西藏冈底斯南缘汤白斑状花岗岩锆石U-Pb年代学、地球化学及地质意义

摘要

In order to discuss the genesis of Late Triassic to Early Jurassic magmatic rocks and its relationship with the early-stage evolution of the Neo-Tethys in the southern margin of the Gangdese, we report petrography, geochronology and geochemistry of the porphyritic granites from Tangbai in southern margin of Gangdese, Tibet. Zircon U-Pb dating yields an age of(190.37 ± 0.87) Ma (MSWD=0.58) for the porphyritic granites, indicating that they were formed in Early Jurassic. Geochemically, the rocks are characterized by high contents of SiO2(75.20%~75.97%),Na2O(3.39%~4.12%)and ratios of Na2O/K2O(1.40~2.00)and low contents of MgO(0.32%~0.38%), thus can be grouped into calc-alkaline I-type granites. The trace elements are characterized by enrichment of LILEs(such as Rb,U and K)and LREEs and depletion of HFSEs(such as Nb,Ta and Ti)and HREEs,indicating that they were formed in magmatic arc setting related to the northward subduction of the Neo-Tethys oceanic slab. Furthermore, the granites have low Mg#values(26.71~41.34,Mean=35.26),and with Nb/Ta ratios similar to the lower crust,indicating that the granites were mainly generated from partial melting of the lower crust. Combined with previous research results, it is shown that the Late Triassic to Early Jurassic magmatic rocks in the southern Gangdese were formed in magmatic arc related to the northward subduction of the Neo-Tethys oceanic slab,and the initial age of northward subduction of the Neo-Tethys oceanic crust should prior to the crystallization age(190.37±0.87 Ma) of the porphyritic granites reported in this paper.%为了探讨冈底斯南缘晚三叠世—早侏罗世时期岩浆岩的成因及与新特提斯洋早期演化的关系,文章对冈底斯南缘汤白地区斑状花岗岩的岩相学、年代学和地球化学特征进行研究.LA-ICP-MS锆石U-Pb定年结果显示,斑状花岗岩的结晶年龄为(190.37±0.87)Ma(MSWD=0.58),形成于早侏罗世.岩体以高SiO2(75.20%~75.97%)、Na2O(3.39%~4.12%)、Na2O/K2O值(1.40~2.00)和低MgO(0.32%~0.38%)为特征,属于钙碱性I型花岗岩.微量元素地球化学特征显示,富集大离子亲石元素(LILEs:如Rb、U和K)和轻稀土(LREEs),亏损高场强元素(HFSEs:如Nb、Ta和Ti)和重稀土(HREEs),表明其形成于新特提斯洋北向俯冲相关的岩浆弧环境.同时岩石具有较低的Mg#值(26.71~41.34,平均值为35.26)和与下地壳接近的Nb/Ta值,指示岩浆主要起源于新生下地壳部分熔融.结合前人最新的研究成果表明,南冈底斯晚三叠世—早侏罗世时期的岩浆岩形成于新特提斯洋北向俯冲相关岩浆弧环境,新特提斯洋向北俯冲起始时间至少早于本文报道的汤白斑状花岗岩的结晶年龄(190.37±0.87 Ma).