Tin polymetallic deposits are the most important type of tin deposit in the Nanling region. Manyresearchers both at home and abroad consider this type of tin deposit to be the product of differentiation andevolution of granite magmas resulting from anatexis of continental crust and to be genetically related to thetransformation-type (S-type) granitoids. In this paper, on the basis of the geological settings, petrology, REEgeochemistry and strontium and oxygen isotopic compositions of 6 granite intrusions associaied with tinpolymetallic deposits in the Nanling region, the authors suggest that the ore-bearing granites of this type areprobably the products of differentiation and evolution of acid magmas resulting from 40-50‰ fractionalcrystallization of magmas formed by partial melting of the pre-existing intermediate-basic volcanic rocks ofmantle origin in the lower crust and a small amount of sialic material and belong to crust-mantle-derivedgranitoids (approaching I-type of B. W. Chappell and A.J.R. White, but being evidently different from theS-type granitoid related to W, Sn, Nb, Ta and REE deposits).