GEOCHRONOLOGIC LINKS BETWEEN THE INGALLS OPHIOLITE, NORTH CASCADES, WASHINGTON AND THE JOSEPHINE OPHIOLITE, KLAMATH MTS., OREGON AND CALIFORNIA

Study of Mesozoic North American ophiolites has been important in deciphering orogenesis and plate margin evolution of the North American Cordillera but the tectonic setting and geologic history of some Cordilleran ophiolites remains controversial and/or poorly known. New isotopic and geochronologic data for the Ingalls ophiolite (North Cascades, WA) clarify its age, setting, and relationship to the better-studied and probably correlative Josephine ophiolite in the Klamath Mts. (OR and CA).

Multi-fraction TIMS zircon from a hornblende pegmatite gabbro of the Ingalls ophiolite (MORB-transitional IAT unit) yielded a concordant age of 161±1 Ma, which is indistinguishable from the published age of the Josephine ophiolite. Ion microprobe (SHRIMP-RG) dating of zircons from the same sample comfirms this age. We have also conducted SHRIMP dating of detrital zircons from graywackes overlying both the Ingalls (Peshastin Formation) and Josephine ophiolites (Galice Formation) to better characterize the age and tectonic setting of both ophiolites and their overlying sedimentary units. Zircon age distributions and age-probability curves for both graywacke samples are remarkably similar and bimodal, with a clear age minimum peak at 153 Ma and a less well-defined peak at approximately 227 Ma. Proterozoic zircons, which are found sparsely in the Galice greywacke, have not been found in the Peshastin greywacke. In both units, low-U zircons are predominant in the Triassic group (<200 ppm generally), whereas Jurassic zircons range more broadly in U concentration (100-600 ppm). Age-corrected 143Nd/144Nd for the 2 samples is also similar (0.51278±1 for the Peshastin greywacke and 0.51273±1 for the Galice greywacke).

These data suggest that the Ingalls and Josephine ophiolites form a common ophiolitic terrane. Sediment deposited on top of both ophiolites was derived from a similar Late Jurassic arc source (Rouge-Chetco arc?), although the Josephine received detritus from the North American craton, whereas the Ingalls sample was apparently shielded from the craton. The source of the Triassic zircons is unclear but could perhaps be related to immature arc basement that was rifted apart during formation of the Josephine and Ingalls ophiolites (Rattlesnake Creek Terrane and possible equivalent rocks of the Ingalls ophiolite?).