remelting of metasedmentary with rich Al in mid-upper crust, and which keeps a supply of inheritance zircon. Through the research of the source and the petrogenesis, not only can be revealed in the material composition of crust, and will also provide important constraint to construction the origin area of metasediment in mid-upper crust. In the early pre-research of applicants and other researchbased, there are promote field survey and detailed mineralogical research of rock., As research Objects, peraluminous granites exposure in different areas and different age, (early, middle Jurassic, and early cretaceous, Eocene, Miocene), analyzed the geochemical composition and zircon Hf isotopes of peraluminum granite, and discuss the magma origin and petrogenesis. Then focused on the age of inherited zircon in peraluminum granite and Hf isotope composition, through the contrast between inherited zircon and Hf isotope of metasediment in Gangdese, build the zircon age and Hf isotope index of peraluminous granite and metasediment in Gangdese. Through the contrast with neighboring continental block, analyse the crustal growth of Gangdese. Project achievement not only to provide important constraints to invasion era and petrogenesis of peraluminous granite in Gangdese, but also provides the evidence for the structure origin of the Gangdese belt.

过铝花岗岩一般与中上地壳富Al变沉积岩的重熔有关，其中保存了大量继承锆石。通过研究其来源和岩石成因，不但可揭示中上地壳的物质组成，而且还将为构造中上地壳的变沉积岩物源区提供重要约束。在申请者前期预研和其它研究基础上，以出露于冈底斯不同地区不同时代（早、中侏罗世,早白垩世、始新世、中新世）过铝花岗岩为对象，进一步开展野外调研和详细的岩石矿物学研究，分析过铝花岗岩的地球化学组成和锆石Hf同位素，讨论其岩浆起源和岩石成因；再重点针对过铝花岗岩的继承锆石年龄和Hf同位素成分，通过与冈底斯带变沉积岩中碎屑锆石年龄和Hf同位素的对比，建立冈底斯带过铝花岗岩和变沉积岩的锆石年龄和Hf同位素指标，通过与邻近陆块的对比，分析冈底斯的地壳生长。项目成果不但将对冈底斯带过铝花岗岩的侵位时代和岩石成因提供重要约束，而且还将为研究冈底斯带的构造起源提供依据。

本项目在早期研究基础之上开展了细致深入研究工作，根据西藏冈底斯新资料，以过铝花岗岩作为探测青藏高原岩石圈的“窗口”与“探针”，采取以岩石学、岩石地球化学、同位素地球化学及构造地质学研究为主，有针对性地开展精细的野外调查研究工作。在冈底斯中部厘定出东西向延展的罗扎过铝花岗岩，具碰撞花岗岩的特点为陆壳部分熔融的产物。在冈底斯带新厘定出早白垩世青龙岩体和买巴岩体，提出早白垩世花岗岩属于与班公湖－怒江洋壳岩石圈南向俯冲有关的同碰撞岩浆岩。提出与雅鲁藏布江洋俯冲-消减带向南迁移密切相关的朗县英云闪长岩体形成于增生楔岛弧环境，是90Ma±的洋脊俯冲地球动力背景下亏损地幔和增生楔熔体混合的产物；而夏如岩体的形成很可能与原特提斯洋向冈瓦纳大陆北缘俯冲过程中的安第斯型造山作用有关, 是地壳加厚、上地壳重熔形成的强过铝质花岗岩。在冈底斯带中段，发现二叠纪花岗岩，侵位于前泥盆系及上石炭统—下二叠统岩层中，为二云母花岗岩，为陆壳部分熔融作用产物；分析了皮康过铝花岗岩中锆石的LA-ICP-MS U-Pb年龄及Sr-Nd同位素组成，初步探讨了中二叠世末碰撞造山事件。本项目研究成果为研究西藏冈底斯构造－岩浆演化和地壳生长提供了新思路和新视角。