THE POLYGENETIC INGALLS OPHIOLITE COMPLEX AND ITS RELATIONSHIP TO THE JOSEPHINE AND COAST RANGE OPHIOLITES

The Ingalls Ophiolite Complex in the central Cascades, Washington, is one of several Mid to Late Jurassic ophiolites of the North American Cordillera. It is a polygenetic complex consisting of Early and Late Jurassic assemblages, and has been interpreted to have formed in a fracture zone setting. The Ingalls consists primarily of mantle tectonites, with harzburgite and dunite in the south, and lherzolite in the north. High-T mylonitic peridotite, overprinted by a serpentinite mélange, separates these two mantle types. Other units within the Ingalls include the Iron Mountain, Esmeralda Peaks, and the Ingalls sedimentary rocks. The Iron Mountain unit is ~192 Ma, consists of within-plate affinity (WPB) volcanic rocks and minor oolitic limestone, and is interpreted as the rifted basement of the complex. The Esmeralda Peaks unit consists of ~161 Ma basalt, diabase, and gabbro, minor sedimentary rocks, has chemical affinities similar to modern supra subduction zones (SSZ), and has been interpreted to have formed after basement rifting. Ingalls sedimentary rocks are predominantly mudstones with minor graywacke, conglomerate, chert, and ophiolite derived sediments. Radiolaria from chert give lower Oxfordian ages, and detrital zircon populations from a graywacke reveal a bimodal age distribution of ~153 Ma and ~227 Ma.

The Ingalls has geochemical similarities to the Coast Range ophiolite, as well as the Josephine ophiolite and its basement the Rattlesnake Creek terrain (RCT), CA-OR. All three have SSZ and WPB affinities; however, the WPB locality of the Coast Range ophiolite (Point Sal) is ~30 m.y. younger than the WPB Iron Mountain unit. Undated WPB volcanics of the RCT underlie rocks that are similar in age to the Iron Mountain unit. Radiolarians from the Ingalls are identical to those that directly overlie the Josephine (Pessagno, per. com.), and the ~153 and ~227 Ma age populations of detrital zircons also occur in sediments above the Josephine (Galice Fm). The Coast Range ophiolite is unconformably overlain by Tithonian Great Valley Group, which is younger than the sediments that overlie the Ingalls. We conclude that the Ingalls correlates to the Josephine ophiolite and its basement, and probably not to the Coast Range ophiolite.