Isotopic variations in Cenozoic mafic volcanic rocks in the Basin and Range Province of the southwestern United States are considered by many to be due to differences in mantle sources, either subduction-enriched subcontinental lithosphere or the asthenosphere. This is a detailed geochemical investigation of the magmatic evolution and mantle source(s) of Quaternary mafic volcanism at Long Valley Caldera and the Devils Postpile National Monument, Western Great Basin. The Quaternary mafic lavas are dominantly post-Bishop Tuff (7%, assimilation of Sierra Nevada granitoids becomes increasingly important at MgO7% MgO, 87Sr/86Sr and La/Sm decrease as 143Nd/144Nd, Nb/La, Zr/Ba and (to a lesser extent) Pb isotope ratios increase from the oldest to the youngest Quaternary lavas. The oldest lavas are chemically similar to other lavas in the western Basin and Range thought to have an enriched lithospheric mantle source. Whereas the shifts in Sr and Nd isotope ratios from oldest to youngest basalts might be consistent with a progressively increasing asthenospheric component in the lavas, the shifts in incompatible element ratios and lack of shifts in Pb isotope ratios with time are not. Instead, they indicate that with time the mafic lavas may have increasingly interacted with mafic crust, perhaps gabbroic/dioritic rocks at depth within the Sierra Nevada Batholith. Alternatively, a second, less enriched lithospheric mantle source is present beneath the Long Valley area that has only recently begun to undergo melt generation, and this source has made progressively larger contributions to the basaltic magmas erupted at the surface. ¿ American Geophysical Union 1996