Petrology of the Chetco Complex, Klamath Mountains, Oregon



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The Chetco complex is a late Jurassic mafic-ultramafic batholith in the northwest Klamath Mountain province of south eastern Oregon. It has previously only been mapped at large scale north of the Illinois River. Mapping of the York Butte quadrangle at 1:24,000 scale indicates that there are two mineralogically and compositionally distinct units: a quartz hornblende gabbro (the Red Dog Creek gabbro) and a two-pyroxene gabbro and troctolite unit (the York Butte gabbro). Both units are intruded by tonalite. Field and textural evidence suggests that a substantial portion of the York Butte gabbro is a cumulate. Smooth compositional trends in major and trace elements as well as inverse rare earth element patterns suggest that both gabbros are related via fractional crystallization from a high-alumina basaltic parent. Trace element patterns are consistent with an island arc setting for the pluton. The two-pyroxene and troctolitic York Butte gabbro forms the core of the pluton and the quartz-hornblende Red Dog Creek gabbro forms the rim. This relationship, with a mafic core and more felsic rim, has been described as a reversely zoned pluton. Characteristics similar to those seen in sheeted plutons such as steep sub-parallel internal contacts and foliations indicate that re-assessment of the pluton’s emplacement history may be necessary. Overlap in most major elements and similarly shaped rare earth element patterns between the Chetco complex and Rogue Formation are consistent with the hypothesis that these units are the plutonic and volcanic portions, respectively, of the same magmatic system.