College of Earth Sciences��Jilin University��Changchun 130061��China

Wushijiazi Basin is located in northern Linxi area��Inner Mongolia�� In order to determine the stra- tigraphic attribution and formation environment of volcanic rocks in the Wushijiazi Basin��we analyze the volcanic rock association and the rhyolites of the basin�� The LA- - ICP- - MS Zircon U- - Pb dating of the rhyolites is 151. 4 ±1. 4 Ma��which indicates its eruption in the late Late Jurassic�� Geochemical analysis indicates that the samples high in SiO 2 and alkaline content�� The ��EE distribution patterns show right deviated and Eu negative�� The elements are enriched in ��b��K and Th��while depleted in Ba��Sr��Ti and Nb�� Summarizing the isotopic dating data of volcanic rocks in the south of Da Hinggan Mountains published for the recent years��which suggests 160 �� 150 Ma as the peak period of the Manketouebo Formation volcanic activity��and combining the stratigraphic sequence with the chronological characteristics��we consider the volcanic rocks belong to the Manketouebo Formation�� The geochemi- cal features of rhyolites are consistent with A �� type granite��indicating the Linxi area was in an extensional environ- ment during the late Late Jurassic��which was possibly related to the lithosphere extension after the closure of the Mongol- - Okhotsk Ocean��