Field, petrographic, and trace-element data indicate that volcanic rocks of the Quaternary San Pedro–Pellado complex (lat 36°S, Chile) have incorporated a silicic component that may be derived from the crust. Isotope ratios of Sr, Nd, and Pb vary little among the volcanic rocks. This is due primarily to the limited isotopic contrast between the Mesozoic-Tertiary crust and the young magmas. Isotope and element compositions of San Pedro–Pellado rocks are similar to those reported from other localities in the southern volcanic zone of the Chilean Andes. We suggest that crustal assimilation is a significant process in the genesis of differentiated volcanic rocks in the southern volcanic zone. The San Pedro–Pellado example illustrates the importance of considering all types of evidence, not just isotopic data, in the evaluation of crustal contributions to arc magmas.