Abstract:The Yanchangbeishan tonalite develop in Lenghu area of Qinghai, which located in the west segment of the north margin of Qaidam. A study of their petrochemical, geochemical shows that they have geochemical characteristics of adakite, such as: High Sr(>400×10-6,409μg/g in average), low Y(varies from 3.09 to 6.6μg/g ),Yb (varies from 0.4 to 0.58 μg/g), Si2O>56% ( ranging from 62.86% to 64.83%), A12O3>15% ( ranging from 17.25% to 18.15% ), MgO<3%(ranging from 1.73% to 1.96%),Mg#<50 (ranging from 33.4 to 37.0); meanwhile, they riched Na and depleted in K(Na2O/K2O=2.39~2.73), Eu and Sr positive anomaly, display characteristics of O-type adakitic rock. Moreover, These samples are depleted in HFSEs (Nb, Ta, P) intensively, riched LILEs (K, Rb, Rb, Ba), reflecting the characteristics of arc granite. LA-ICPMS zircon U–Pb dating shows that the formation age of the rock is 263±2Ma. Based on the regional geological backgroud and the geochemical characteristics of the masses, we consider the adakitic granite of the west segment of the north margin of Qaidam may formed in volcanic arc setting of active continental margin underneath subduction. it is believed that the adakitic magma originated by melting of subducting plate, and which indicated the north margin of Qaidam was in a tectonic evolution stage of oceanic crust subduction during middle Permian epoch