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

The authors studied zircon LA-ICP-MS U-Pb dating，the zircon Hf isotope and geochemistry of acidic volcanic rocks in Baiyingaolao Formation of Keyouzhongqi area，Inner Mongolia，and discussed the chronology， source region and tectonic setting of the volcanic rocks in the studied area． The clear oscillatory zoning of zir- cons indicates a typical magmatic origin，and the results of dating show that the volcanic rocks of Baiyingaolao Formation were formed in Early Cretaceous (121.5 ±1.0 Ma)． The features of major and trace elements show that the rocks are alkali-rich，poor in calcium and magnesium with enrichment in LILEs like Th，U，K and Gd and depletion in HFSEs，e.g. Nb，Ta，Sr and Ti． The fact implies that they were the products of partial melting of the crust． εHf(t) = ( +6.30--+9.06) and TDM2=600--835 Ma，suggest the magma originated from par- tial melting of the young crust． Combined with the evolution of regional tectonic structure，the authors conclude that the acidic volcanic rocks of Baiyingaolao Formation may be formed under the extensional environment relat- ed to the subduction of Paleo-Pacific Plate．