The Grayback Pluton: Magmatism in a Jurassic back-arc environment, Klamath Mountains, Oregon

The Jurassic Grayback pluton was emplaced in a back-arc setting behind a contemporaneous oceanic arc. Th\alphae main stage of the pluton consists of an early, reversely zoned tonalite to gabbro that was intruded by synplutonic noritic and gabbroic magmas. Late-stage activity was characterized by intrusion of tonalitic and granitic dikes, many of which contain mafic enclaves and hybrid zones. Most mafic rocks in the pluton are calc-alkaline, with characteristic magnesian clinopyroxene, calcic cores in plagioclase, and elemental abundances similar to H₂O-rich arc basalts. However, some mafic rocks contain relatively Fe-rich clinopyroxene, lack calcic cores in plagioclase, and are compositionally similar to evolved high-alumina tholeiite.

Compositional variation in the main stage can be modeled in part by fractional crystallization and crusted assimilation during which parental calc-alkaline basalt evolved to granitic compositions. Cumulates related to this process are represented by modally variable melagabbro and pyroxenite. Mixing of basaltic and tonalitic magmas accounts for the compositions of most main-stage intermediate rocks, but mixing of basaltic and granitic magmas was uncommon until late in the pluton's history.

Oxygen, Sr and Nd isotopic data indicate that virtually all main-stage magmas in the pluton contain a crustal component. Isotopic and trace element data further suggest that late-stage tonalitic dikes represent melts derived from older, metavolcanic arc crust Deep crustal contamination of main-stage rocks took place below the level of emplacement, probably in a magma-rich zone where basalts ponded and mixed with crustal melts.

The Grayback pluton illustrates the diversity of Jurassic back-arc magmatism in the Klamath province and demonstrates that ancient magmatism with arc-like features need not be situated in an arc setting.