The nature and significance of Sr and Nd isotopic variations in late Cenozoic basalts from the western U.S. Cordillera region are examined in the light of major and trace element characteristics of two end member suites from the Lunar Crater (LCVF) and the Snake River Plain (SRP) volcanic fields. Like many other late Cenozoic Basin and Range basalts, those from LCVF are mildly alkalic and have isotopic (Sr. Nd) compositions similar to those of many oceanic island basalts (OIB). SRP basaltic lavas are tholeiitic and have lower incompatible trace element contents and systematically higher 87Sr/86Sr and lower 143Nd/144Nd isotopic ratios compared to LCVF lavas. These two suites are unlikely to have any genetic relation to one another but are representative of the range of isotopic variations observed in the region. We specifically address the question of how such isotopic diversity arises. For example, (1) could all of these magmas be generated from isotopically similar source materials and then be variably contaminated by crustal material during ascent, or (2) do their isotopic compositions reflect actual heterogeneities within the underlying mantle? To answer these questions, processes of contamination were evaluated using combined major and trace element and isotopic data, and it is concluded that isotopic distinctions between SRP and LCVF basalts cannot be entirely due to contamination. Indeed, they seem to reflect compositional differences in the respective magma source regions. We interpret the LCVF magmas to represent partial melts of dominantly ''asthenospheric'' mantle (i.e., similar to OIB sources) which, in the case of the Basin and Range province, has presumably presumably upwelled beneath a region of major lithospheric extension. In contrast, isotopically distinct SRP basalts could be derived from ancient or enriched mantle that most likely resides in the continental lithosphere. Thus lateral and/or vertical heterogeneities inferred to exist in the mantle beneath the western United States can be reconciled with geodynamic models that are consistent with regional tectonic evolution. ¿ American Geophysical Union 1989