TECTONIC SETTING OF OROGENIC GRANITES IN THE BEISHAN REGION,GANSU PROVINCE

Rb-Sr isotopic studies of orogenic granites in the Beishan region,Gansu province,gave threeisochron ages:278 Ma（granodiorite south of Langwa Mountain in the northern part of theregion）,351 Ma （plagiogranite north of Yemajie in the southern part of the region）and 311 Ma（plagiogranite south of Yemajie）. On the basis of these data coupled with large amounts of iso-rope data available in the region and the tectonic setting,the author proposes the existence of theLate Paleozoic significance.Through a study of the Sr,Nd and Pb isotopic compositions, it issuggested that the Kazakhstan plate has extended into the region and that its basement is mainlycomposed of oceanic crust.