青海祁漫塔格地区晚泥盆世辉绿岩墙群LA-ICP-MS锆石U-Pb年龄及其构造意义

东昆仑造山带是横亘青海省中部的一条复合造山带,北部祁漫塔格一带有大量NW向延伸的辉长岩、辉绿岩及辉绿玢岩呈岩墙分布.其中,有一期侵入到元古宙—志留纪地质体中的基性岩墙TiO2平均含量为1.63％,为典型的高钛基性岩,岩石地球化学成分与板内玄武岩一致,大离子亲石元素富集,高场强元素较低,微量元素蛛网图呈现出明显的Ba、Ta和Nb“谷”.利用激光等离子体质谱法(LA-ICP-MS),获得辉绿岩中22颗岩浆锆石的206Pb/238U年龄加权平均值为380.3Ma±1.5Ma (MSWD=1.4),精确地表明中晚泥盆世东昆仑—祁漫塔格地区进入陆内伸展阶段,区域构造由志留纪—早泥盆世的碰撞挤压体制转变为后造山伸展环境.

LA-ICP-MS zircon U-Pb dating of Late Devonian diabase dike swarms in Qimantag area

East Kunlun orogenic belt is a composite orogenic belt in the middle part of Qinghai Province. There exist a large number of gabbro, diabase and diabase dykes in NW direction in northern Qimantag area. Due to the invasion of the mafic dyke into the geological body in Proterozoic-Silurian period, the average TiO2 content of the mafic dyke is 1.63%, suggesting typical high titanium rocks. The geochemical composition of the rocks is consistent with the intraplate basalt, characterized by the enrichment of large ion lithophile elements and relatively low high field elements. U-Pb isotopes of zircon from the diabase were determined by LA-ICP-MS and 206Pb/238U dating of 22 zircon samples, which yielded an isotopic age of 380.3Ma±1.5Ma(MSWD=1.4), evidently suggesting that Eastern Kunlun-Qimantag region had entered the stage of intracontinental extension in Late Devonian.