Tectonic implications of isotopic variation among Jurassic and early Cretaceous plutons, Klamath Mountains

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. Tonalitic and trondhjemitic plutons intruded after Nevadan thrusting are typically calcic, contain low Rb and K ₂ O contents, and have δ ¹⁸ O less than +8.0‰. 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 Nevaden thrusting. Post-Nevadan plutons show little evidence of assimilation of metasedimentary crust but display trace-element evidence (low K ₂ O, Rb, and rare-earth-element concentrations) consistent with an origin by partial melting of young ophiolitic rocks underthrust during the Nevadan orogeny. -from Authors