Jurassic and Early Cretaceous plutons in the Klamath Mountains province of California and Oregon were intruded both before and after thrusting during the Nevadan orogeny (ca. 155-148 Ma). Plutons intruded before Nevadan thrusting range from calc-alkalic to calcic and display a range of compositions that changes consistently with time. Among this group, Rb, K2O, and initial 87Sr/86Sr (Sri; range 0.7034 to 0.7047) decrease and ϵNd (range 2.0 to 4.8) increases in younger plutons. Within individual plutons, Sri varies considerably, whereas ϵNd is nearly constant. The Sr and Nd isotopic data typically plot to the right of the mantle array. δ18O values in these plutons are greater than +8.0‰. Tonalitic and trondhjemitic plutons intruded after Nevadan thrusting are typically calcic, contain low Rb and K2O contents, and have δ18O less than +8.0‰. These plutons have a narrow range of SRi (0.7033-0.7036) and ϵNd(4.7 to 7.4); values that plot in the mantle array. The isotopic compositions of the pre-Nevadan plutons requires assimilation of metasedimentary rocks or mixing with crustal melts at deep-crustal levels. The deep-crustal component is not exposed in the province and was presumably truncated during Nevadan thrusting. Post-Nevadan plutons show little evidence of assimilation of metasedimentary crust but display trace-element evidence (low K2O, Rb, and rare-earth-element concentrations) consistent with an origin by partial melting of young ophiolitic rocks underthrust during the Nevadan orogeny.