The Grayback Pluton: Magmatism in a Jurassic Back-Arc Environment, Klamath Mountains, Oregon

The Jurassic Grayback pluton was emplaced in a back-arc settingbehind a contemporaneous oceanic arc. Th\alphae main stage ofthe pluton consists of an early, reversely zoned tonalite togabbro that was intruded by synplutonic noritic and gabbroicmagmas. Late-stage activity was characterized by intrusion oftonalitic and granitic dikes, many of which contain mafic enclavesand hybrid zones. Most mafic rocks in the pluton are calc-alkaline,with characteristic magnesian clinopyroxene, calcic cores inplagioclase, and elemental abundances similar to H₂O-rich arcbasalts. However, some mafic rocks contain relatively Fe-richclinopyroxene, lack calcic cores in plagioclase, and are compositionallysimilar to evolved high-alumina tholeiite. Compositional variation in the main stage can be modeled inpart by fractional crystallization and crusted assimilationduring which parental calc-alkaline basalt evolved to graniticcompositions. Cumulates related to this process are representedby modally variable melagabbro and pyroxenite. Mixing of basalticand tonalitic magmas accounts for the compositions of most main-stageintermediate rocks, but mixing of basaltic and granitic magmaswas uncommon until late in the pluton's history. Oxygen, Sr and Nd isotopic data indicate that virtually allmain-stage magmas in the pluton contain a crustal component.Isotopic and trace element data further suggest that late-stagetonalitic dikes represent melts derived from older, metavolcanicarc crust Deep crustal contamination of main-stage rocks tookplace below the level of emplacement, probably in a magma-richzone where basalts ponded and mixed with crustal melts. The Grayback pluton illustrates the diversity of Jurassic back-arcmagmatism in the Klamath province and demonstrates that ancientmagmatism with arc-like features need not be situated in anarc setting. KEY WORDS: Grayback Pluton; Klamath Mountains; Oregon; back arc; crustal contamination ^*Corresponding author