SANDSTONE PETROFACIES AND DETRITAL ZIRCON GEOCHRONOLOGY OF THE PERMIAN–EARLY JURASSIC BAKER TERRANE MÉLANGE BELT, NORTHEASTERN OREGON

The relationship between the late Paleozoic–early Mesozoic Baker terrane mélange belt of the Blue Mountains Province, eastern Oregon, and surrounding lithotectonic assemblages is unresolved. The Baker terrane consists of numerous deformed subterranes including (1) argillite-matrix mélange of the outboard Bourne subterrane, and (2) serpentinite-matrix mélange of the inboard Greenhorne subterrane. To resolve Baker terrane–and subterrane–relationships to North America, to the outboard Wallowa terrane, and to each other, we analyzed the petrography of chert-grain sandstone samples from both mélange units of the Baker terrane for comparison to detrital zircon ages from the same units. Baker terrane sandstone samples all represent chert-rich petrofacies indicative of a subduction complex, lithic recycled orogenic provenance. In addition, all samples contain volcanic grains, single-grain plagioclase, and low amounts of monocrystalline quartz. We recognize three distinct chert-rich variations: 1) argillaceous-chert dominated samples (>90%), 2) samples with an elevated percentage of volcanic grains (>10–20%), and 3) samples with abundant detrital carbonate grains and elevated percentage of volcanic grains. Samples from the outboard Bourne subterrane represent group 2 and 3 variants and are generally dominated by Devonian–Permian ages with variable amounts of Precambrian grains ca.2.7–2.5 and 2.2–1.8 Ga. Samples from the inboard Greenhorne subterrane are group 1 and 2 sandstones, and have abundant Precambrian detritus (2.7–2.5 and 2.2–1.8 Ga) and low abundance of Pz or Mz ages. These observations support a well-established distinction between the inboard Greenhorn and outboard Bourne subterranes. The ubiquitous presence of Precambrian grains and argillaceous chert grains suggests that (1) both the Greenhorn and Bourne subterranes originated in a pericratonic setting adjacent to the Olds Ferry arc, and (2) the sediment source was an uplifted accretionary prism. Provenance data are not consistent with derivation from the Wallowa arc which lacks significant chert or Pre-Mississippian ages. The contact between the Borne and Greenhorn subterranes represents an important boundary within an evolving continent-fringing forearc region.