摘要
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,对其进行了锆石LA-ICP-MSU-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(n=15,MSWD=11.0)(95%置信度),代表了岩体结晶年龄。表明该花岗岩体的形成时代属于印支中期,而不是前人认为的燕山晚期。岩石地球化学研究表明,龙塘二长花岗岩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,岩石属钾玄岩系列,为强过铝质花岗岩。稀土元素总量为88.21～506.96μg/g,稀土元素配分曲线呈右倾型,具有弱的负铕异常。Ba、Nb、Sr、Eu等元素具有明显的亏损,而Rb、U、Th等大离子亲石元素具有明显的富集。地球化学特征和微量元素构造判别图解表明,该花岗岩是以富含黏土的变质杂砂岩部分熔融形成的花岗质岩浆上升侵位形成的,是一种典型的壳源成因类型,具后碰撞花岗岩特征。形成于板块间从挤压体制向拉张体制转变的构造环境。腾冲地块中三叠世后碰撞花岗岩的出现,表明西南三江古特提斯洋在印支早期曾发生板块碰撞造山作用,并预示在中三叠世班公湖-怒江-龙陵-瑞丽板块缝合带主造山作用的结束,冈底斯-腾冲构造带进入造山后伸展构造环境,并一直持续至晚三叠世末。西南三江印支期的构造岩浆造山事件与古特提斯洋的闭合及板块间的俯冲、碰撞造山作用密切相关。龙塘二长花岗岩体形成时代的厘定,为进一步探讨腾冲地块构造演化提供了新证据。
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.
出处
《大地构造与成矿学》
EI
CAS
CSCD
北大核心
2011年第3期439-451,共13页
Geotectonica et Metallogenia
基金
云南1:5万梁河县
江东
帕底幅区域地质调查项目(项目编号:1212010784007)的成果
