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

The volcanic rocks from Baiyingaolao Formation in the northeastern Hailar Basin are mainly com- posed of rhyolite and trachydacite�� U- Pb dating of zircon shows these volcanic rocks formed in Early Cretaceous (128- - 124 Ma)�� Geochemical data indicate that they are sub- alkaline series in composition and rich in alkali and potassium�� All samples have similar rare earth element patterns characterized by high total rare earth ele- ments contents (∑��EE = 113.96 × 10^-6��204.33 × 10^-6 )��significant fractionation of heavy and light rare earth elements (∑L��EE/∑H��EE = 3.10��4.52) with middle negative Eu anomalies (δEu = 0.46��0.76)�� The trace elements are characterized by enrichment in large- ion lithophile elements such as K����b��L��EE and depletion in high field strength elements e.g. Nb, Ta, H��EE, P and Ti�� while enriched in Th and U�� ��hyolite and trachydacite contain low initial 87 Sr/ 86 Sr ratios (0.704 9��0.7 053) and positive ε Nd (t) values (ca��4.15)�� These data suggest that the magma of rhyolite and trachydacite were derived from mafic lower crust newly accreted from mantle��with the evolutional trend of comagmatic fractional crystallization��