The rock associations of the and second cycle of early Cretaceous at Yunshan caldera,Yongtai County,are composed of sedimentary rocks-basic lava-acidic volcanic rocks.Intermediate volcanic rocks are absent in these rock associations; therefore, they can beclassified as typical bimodal volcanic rock associations, Geochemically;basaltic volcanic rocks in these bimodal rock associations are rich in alkali (especially potassium),aluminum, iron LILE and LREE and poor in magnesium,HFSE and siderophile elements ,and have relatively high Isr ratios and obviouslv low εNd (T) values (Isr = 0.70682~0.70770,εNd(T) = -7.42~3.22).Rhyolitic volcanic rocks are generally characterized by rich alkali, silicon, aluminum,and poor calcium, magnesium. They have relatively high Rb,Th and rather low Ba,Sr,Ti,P,and exhibit obviouslv negative europium anomaly (δEu=0.16~0.44).The εNd(T) values of the rhyolitic volcanic rocks range from -5.93 to -2022.Basalts and rhyolites in different cycles show clear geochemical variance.On the basis of an integrated analysis of the geochemical features of these volcanic rock associations,this paper concludes that thebasalts originated mainly from interaction between mantle-derived magma and low-crustal materials, and rhyolitic volcanic rocks resulted from thermal and chemical interaction between basaltic magma and the upper part of crustal materials.Basalts and rhyolites in different cycles have different source regions,and they are products of multiperiodic pulsive layer partial melting.