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 (∑ＲEE = 113.96 × 10^-6－204.33 × 10^-6 )，significant fractionation of heavy and light rare earth elements (∑LＲEE/∑HＲEE = 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，Ｒb，LＲEE and depletion in high field strength elements e.g. Nb, Ta, HＲEE, P and Ti， while enriched in Th and U． Ｒhyolite 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．