The Dashishan intrusive in Beishan area of Gansu Province mainly consists of biotite potassium-feldspar granites. The granites of Dashishan are metaluminous (Al2O3=12.18%～13.37%) and are characterized by high SiO2（74.22%～78.38%），Na2O+K2O（7.60% ～8.50%)，FeO T/MgO ratio，REE（102.3×10-6 ～120.02×10-6），Y，Nb and F，and low CaO（0.01%～0.99%），MgO（0.05％～1.15％），Sr and Ba．These characteristics are very different from characteristics of I_type and S-type granites in Beishan area，but quite similar to A2 type granites．The Dashishan intrusive has a Rb-Sr isochron age of 273 Ma. Itis therefore suggested that crustal rifting might have started in late Hercynian in that area．