Geochemistry and Tectonic Setting of the Ophiolitic Ingalls Complex, North Cascades, Washington: Implications for Correlations of Jurassic Cordilleran Ophiolites

Abstract

This study synthesizes new and existing chemical data to interpret the tectonic setting of the Late Jurassic Ingalls Complex of the Northwest Cascades, Washington, and to compare it with other Cordilleran ophiolites of similar age. Mafic rocks of the Ingalls Complex represent a spectrum of magma types including normal‐type and enriched mid‐ocean ridge basalt, within‐plate basalt, and island arc basalt. Based on mafic magma chemistry, compositions of Cr‐spinels in associated peridotite, and field relations, the Ingalls Complex is a suprasubduction zone ophiolite formed in a back‐arc basin cut by an oceanic fracture zone. Regional tectonic relations also support this hypothesis. Our synthesis of available chronologic, geochemical, and stratigraphic data for all Jurassic Cordilleran ophiolites shows that the Ingalls Complex has similarities and differences with each. The most compelling likeness is between the Ingalls Complex and Josephine ophiolite, both of which exhibit a wide range of magmatic affinities, Cr‐spinel compositions that are consistent with a suprasubduction zone origin in a back‐arc basin setting, and positions inboard of a coeval Jurassic arc.