Isotopic dating for the volcanic rocks of the Aikendaban Formation in West Tianshan, China and its tectonic implication

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.