云南双江县勐库地区三叠纪火山岩的发现及其意义

THE DISCOVERY AND SIGNIFICANCE OF T VOLCANIC ROCK IN MENGKU AREA OF SHUANGJIANG,YUNNAN

经过1∶5万区域地质填图,在滇西双江县勐库地区新识别出一套三叠纪火山-沉积岩系地层,该套地层呈断块分布于湾河蛇绿混杂岩带内。笔者对该套地层中的灰绿色英安岩样品进行LA-ICP-MS锆石U-Pb年龄测试,其协和年龄值为230. 4±2. 3Ma和227. 7±3. 6Ma,表明该套中酸性火山岩的喷发形成于三叠纪。岩石地球化学具有高硅(SiO_2=62. 95～68. 11%)、高铝(Al2O3=10. 94～14. 69%)和低钛(TiO_2=0. 53～0. 80%)的特征,属钙碱性岩类;该套火山岩还具有LREE强烈富集的"L型"稀土元素配分模式。总体上看,该套火山岩具弧火山岩的特征,可能代表了古特提斯洋盆闭合后形成的一个小型弧间盆地。

In the 1 ： 50000 regional geological mapping, a suite of T volcanic rock-sedimentary rock series strata have been divided recently in Mengku area of Shuangiang, W Yunnan, which are in Wanhe ophiolite melange zone. We have the determination of the LA- ICP-MS zircon U-Pb dating, the concordia age values are 230. 4±2. 3Ma and 227. 7±3. 6Ma, showing the eruption age of tlie intermediate-acid volcanic rock suite is T. They are characterized by high Si （ SiO2 = 62. 95-68. 11% ）, high Al （ Al2O3 = 10. 94-14. 69%） and low Ti （TiO2 = 0.53-0. 80%） in geochemistry, belonging to the alkaline rocks. And this volcanic rock suite has the“ L type” REEdistribution pattern very rich in LREE. Generally speaking, this volcanic rock suite is characterized by the arc volcanic rock, probably standing for a small basin between arc forming after tlie close down of Tetliys basin.