MIDDLE JURASSIC SEXTON MOUNTAIN SUPRA-SUBDUCTION OPHIOLITE COMPLEX, KLAMATH MOUNTAINS, OREGON

The Sexton Mountain Ophiolite (SMO) and an associated mélange structurally overlies the Jurassic Western Klamath terrane (Galice Fm.) along the Orleans thrust and structurally underlies metamorphic rocks of the May Creek terrane (~155 Ma cooling ages) along the inferred Grave Creek thrust. Mafic/ultramafic rocks of the SMO preserve an ophiolite pseudo-stratigraphy with basal hornblende/pyroxene gabbro (and local pyroxenite) overlain by basalt/andesite bodies; dikes of similar basalt/andesite also cut the gabbro section. Portions of the gabbro and basalt/andesite bodies contain mylonite shear zones which collectively are cut by pyroxene/hornblende phyric-basaltic dikes and records a protracted episode of syn-magmatic deformation (extension?). The plutonic portion of the ophiolite was intruded through and constructed on top of, underlying serpentinized peridotite and serpentinite-matrix mélange. Overlaying the ophiolite basalt/andesite bodies are meta-sedimentary rocks consisting of stretched-pebble conglomerate containing chert clasts that are overlain by mica-rich phyllite of the Upper Galice Formation. Magmatic rocks of the SMO are predominately low K tholeiites; new trace element and Nd isotopic data are consistent with generation by partial melting of a depleted MORB mantle source. Variable LILE enrichment records a subduction component. Petrographic data are consistent with fractionation of wet tholeiite magma. Sr isotopic data record an episode of sea-floor interaction. Collectively, these data suggest that the SMO formed in a supra-subduction zone oceanic basin. Zircons from Sexton Mountain gabbro yielded a Middle Jurassic age (177 ± 4 Ma) suggesting correlation with the volcanogenic rocks of the western Hayfork terrane. Chert blocks from the underlying mélange are reported to contain 202-190 Ma radiolarian (Irwin and Blome, 2004) permitting correlation with mélange of the Rattlesnake Creek terrane. The SMO appears to record supra-subduction magmatism in an extensional oceanic basin and probably represents the initiation of the western Hayfork volcanic arc (~174-166 Ma). Emplacement of the SMO as a thrust sheet during Nevadan contractional deformation occurred between 156-139 Ma based on reported ages for stitching plutons.