1�� College of Earth Science��Jilin University��Changchun 130061��China; 2�� Institute of Geology��Chinese Academy of Geological Sciences��Beijing 100037��China; 3�� Shenyang Institute of Geology and Mineral ��esources��China Geological Survey��Shenyang 110032��China; 4�� Jiangxi Geological Exploration and Exploitation Institute for Nonferrous Metal Ore Deposit��Nanchang 330001��China; 5�� Shenyang Tests ��esearch Centre��Northeast China Coal Field Geology Bureau��Shenyang 110016��China

By using petrological��isotope chronological��and geochemical methods��the authors studied the vol- canic rocks in the studied area��mainly including dacites and trachytes�� The results show that they formed dur- ing the late Early Cretaceous�� Geochemically��the volcanic rocks are relatively enriched in large- ion lithophile elements (��b��K��and Th) and depleted in high field strength elements (Nb��Ta��and Ti)��and rich in light rare earth elements�� and depleted in heavy rare earth elements�� The fact indicates that the main body of the vol- canic rocks in the Qushenla Formation was derived from the partial melting of lower crust�� The lithological as- semblages are characterized by continental high- K calc- alkaline and shoshonitic series��suggesting that the southward- subducting oceanic slab in southern Bangong Lake had break off and that the Bangong Lake- Nujiang Ocean had closed before 107 Ma�� The main dynamic mechanisms for the genesis of this set of intermediate- acid- ic volcanic rocks were upwelling of the asthenosphere and partial melting of the lower crust caused by slab break- off��