西天山艾肯达坂组火山岩系同位素定年及其构造意义

ISOTOPIC DATING FOR THE VOLCANIC ROCKS OF THE AIKENDABAN FORMATION IN WEST TIANSHAN,CHINA AND ITS TECTONIC IMPLICATION

西天山艾肯达坂地区较好发育了艾肯达坂组红色陆相火山岩建造,它不整合在下石炭统大哈拉军山组之上,未经变形和变质,属于陆陆碰撞晚期的橄榄安粗岩系,其年龄确定是厘定从碰撞造山向陆内构造演化的关键。因此,通过16件新获得的钾氩年龄测值,确定艾肯达坂组火山岩系形成在260Ma～270Ma之间,属早二叠世,而不是过去认为的石炭纪;西天山的陆陆碰撞应在二叠纪末结束,此后进入陆内造山阶段。

The Aikendaban Formation ofred-colored, continental volcanic sequence well developed in the Aikendaban area, West Tianshan. Unconformably overlying the Dahalajunshan Formation of Early Carboniferous,it shows no regional deformation and metamorphism. This sequence consisted of volcanic rocks of shoshonite series resulted from late continental collision.Its isotope age of Carboniferous was reported in previous studies. Timing of the volcanic rocks is a key to the determination of transition from continental collision to intracontinental tectonism. Therefore, this paper reports 16 K-Ar ages newly obtained from the Aikendaban Formation. New isotope ages show that volcanic rocks of the Aikendaban Formation were erupted during 260 Ma～270 Ma of Early Permian instead of Carboniferous,and that at the end of Permian the continental collision orogeny in West Tianshan demised to an intracontinental orogenic evolution.