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

Petrographically, the volcanic rocks in Manketouebo Formation from keyihe area northern Da Hinggan Mountains are mainly composed of rhyolitic and dacitic volcanic rocks. The TAS diagram displays that they are rhyolite. LA-ICP-MS zircon U-Pb dating results show that they formed at 139±2Ma. The volcanic rocks, which are rich in SiO₂ and alkalis, belong to high K Calc alkaline series. They are low in total rare earth elements(ΣREE=137.34×10^-6~191.18×10^-6), whilst highly fractionated LREE from HREE ((La/Yb)_N=12.57~22.25),with moderately negative Eu anomalies (δEu=0.40~0.72). They are characterized by the enrichment of large ion lithophile elements(such as Rb,Th and U) and LREE, and deplection of high field strength elements(such as Nb, Ta, P, and Ti). The average value of Sr is 115×10^-6, Yb is 1.70×10^-6, Y/Yb is 9.46, (Ho/Yb)_N is 1.04, with negative Eu indicating that the magmas of these volcanic rocks are derived from partial melting of the granulite fracies of thickened lower crust. Integrating regional geology, it is suggested that the formation of the volcanic rocks were related to the crustal thickening during the closure of Mongol-Okhotsk Ocean.