滇西北贡山地块始新世花岗岩的成因及其构造意义
Origin and Tectonic Implications of the Eocene Granite from the Gongshan Block in Northwest Yunnan Province

摘要： 滇西三江地区发育古近纪花岗岩，记录了印度－欧亚大陆碰撞的岩浆活动信息。对贡山地块福贡花岗岩开展岩石地球化学及锆石UPbHf系统研究，结果表明，该花岗岩为钙碱性、过铝质特征的IS型花岗岩。锆石UPb同位素分析表明，福贡马吉花岗岩侵位于55 Ma，并含有252~77 Ma的继承锆石。锆石Hf同位素分析表明，该区岩浆锆石具有与青藏高原及东其南缘同时代长英质侵入体相似的Hf同位素组成，暗示其相似的岩浆起源。微量元素和同位素组成模拟计算结果表明，马吉花岗岩的原生岩浆是由53%的新生地壳组分和47%古老地壳基底物质混合而成的原岩经 5%~15%(F=005~015)的部分熔融而成。贡山地块福贡马吉花岗岩与冈底斯地块和腾冲地块早始新世岩浆岩(约55 Ma)具相似的年龄及地球化学特征，暗示它们之间可能存在类似的成因机制，均为新特提斯洋俯冲板片断离引起的壳内减压熔融的产物。
Abstract: Paleogene granite developed in the Sanjiang area，Yunnan Province, which records magmatism information during the collision between Indian and Asian continents. Integrated geochemistry and zircon UPbHf isotopic analyses were carried out on the granite in the Gongshan block. Results show that the granite is IS type with calcalkaline, peraluminous features. Zircon UPb isotopic analyses show that the granite emplaced at 55 Ma with 252-77 Ma inherited zircon grains in it. Magmatic zircons from the granite in the Gongshan show resemblance Hf isotope compositions with those from coveal felsic intrusions in the Lhasa block of Tibetan Plateau and its extended southeastern margin, implying their similar magmatic origin. The mixing calculation results under constraints of isotopic and trace element compositions confirm that the primary magma of the Eocene granite in the study area originate from the partial melting (5%-15%) of mixed protolith between juvenile crust material (53%) and Mesoproterozoic crustal basement component (47%). Similarities of the Eocene granite in the Gongshan block and the coeval Gangdese block and Tengchong block in geochemical features imply their common petrogenesis. We suggest that the Eocene granite in the Gongshan block was most likely derived from intracrustal decompressional melting related to breakoff of the NeoTethyan oceanic slab during postcollision