THE WEST MOUNTAIN ARC: EARLY-MIDDLE CRETACEOUS MAGMATISM IN THE NORTHWEST U.S. CORDILLERA

Continuous subduction-related magmatism from ~128-85 Ma indicates the presence of an extensive Cretaceous magmatic arc – named the West Mountain arc – in the Pacific Northwest of the United States. The West Mountain magmatic arc is most continuously exposed in the Hazard Creek complex and eastern part of West Mountain, both east of New Meadows, ID. This arc occurred after older magmatism associated with the collision of the Blue Mountain terranes with North America. However, the West Mountain arc is earlier, and compositionally distinct from the peraluminous granites of the Idaho batholith. The granitic rocks of the West Mountain arc have compositions varying from tonalites to granodiorites, and are metaluminous. The West Mountain arc includes: 1) The Hazard Creek complex, except for those intrusions that are older and part of a distinct Late Jurassic magmatic arc; 2) The Little Goose Creek complex; 3) Igneous intrusions of the western part of West Mountain; 4) Intrusions into the Olds Ferry and/or Baker terrane in eastern Oregon (including the Lookout Mountain); and 5) The older parts (~90-85 Ma) of the Idaho batholith – the early metaluminous suite and border zone suite – that are metaluminous and show similar compositions to the older Hazard Creek complex. New data from the Hazard Creek complex and the West Mountain area yields U-Pb zircon crystallization dates ranging from 128 to 109 Ma, with εHf isotope compositions ranging from +12.1 to +4.7. Given the strong deformation and exhumation of these eastern margin of the West Mountain arc associated with the ~100-85 Ma western Idaho shear zone, these granites were likely a significant source of detrital zircons in the central North American Cordillera.