RECONSTRUCTING THE WESTERN IDAHO SHEAR ZONE: A GEOCHRONOLOGY FRAMEWORK

The North American Cordilleran Cretaceous magmatic arc has remarkable similarities along its length: it intrudes across accreted terranes and continental crust, resulting in a gradational geochemical boundary marked by the ⁸⁷Sr/⁸⁶Sr = 0.706 isopleth, and it underwent several magmatic flare-up events, including during the mid-Late Cretaceous (110-85 Ma), with magmatism becoming increasingly felsic and younger to the east. In contrast, the Idaho batholith intruded into continental crust further east than other Cordilleran arcs, and present exposures are dominated by plutonic rocks younger than 87 Ma. However, Gaschnig et al. (2016) document 98-87 Ma inherited zircon in younger plutons across the current footprint of the Idaho batholith, and suggest the existence of an older arc that has been partly assimilated into younger intrusions and largely eroded away. Moreover, the Western Idaho shear zone (WISZ) forms the western boundary of the Idaho batholith and its steep isotope gradient marks the western boundary of the continent. Limited geochronology of WISZ orthogneiss indicates plutons intruded 118-104 Ma and underwent intense transpression and metamorphism from 105-90 Ma. Here, we present new zircon U-Pb ages and Hf isotope results from the WISZ to provide a framework for better understanding the pre-deformational magmatic history of the middle Cretaceous arc in western Idaho.

We collected eight samples from the Hazard Creek and Little Goose Creek complexes, which form north-south trending belts of the WISZ and are mapped in detail near McCall, Idaho. Seven samples from the Hazard Creek complex yield dates of approximately 162 Ma, 128 Ma (with a well-defined 203 Ma population of inherited zircon), and 120-109 Ma; the Little Goose Creek sample yields a date of 113 Ma. Two samples from South Mountain, located south of McCall, yield dates of approximately 113 Ma (Council Mountain pluton) and 180 Ma (No Business pluton). Although there is no obvious spatial trend to U-Pb ages or composition of plutons, Hf data indicate that these samples are less radiogenic to the east (ε_Hf =+10.1 to +4.7), consistent with previous Nd and Sr studies. Our data suggest that the Hazard Creek and Little Goose Creek complexes overlap in age and that the 120-100 Ma Cretaceous arc in Idaho extended the length of the WISZ deformational belt.