SUPRA-SUBDUCTION ZONE OPHIOLITE FORMED IN AN EXTENSIONAL FORE-ARC - TRINITY TERRANE, KLAMATH MOUNTAINS, CALIFORNIA

The Paleozoic Trinity terrane is a massif of ultramafic rock and mafic intrusive complexes that forms the basement of eugeoclinal rocks in t he eastern Klamath Mountains. The origin of the Trinity terrane has be en particularly enigmatic and several mutually exclusive origin models have been proposed: (1) mantle diapir; (2) slice of obducted monogene tic oceanic lithosphere; (3) monogenetic backarc-basin lithosphere; (4 ) forearc lithosphere; and (5) volcanic are basement. We have compiled new and existing field, petrographic, geochronologic, isotopic, and w hole-rock chemical data for the Trinity terrane to evaluate these mode ls and conclude that the Trinity terrane is an supra-subduction zone o phiolite whose mafic intrusive complexes record subduction-related mag matism and forearc extension that occurred during the inception of int raoceanic subduction According to this extensional forearc model, all four terranes in the eastern Klamath Mountains evolved principally dur ing the inception and evolution of a single, E-dipping, mid-Paleozoic convergent margin analogous to the Eocene Izu-Bonin-Marianas are syste m. Conventional wisdom has been that the Devonian volcanic succession in the Eastern Klamath terrane records the inception of Paleozoic subd uction-related magmatism in California. New U-Pb zircon age data for g abbros in mafic intrusive complexes of the Trinity terrane range from Early Silurian to Early Devonian (431 to 404 Ma), indicating that subd uction-related magmatism began earlier than previously believed. Silur ian and Devonian igneous rocks of the Trinity and Eastern Klamath terr anes are coeval, broadly cogenetic, and represent different levels of exposure of the same ophiolite. This interpretation provides the first integrated explanation of the origin of seemingly disparate Paleozoic lithotectonic elements in the eastern Klamath Mountains.