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

Abstract

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, K (sub 2) O, and initial (super 87) Sr/ (super 86) Sr (Sr (sub i) ; range 0.7034 to 0.7047) decrease and epsilon Nd (range 2.0 to 4.8) increases in younger plutons. Within individual plutons, Sr (sub i) varies considerably, whereas epsilon Nd is nearly constant. The Sr and Nd isotopic data typically plot to the right of the mantle array. delta (super 18) O values in these plutons are greater than +8 per mil. Tonalitic and trondhjemitic plutons intruded after Nevadan thrusting are typically calcic, contain low Rb and K (sub 2) O contents, and have delta (super 18) O less than +8 per mil. These plutons have a narrow range of Sr (sub i) (0.7033-0.7036) and epsilon Nd (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 K (sub 2) O, Rb, and rare-earth element concentrations) consistent with an origin by partial melting of young ophiolitic rocks underthrust during the Nevadan orogeny.

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