WITHIN-PLATE MAGMATIC AFFINITIES OF A LOWER PILLOW UNIT IN THE INGALLS OPHIOLITE COMPLEX, NORTHWEST CASCADES, WASHINGTON

The Ingalls Complex consists of kilometers scale blocks within sheared serpentinite and has been interpreted as having formed in a fracture zone. Based primarily on an U/Pb zircon age of ~155 Ma (Miller et al, 1993) and Late Jurassic radiolarian chert, it was thought that the Ingalls ophiolite correlated with the Josephine ophiolite in the Klamath Mountains. Further investigation within the eastern portion of the Ingalls Complex has revealed that a lower pillow basalt unit, that has a within plate basalt (WPB) geochemistry, underlies an argillite that contains Early Jurassic cherts. The lower WPB pillow basalt unit is distinct from the Late Jurassic IAT-MORB pillow basalt unit, which overlies the argillite, due to its WPB geochemistry and interpillow limestones. New Nd isotopic data for the Ingalls ophiolite also apparently show this distinction. Two samples from the WPB unit of the Ingalls have eNd(t) of +6 to +7, whereas samples of the Late Jurassic IAT-MORB unit of the Ingalls have eNd(t) of +8 to +9. The eNd(t) values of the IAT-MORB unit of the Ingalls are also similar to eNd(t) values for the Josephine ophiolite (+8 to +9). Mapping has shown the lower pillow basalt unit, with WPB geochemistry and associated limestones, is continuous for 5.2 km west, and an isolated outcrop occurs 5.8 km farther west. A bedded limestone is oolitic, indicating a very shallow water environment. This, along with the WPB geochemistry, suggests an origin of this lower pillow unit as seamounts. These Early and Late Jurassic ages indicate that the Ingalls is a polygenetic ophiolite formed by rifting of an older accreted terrane during the Late Jurassic. The Ingalls ophiolite is similar to the well-documented rift facies associated with the Josephine ophiolite (e.g., Devils Elbow Remnant).