The Sexton Mountain area, southwestern Oregon: A tectonically imbricated Middle Jurassic supra-subduction zone ophiolite complex

The Sexton Mountain area is a tectonically imbricated, supra-subduction zone (SSZ)/island are ophiolite that formed at 177+/-3.5 Ma during inception of a Middle Jurassic subduction zone which produced the western Hayfork island arc (177-166 Ma).; All pyroxenite, gabbronorite and hornblende-gabbronorite bodies of the Sexton Mountain ophiolite were generated as SSZ magmas that were formed as wet melts from a variably depleted, residual N-MORB type mantle source. These plutonic bodies were built on top of and intruded through serpentinized peridotite and a serpentinite matrix melange unit that shares similarities with melange of the Rattlesnake Creek terrane.; Following emplacement of the plutonic bodies, aphanitic mafic bodies and dikes were emplaced during an interval of protracted mylonite formation. Aphanitic mafic rocks and plagiogranite was generated from the same variably depleted, residual N-MORB type mantle source as hornblende-gabbronorite bodies. Aphanitic mafic rocks have geochemical signatures and mineralogies that vary from SSZ to N-MORB types with minor SSZ components. This observed contrast in geochemical signatures and mineralogy is thought to document the diminishing effect of subduction zone fluids entrained into the zone of melting during continued lithospheric extension and inter-arc basin formation. Collectively, the Sexton Mountain ophiolite underwent varying degrees of seafloor hydrothermal alteration which overprinted magmatic rocks and mylonitic fabrics.; The Sexton Mountain area documents a period of lithospheric extension which generated SSZ/island are crust that was followed 22 m.y. later by a period of contraction, imbrication and accretion onto the continental margin. The formation and emplacement history for the Sexton Mountain area is similar to the tectonic switching model of Collins (2002b) and can be applied to the Middle Jurassic convergent margin tectonic history of the Klamath Mountains province.