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��