NEW SHRIMP U-PB AGES BEARING ON LATE CRETACEOUS EVOLUTION OF CENTRAL IDAHO

In Cretaceous-Paleocene time, central Idaho was above the active, SW-deepening basal detachment fault of the Montana segment of the Sevier thrust belt. We used the Stanford-USGS SHRIMP-RG to analyze sectioned zircons and apatites and the faces of unpolished zircon crystals embedded in indium for U-Pb age systematics to provide constraints on the Cretaceous structural and magmatic evolution of central Idaho. Near Salmon Idaho, the Brushy Creek (BC) and Poison Creek (PC) faults are top-to-the-east mylonite zones developed from upper plate Middle Proterozoic granites, and the faults are inferred to be Cretaceous in age. Zircons from the BC mylonite yielded a slightly discordant concordia array with an upper intercept near 1400 Ma and a poorly constrained Mesozoic lower intercept. Apatite from the same sample, with Pb closure temperature around 450^oC, yielded an early Neoproterozoic age (888±41 Ma), indicating Mesozoic mylonitization did not reset their U-Pb dates. Zircons from the PC mylonite gave a strongly discordant concordia array with an upper intercept near 1400 Ma and a lower intercept of 92±34 Ma. Pb loss during mylonitization is inferred to have resulted in the rough mid- to Late Cretaceous lower intercept age. About 200 km to the west near McCall Idaho, the Western Idaho Shear Zone (WISZ) is a subvertical, ductile, dextral-slip zone active between 105-90 Ma (Giorgis et al., 2008). A sample from the Little Goose Creek pluton in the WISZ (zircon age of 105.5±2.0 Ma) yielded an apatite age of 82.3±4.6 Ma, compatible with previous ⁴⁰Ar/³⁹Ar mineral cooling constraints. The Idaho batholith between the above areas was sampled near the Au-W-Sb Yellow Pine deposit 80 km east of McCall. Three distinct petrologic phases of the batholith with uncertain relationships occur in that region: Bt granodiorite, Bt-Ms granite and Ms granite. The Bt-Ms granite yielded a zircon age of 94.9±0.8 Ma, the oldest phase of the batholith in that area. Bt granodiorite gave a zircon age of 87.2±0.7 Ma. Previous TIMS zircon ages of Ms granite and alaskite dikes were 85.7±0.1 and 83.6±0.1 Ma (Gillerman et al., 2014), the youngest plutonic phases in the region. Hydrothermally altered Bt granodiorite above Hangar Flats yielded a bi-modal distribution of ages, with a younger age of 77.9±0.8 Ma that may reflect resetting during mineralization.