滇西梁河龙塘花岗岩体LA-ICP-MS锆石U-Pb年代学及其构造意义

为了查明出露于滇西梁河县的龙塘二长花岗岩体的形成时代和构造背景,对其进行了锫石LA-ICP-MS U-Pb定年和岩石地球化学研究.结果显示:该花岗岩的锆石具有清晰的生长振荡环带,其Th/U值为0.11 ～1.79,属于典型的岩浆成因锆石.锆石的15个测点206pb/238U加权平均年龄为(232.1±5.6) Ma (...

Geochronology and Geochemistry of the Longtang Granite in the Lianghe Area, Western Yunnan and its Tectonic Implications

LA-ICP-MS zircon U-Pb dating and geochemical analysis of the Longtang granite in the Lianghe area of Western Yunnan were carried out in order to determine the age and to discuss the tectonic environment under which the granite was formed.The results show that the zircons from the granite have clear oscillatory zones,with Th/U ratio between 0.11～1.79,and give a weighted average 206Pb/238U age of(232.1±5.6)Ma(n=15,MSWD= 11.0).The Longtang granite has following geochemical characteristics:SiO2=69.74%～74.44%,Al2O3= 13.91%～14.52%,MgO=0.26%～0.87%,CaO=0.72%～1.36%,Na2O=2.63%～2.82%,K2O=5.24%～5.72%,K2O/Na2O=1.87～2.20,and A/CNK=1.11～1.20.All these characteristics suggest that the Longtang granite belongs to high K peraluminous granite.The ΣREE ranges from 88.21～506.96μg/g,REE diagrams show right dipping lines with obvious negative Eu anomalies.The granitic rocks are depleted in Ba,Nb,Sr and Eu while enriched in Rb,U and Th.The magma was probably derived from partial melting of greywacke.Geochemistry of the Middle Triassic granitoids indicates that the granite are of post-collision type,which formed in a transitional tectonic setting from the compressional system to the tensional system.The occurrence of the Middle Triassic Longtang granite marked an orogenic event during early Indosinian in the Sanjiang Paleo-Tethys Sea,and the end of the main orogenic event in Bangonghu-Nujiang-Longling-Ruili suture.From Middle Triassic,Gangdese-Tengchong tectonic belt entered the tensional stage until the end of Late Triassic.The Sanjiang Indosinian tectono-magmatic orogenesis was closely related to the closure,subduction,and collision of the Paleo-Tethys Sea.The crystallization age of the Longtang granite provides new evidence for the Paleozoic-Mesozoic tectonic evolution of the Tengchong massif.