Abstract: The early-middle Cambrian volcanic rocks from Baiyin mining field are composed mainly of calc-alkaline basalts，rhyolites and minor andesites． They were considered to be resulted from magmatism of continental rift．The results of the geochemial characteristics show that，compared with oceanic ridge basalt (MORB)，the basalts of study area are characterized by low Ti and Ni(Ni<25 at Mg’=40)，obviously negative anomaly of Nb-Ta，enrichment of LREE。re1ative depletion of HFSE，high ratios of Th／Ta(9～15)，Th／La(0.32-0.56) and La／Nb(>1.4)，highly variable range of Ce／Zr，La／Zr and Nb／Zr，and with low Zr／Nb ratio(<16)．(87Sr／86Sr)i=0.704～0.707， Nd(t)= -6.46 and δEu≈ 1.0．However，the rhyolitic rocks have (87Sr／86 Sr)i=0.7026～0.7048，εNd(t )=+5.60～ +6.14，δEu = 0.5～ 1.1，Th／Ta =6～12 and Ti／Zr 6.1～53．Their REE distribution patterns have a negative anomaly of MREE． indicating the presence of amphibole as a residual phase in the mafic lower crust(pre－existing island arc basalts)． According to the geochemistry of the volcanic rocks，it can be inferred that the basalts and rhyolites were resulted from the ma gmatism in normal continental margin arc， but with different sources． The basalts stemmed from enriched sub-continental lithospheric mantleand the mantle-derived magma was strongly contaminated by continental crust． However, the rhyolitic rocks were resulted from mixing of mafic lower crust generated by partial melting of depleted mantle wedge and subduction-related mantle-derived underplating magmas at the same tectonic episode