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The polygenetic Ingalls ophiolite complex in the central Cascades, Washington, is one of several Middle to Late Jurassic ophiolites of the North American Cordillera. It consists primarily of mantle tectonites. High-temperature mylonitic peridotite, overprinted by serpentinite mélange (Navaho Divide fault zone), separates harzburgite and dunite in the south from lherzolite in the north. Crustal units of the ophiolite occur as steeply dipping, kilometer-scale fault blocks within the Navaho Divide fault zone. These units are the Iron Mountain, Esmeralda Peaks, and Ingalls sedimentary rocks.

Volcanic rocks of the Iron Mountain unit have transitional within-plate–enriched mid-ocean-ridge basalt affinities, and a rhyolite yields a U-Pb zircon age of ca. 192 Ma. Minor sedimentary rocks include local oolitic limestones and cherts that contain Lower Jurassic (Pliensbachian) Radiolaria. This unit probably formed as a seamount within close proximity to a spreading ridge. The Esmeralda Peaks unit forms the crustal section of the ophiolite, and it consists of gabbro, diabase, basalt, lesser felsic volcanics, and minor sedimentary rocks. U-Pb zircon indicates that the age of this unit is ca. 161 Ma. The Esmeralda Peaks unit has transitional island-arc–mid-ocean ridge basalt and minor boninitic affinities. A preferred interpretation for this unit is that it formed initially by forearc rifting that evolved into back-arc spreading, and it was subsequently deformed by a fracture zone. The Iron Mountain unit is the rifted basement of the Esmeralda Peaks unit, indicating that the Ingalls ophiolite complex is polygenetic. Ingalls sedimentary rocks consist primarily of argillite with minor graywacke, conglomerate, chert, and ophiolite-derived breccias and olistoliths. Radiolaria from chert give lower Oxfordian ages.

The Ingalls ophiolite complex is similar in age and geochemistry to the Josephine ophiolite and its related rift-edge facies and to the Coast Range ophiolite of California and Oregon. The Ingalls and Josephine ophiolites are polygenetic, while the Coast Range ophiolite is not, and sedimentary rocks (Galice Formation) that sit on the Josephine and its rift-edge facies have the same Radiolaria fauna as Ingalls sedimentary rocks. Therefore, we correlate the Ingalls ophiolite complex with the Josephine ophiolite of the Klamath Mountains. Taking known Cretaceous and younger strike-slip faulting into account, this correlation implies that the Josephine ophiolite either continued northward ~440 km—thus increasing the known length of the Josephine basin—or that the Ingalls ophiolite was translated northward ~440 km along the continental margin.

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