GEOCHEMISTRY AND POSSIBLE TECTONIC SETTING OF THE ESMERALDA PEAKS UNIT, AND RELATED ROCKS, INGALLS OPHIOLITE COMPLEX, WASHINGTON

The Ingalls Ophiolite Complex is the largest and most complete Middle to Late Jurassic ophiolite in Washington. It is the northern extension of a Middle to Late Jurassic ophiolite belt that extends along the western cost of North America and has been interpreted to have formed in a fracture zone setting. The Esmeralda Peaks unit of the Ingalls Ophiolite Complex occurs as km-scale blocks within a serpentinite matrix mélange, and consists of gabbro (161±1 Ma by U-Pb zircon), diabase (including sheeted dikes), pillow basalt, broken pillow breccia, and rare plagiogranite. Intra-pillow chert occurs within this unit, which is overlain by chert, argillite, minor graywacke, conglomerate, ophiolite breccia and rare sedimentary serpentinite. The chemical affinities of the mafic volcanic rocks are transitional between mid-ocean ridge basalt and island arc tholeiite (MORB-IAT). Two boninites were also analyzed from the unit. These mafic rocks have Ta/Yb ratios that suggest they originated from a mantle source slightly enriched to N-MORB. Plagiogranites have IAT to calc-alkaline affinities, and have fractionated from the mafic rocks. Gabbros have similar geochemical affinities as the other mafic rocks but display the effects of mineral accumulation. Local amphibolites within the ophiolite have MORB-IAT chemical affinities. They are cut by undeformed diabase dikes; one of these dikes has MORB-IAT chemical affinities, while another dike is a picrite. Also, dikes that cut ultramafic tectonites have predominantly MORB affinities, one being a Fe-Ti basalt. The chemical affinities of the Esmeralda Peaks unit support the inferred back-arc basin setting for the Ingalls. A possibly younger arc complex is located ~33 km southwest of the Ingalls and it, along with the boninites, may suggest that the Esmeralda Peaks unit originated in a fore-arc setting. The amphibolites and undeformed dikes are chemically similar to the Esmeralda Peaks unit and may represent equivalents that were deformed in a fracture zone setting. The age, chemical affinities, possibly late MORB basalts, including the Fe-Ti basalt, and possible fore-arc setting support the Ingalls correlation with the 162 Ma Josephine Ophiolite of California-Oregon.