In the Cordilleran region of the western United States, Mesozoic and Tertiary peraluminous granitic rocks display regional variations in initial 143Nd/144Nd(&egr;Nd); &egr;Nd=-10 to -12 in southern Arizona, -17 to -19 in the northern Great Basin (NGB), and -30 in the northern Rocky Mountains Initial 87Sr/86Sr values are between 0.710 and 0.721 and show no regional pattern. Metaluminous granitic rocks have a wider range of &egr;Nd values extending from values similar to those of the peraluminous granites to much higher values. The 87Sr/86Sr values are mostly fairly low, between 0.705 and 0.710 except in the NGB where values as high as 0.7157 are observed. No systematic differences between the &egr;Nd or 87Sr/86Sr values of Cu- or Mo-mineralized and unminearlized granite were discerned, except for Cu-minearlized granite in eastern Nevada and Mo-mineralized granite in Colorado, which have &egr;Nd values higher (~0) and lower (~-10.0), respectively, than unmineralized granite in the same region. Comparison to &egr;Nd values of exposed Precambrian rock suggests that the peraluminous granite, and the Mo granite in Colorado, were derived exclusively from felsic Precambrian basement rocks and that the regional variations in the &egr;Nd values reflect the regional variation in the average crustal age. The Nd data confirm that the Precambrian basement underlying the NGB and eastern California is isotopically distinct from Precambrian crust in the remainder of the western United States. The similarity between the &egr;Nd values of peraluminous granite and Precambrian crust also suggests that the high 47Sm/144Nd(>0.13) and the low total light rare earth element (LREE) abundances characteristic of peraluminous granite in southern Arizona were imposed during the chemical evolution of the magmas. Metaluminous granite are interpreted to have formed via mixing of mantle-derived magma and large proportions of low 87Sr/86Sr (granulite facies) lower crust, except in the eastern NGB where the mantle magmas mixed with a lower crustal source with a significantly higher 87Sr/86Sr ratio. REE abundance patterns for metaluminous granite in the NGB are characterized by extreme LREE enrichment, which supports the proposed origin for these rocks by mantle/crust mixing or by remelting of the lower crust alone. No systemaic difference exists between the sources of Cu- or Mo-mineralized and unmineralized metaluminous granite, but the data suggest that the Cu sources are in the mantle and the Mo sources are in preexisting crust. Overall, the Nd data indicate that continental interior granite in the western United States was primarily derived from preexisting crust, and although changes in the thermal structure of the continental mantle may have triggered magma formation, the resulting granites do not represent significant juvenile additions to the continental crust.