The Sr,Nd,Pb Isotopic Compositon and its Implication for the Magmatic Source of the Baimashan Rock Mass,Southern Qinling

This paper studies Sr,Nd,Pb,isotopic compositions of 7 granitoids from South Qinling,which were all formed about 200 Ma B.P.The results indicate that the granitic rocks including the Baimashan rock mass from the South Qinling are characterized by low radiogenic Pb isotopic composition.Their initial(87Sr/86Sr)t ratios range from 0.70495 to 0.70908,eNd(t)values from-2.40 to-8.55,and Nd isotopic model ages(TDM)from 1.2 Ga to 1.7 Ga.their eNd(t)values gradually decrease from east to west,whereas TDMvalues gradualy increase.The Baimashan rock mass has much lower(87Sr/86Sr)t(0.70637),lower eNd(t)(-6.75)and much higher TDM(1.53 Ga).This study reveals that the magmatic source of the Baimashan rock mass was derived from the deep continental crust.In the magmatic source,basic magmatic material in the crust similar to the Yaolinghe Group basic volcanic rocks in South Qinling,is the major source for the formation of granitoids with less significant old continental crust material.The Baimashan adakite which is in favor of copper and gold mineralization,may resulted from partial melting of mafic lower crust in the thickened lower crust due to the lithospheric delamination and the asthenospheric upwelling,which can provide some clues to the direct copper-gold prospecting.