Abstract
The ophiolitic De Roux Unit, located in the central Cascades, Washington, consists of low-grade metabasalt, serpentinite, metasedimentary rocks (argillite, chert, minor graywacke, tuff and limestone) and rare amphibolite. This unit is in fault contact with the Ingalls ophiolite complex to the north and is overlain by the Eocene Swauk Formation to the south. The Ingalls ophiolite complex consists predominantly of ultramafic rocks, approximately 161 Ma mafic complexes (U/Pb zircon age; J. S. Miller personal comm.), and an overlying argillite unit containing metagraywacke and ophiolite breccia. Along the southern part of the Ingalls, and east of the De Roux, an older (Early Jurassic?) Iron Mountain Unit consists of pillow lavas and minor limestone. The De Roux Unit has never been dated and its tectonic origin and regional relationships are enigmatic. New XRF and ICP-MS data for metabasalts, along with existing XRF data, from the De Roux unit indicate normal mid-ocean ridge basalt (N-MORB) magmatic affinities. This differs from the transitional island-arc tholeiite to N-MORB affinities of the Ingalls ophiolite approximately 161 Ma mafic complexes (Metzger et al., 2002, J. Geol.; MacDonald and Harper, unpubl. data) and the within-plate magmatic affinities of the Iron Mountain Unit (MacDonald et al., 2002, GSA abs. with prog.). New field and chemical data from the southeastern portion of the Ingalls suggest that the De Roux Unit may underlie the Iron Mountain Unit. These new data, along with the higher metamorphic grade and greater deformation than other units, suggest that the De Roux Unit may locally form basement for the Ingalls ophiolite complex. If so, at least part of the Ingalls Complex would most likely represent a rift-edge facies, with the Iron Mountain and De Roux Units being older rifted ophiolitic basement. This situation for the Ingalls ophiolite complex is similar to the approximately 164 Ma rift-edge facies of the Josephine Ophiolite in the Klamath Mountains, CA-OR. Alternatively, the De Roux Unit could be a higher grade equivalent of the Late Jurassic Ingalls metasedimentary-ophiolite breccia unit or the entire Ingalls ophiolite complex; however, the Ingalls differs from the De Roux by its lack of tuff.