The Ingalls ophiolite complex, central Cascades, Washington;geochemistry, tectonic setting, and regional correlations

Abstract

The polygenetic Ingalls ophiolite complex in the central Cascades,Washington, is one of several Middle to Late Jurassic ophiolites ofthe North American Cordillera. It consists primarily of mantletectonites. High-temperature mylonitic peridotite, overprinted byserpentinite melange (Navaho Divide fault zone), separates harzburgiteand dunite in the south from lherzolite in the north. Crustal units ofthe ophiolite occur as steeply dipping, kilometer-scale fault blockswithin the Navaho Divide fault zone. These units are the IronMountain, Esmeralda Peaks, and Ingalls sedimentary rocks. Volcanicrocks of the Iron Mountain unit have transitionalwithin-plate-enriched mid-ocean-ridge basalt affinities, and arhyolite yields a U-Pb zircon age of ca. 192 Ma. Minor sedimentaryrocks include local oolitic limestones and cherts that contain LowerJurassic (Pliensbachian) Radiolaria. This unit probably formed as aseamount within close proximity to a spreading ridge. The EsmeraldaPeaks unit forms the crustal section of the ophiolite, and it consistsof gabbro, diabase, basalt, lesser felsic volcanics, and minorsedimentary rocks. U-Pb zircon indicates that the age of this unit isca. 161 Ma. The Esmeralda Peaks unit has transitionalisland-arc-mid-ocean ridge basalt and minor boninitic affinities. Apreferred interpretation for this unit is that it formed initially byforearc rifting that evolved into back-are spreading, and it wassubsequently deformed by a fracture zone. The Iron Mountain unit isthe rifted basement of the Esmeralda Peaks unit, indicating that theIngalls ophiolite complex is polygenetic. Ingalls sedimentary rocksconsist primarily of argillite with minor graywacke, conglomerate,chert, and ophiolite-derived breccias and olistoliths. Radiolaria fromchert give lower, Oxfordian ages. The Ingalls ophiolite complex issimilar in age and geochemistry to the Josephine ophiolite and itsrelated rift-edge facies and to the Coast Range ophiolite ofCalifornia and Oregon. The Ingalls and Josephine ophiolites arepolygenetic, while the Coast Range ophiolite is not, and sedimentaryrocks (Galice Formation) that sit on the Josephine and its rift-edgefacies have the same Radiolaria fauna as Ingalls sedimentary rocks.Therefore, we correlate the Ingalls ophiolite complex with theJosephine ophiolite of the Klamath Mountains. Taking known Cretaceousand younger strike-slip faulting into account, this correlationimplies that the Josephine ophiolite either continued northwardapproximately 440 km-thus increasing the known length of the Josephinebasin-or that the Ingalls ophiolite was translated northwardapproximately 440 km along the continental margin.

41 samples from this reference