Abstract

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.

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