LINKING THE WALLOWA TERRANE TO NORTH AMERICA: LATEST JURASSIC–EARLIEST CRETACEOUS COON HOLLOW FORMATION, BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON AND WESTERN IDAHO

New isotopic provenance data from Early Mesozoic sedimentary rocks in the Blue Mountains Province place new constraints on the timing of terrane accretion. Historically, correlation of Middle Jurassic deposits in the Snake River region (Coon Hollow Formation) to those of the John Day region (Snowshoe Formation) has been used to link the terranes of the Blue Mountains Province by Middle Jurassic time. New detrital zircon ages dispute this interpretation. The upper portions of the Coon Hollow Formation include grains ca. 250–220 Ma and ca. 165–140 Ma. Maximum depositional age estimates suggest a late Latest Jurassic–Early Cretaceous depositional age (ca. 153–142 Ma). Thus the Coon Hollow Formation is not correlative with Middle Jurassic rocks of the John Day region, either in depositional age or provenance, and instead represents a basin-filling stage that is younger than, and distinct from that of Triassic to Middle Jurassic deposits in the Blue Mountains Province. These deposits (fluvial and marine) accumulated in a large belt of forearc and intra-arc basins that were fed by arc volcanoes at the newly-formed Late Jurassic–Early Cretaceous Andean-type Cordilleran margin. This period of arc magmatism (ca. 170–140 Ma) is recognized in the Klamath Mountains and Sierra Nevada as the Western Hayfork, Rattlesnake Creek, and Western Klamath magmatic episodes and by several newly recognized suites of plutons in the Blue Mountains Province. This result shows that the Coon Hollow Formation cannot be considered part of the allocthonous Wallowa terrane, but instead is part of a regionally extensive Latest Jurassic–Earliest Cretaceous sedimentary overlap assemblage. Recognition of a North American origin for detritus in the overlapping Coon Hollow Formation shows that the Wallowa terrane was accreted to North America by ca. 155–150 Ma. Previous interpretations of initial accretion ca. 135–118 Ma, based on ⁴⁰Ar/³⁹Ar ages in the Salmon River Belt, represent metamorphic cooling ages. They do not constrain the timing of accretion in the region, but rather, represent stages in the metamorphic evolution of the Salmon River Belt during Sevier-related orogenesis.