Petrologic character of Permian and Triassic greenstones from the Melange terrane of eastern Oregon and their implications for terrane origin

Petrography, mineralogy, and geochemistry of lower greenschist fades Permian and Triassic metavolcanics from throughout the pre-Tertiary Melange terrane of eastern Oregon indicate three different sources for these rocks: island arc, alkalic–within plate, and ocean ridge. Most have island-arc affinities. Metabasalts associated with chert at two widely separated localities have nearly identical alkalic compositions. Knockers in serpentinite-matrix melange have diverse sources. Metagabbros of the melange are mostly tholeiitic and may have an origin different from that of the calc-alkaline, arc-related, coherent ophiolite within the terrane. The compositions and lithologies of metavolcanics and metagabbros, as well as the associated metasediments, suggest that the Melange terrane represents a forearc.