Formation and extent of the Cretaceous West Mountains plutonic complex

Abstract

The West Mountains plutonic complex is a series of intrusive bodies, presently located in Idaho, that formed during a continuous period of Early to mid-Cretaceous magmatism in the northwest North American Cordillera. Granitoids of the West Mountains plutonic complex were emplaced across the boundary between accreted terranes and Laurentia following the Jurassic accretion of the Blue Mountains Province. New U-Pb zircon crystallization dates from the West Mountains plutonic complex ranging from 130 to 109 Ma and new zircon εHf isotopic compositions ranging from +12.1 to +4.7 suggest the complex is older and compositionally distinct from the younger than 85 Ma peraluminous granites of the Idaho batholith to the east. Instead, metaluminous compositions and the timing of magmatism indicate the West Mountains plutonic complex is similar to the subduction-related intrusions of the Sierra Nevada and northwest Nevada regions. We present a model for the West Mountains plutonic complex as part of the ca. 130–85 Ma subduction-related Idaho arc. Using new data from the West Mountains plutonic complex, we characterize the tectonic history of the subduction-related Idaho arc as separate from the peraluminous parts of the Idaho batholith, which resulted from crustal melting. The existence of the Idaho arc supports models of a longitudinally continuous Early to mid-Cretaceous magmatic arc in the western United States. The Idaho arc, including parts of the West Mountains plutonic complex, experienced significant transpressional deformation and intrusive overprinting, resulting in a diminished spatial footprint relative to other Cretaceous magmatic centers. This deformation and associated exhumation resulted in the Idaho arc being a significant source of detrital zircons in the central North American Cordillera during mid-Cretaceous time