REASSESSING TERRANE BOUNDARIES IN THE BLUE MOUNTAINS PROVINCE OF EASTERN OREGON USING DETRITAL ZIRCON U-PB AGES

One of the most complete and least deformed early Mesozoic stratigraphic records in the western U.S. Cordillera is present in the terranes of the Blue Mountains Province of eastern Oregon and western Idaho. Here, in marine rocks of Late Triassic through Cretaceous age, many unambiguous sedimentary structures, depositional contacts, and map relationships are preserved, allowing for detailed analysis of depositional and tectonic history. Traditionally defined terranes in the Blue Mountains Province include the Wallowa, Baker, Olds Ferry, and Izee terranes. The Wallowa and Olds Ferry terranes contain late Paleozoic to early Mesozoic igneous and sedimentary rocks that formed in two facing magmatic arcs. The intervening Baker terrane represents a deformed subduction-accretion complex and the Izee terrane consists primarily of Late Triassic–late Middle Jurassic sedimentary rocks. Detrital zircon U-Pb data challenge these traditional terrane definitions and provide unique insight into possible connections between areas previously considered distinct terranes. In the John Day region of central Oregon, Permo-Triassic rocks of the Baker terrane and Triassic rocks of the Izee terrane (Vester Fm.) contain an identical age distribution of Late Paleozoic and ca. 1.8–2.7 Ga grains. In contrast, structurally adjacent Late Triassic rocks of the Aldrich Mountains Group, typically mapped as Izee terrane contain a distinct age distribution of Silurian and 1.0–2.0 Ga grains. Structural and stratigraphic intercalation of these two age distributions has been used to argue for a northwestern-derived, Laurentian-Baltican origin for numerous subduction-accretionary complexes in the Cordillera. These data indicate a provenance affinity between rocks of the Baker terrane and pre-Early Jurassic rocks of the Izee terrane (Vester Fm. and Aldrich Mountains Group). In contrast, data from the Baker terrane (Burnt River Schist) and Izee terrane in the Huntington region of eastern Oregon are unimodal with ages centered on ca. 200 Ma. These data indicate a provenance affinity between rocks of the Baker terrane and post-Early Jurassic rocks of the Izee terrane (Weatherby Fm.). In both regions, new detrital zircon data suggest that the traditional terrane distinction between Baker and Izee terrane rocks is invalid.