Abstract
The Ingalls ophiolite complex, central Cascades, Washington, mainly consists of mantle-derived ultramafic tectonite, with crustal rocks consisting of gabbro, diabase, basalt, and sedimentary rocks. The crustal rocks occur as faulted blocks within serpentinite melange (Navaho Divide fault zone). Mafic rocks in most of these blocks comprise the Late Jurassic Esmeralda Peaks unit. Herein, we define an older, Early Jurassic unit within the Ingalls ophiolite complex, which we call the Iron Mountain unit. This unit occurs along the southern edge of the complex and consists dominantly of mafic volcanic rocks with minor sedimentary rocks. A rhyolite within the Iron Mountain unit yields a ca. 192 Ma U-Pb zircon age, consistent with an Early Jurassic age assignment based on radiolarians in cherts. The presence of volcanic rocks that have within-plate basalt magmatic affinities and oolitic limestone suggests that the Iron Mountain unit formed as a seamount. Magmatic affinities range from within-plate basalt to enriched mid-ocean-ridge basalt (E-MORB), which is compatible with a mantle plume close to a ridge. The Early Jurassic age of the Iron Mountain unit, which is approximately 30 m.y. older than the Esmeralda Peaks unit, indicates that the Ingalls ophiolite complex is polygenetic. The Iron Mountain unit most likely represents basement that was rifted in a suprasubduction-zone setting in the Late Jurassic during formation of the Esmeralda Peaks unit.
