PERMIAN–TRIASSIC RECORD OF NASCENT SUBDUCTION IN THE BAKER TERRANE AND VESTER FORMATION, BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON

Along the western margin of Laurentia, east-dipping Cordilleran subduction initiated following the Sonoma orogeny and closure of the Slide Mountain-Golconda basins. Post-Sonoma subduction has been interpreted by the presence of Late Permian to Late Triassic plutons in Arizona and California and by ca. 230 Ma blueschists in the accreted terranes of the Sierra Nevada foothills, Klamath, and Blue Mountains. Here, we suggest further that chert-rich clastic rocks in the Baker terrane and Vester Formation (Izee megasequence 1), Blue Mountains Province eastern Oregon, also record the early stages of subduction along the western U.S. plate margin. Previous observations on the petrography and detrital zircon ages of chert-grain sandstone from mélange units of the Baker terrane and from the Vester Formation support a well-established distinction between the inboard Greenhorn and outboard Bourne subterranes. 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. Sandstone samples from the Badger Creek unit of the Baker terrane and from the Vester Formation all represent chert-rich petrofacies with abundant volcanic grains and single-grain plagioclase; all samples contain low amounts of monocrystalline quartz. Samples plot within subduction complex and lithic recycled orogenic provenance fields. Detrital zircon analysis shows that the rocks contain variable amounts of Precambrian grains ca. 2.7–2.5 and 2.2–1.8 Ga and Devonian–Triassic ages. The youngest age modes present in these chert-rich clastic rocks is ca. 230 Ma. These sandstone petrofacies and detrital ages are consistent with erosion of a source area containing both northern Laurentian cratonic zircon and younger arc sources, as well as uplifted oceanic rocks. We suggest that the source area is consistent with Late Triassic subduction-related uplift of late Paleozoic peri-Laurentian arcs and subduction complexes accreted to the plate margin during the Sonoma orogeny. In this scenario, Late Permian to Triassic subduction initiation resulted in the uplift of oceanic chert and the accreted Sonoma collider which shed detritus into evolving Late Triassic basins.