A “FINAL” PULSE OF MAGMATISM IN THE WALLOWA TERRANE, NORTHEASTERN OREGON: LATE TRIASSIC (NORIAN) SLAB DETACHMENT RECORDED IN THE HURWAL FM., SOUTHERN WALLOWA MOUNTAINS

Geologic mapping, detrital zircon U-Pb ages, and sandstone provenance data from sedimentary rocks of the Hurwal Formation (Wallowa terrane, eastern Oregon) provide new information on magmatism and basin subsidence. Detrital zircon U-Pb ages from the Excelsior Gulch unit submarine fan define a unimodal age distribution (n=105) that spans 250–200 Ma (Permian to Late Triassic) with an age mode ca. 212 Ma. This is consistent with published faunal ages and is interpreted to represent a maximum depositional age. All samples plot in the magmatic arc provenance field on a QpLvLsm diagram. On QFL diagrams, samples plot in the undissected arc field and are pulled towards the L pole due to a lack of both mono and polycrystalline quartz grains, a low abundance of plagioclase grains, and a large proportion of volcanic lithic, carbonate mudstone-skeletal wackestone, and chert and chert-argillite grains. On a QmFLt diagram, samples are spread across 1) undissected arc, 2) lithic recycled, and 3) along a mixing line between undissected arc and lithic recycled provenance fields. This mixed provenance signature contrasts with underlying volcanic-only, ca. 230 Ma rocks of the Doyle Creek Formation, which may represent a ridge-subduction event (Kurz et al., 2011). These new data indicate a distinct, Norian-age sediment source composed of syn-depositional volcanic and recycled sedimentary rocks. We interpret these data to reflect the presence of an intrabasinal volcanic high ca. ~212 Ma in the Wallowa forearc region and propose that this Norian volcanic high resulted from a slab-removal event (detachment or tear). Slab removal caused asthenospheric upwelling and emplacement of a Norian quartz diorite pluton in the Sparta complex arc basement, as well as the emergence of a Norian volcanic and sedimentary source to the Wallowa forearc region. Our new results and the work of Kurz et al. (2011) show that magmatism in the Wallowa terrane was episodic and occurred during distinct pulses of (1) ca. 265–249 Ma subduction, (2) ca. 230 Ma slab-window magmatism, and (3) ca. 212 Ma slab-removal. These results are consistent with correlation of the Wallowa terrane to the Kutcho-Cadwallader-Wineglass assemblage, inferred to have entered the Stikinia subduction zone by the end of Norian–Rhaetian time.