Abstract:There are a large number of late Hercynian intermediate- acid magmatic rocks in the Xiaosaishiteng Mountain, located in the western part of the northern margin of Qaidam basin. Their formation age and genetic types are of great significance to reveal the regional tectonic evolution. In this study, the Permian biotite monzonitic granite and quartz diorite in the Xiaosaishiteng Mountain are studied by petrology, zircon U- Pb chronology, petrogeochemistry and Lu- Hf isotopes. The results show that the ages of biotite monzonitic granite and quartz diorite are 272±3 Ma and 273±2 Ma, respectively, indicating that they were formed in Early- Middle Permian. Geochemical results show that biotite monzonitic granite belongs to I- type granite of weakly peraluminous potassium basalt series, and quartz diorite belongs to I- type granite of quasi- aluminous calc- alkaline series. Both of them are enriched in large ion lithophile elements Rb, Th, K to varying degrees, and depleted in high field strength elements such as Nb, Ta, Ti, showing typical arc magma characteristics. Lu- Hf isotopic compositions indicate that biotite monzonitic granite has a positive εHf(t) value (2. 04~8. 16), and younger two- stage Hf model age (tDM2=0. 77~1. 16 Ga). Combined with previous research results, it is considered that the Early- Middle Permian biotite monzonitic granite and quartz diorite in the Xiaosaishiteng Mountain were the products of subduction of the Zongwulong Ocean. The quartz diorite was formed by the melt contaminated by mantle peridotite caused by the partial melting of the basaltic oceanic crust plate, and the biotite monzonitic granite was formed by partial melting of the mixed crust composed of the juvenile crust and the ancient basement crust under the condition of amphibolite facies, which is caused by the magma underplating generated by oceanic crust melting.