GEOLOGY OF THE BEAVER CREEK STOCK IN CLEARWATER COUNTY, IDAHO

The Beaver Creek Stock is located within the Clearwater National Forest. Regionally significant faults cut directly through the study area. The identification of crosscutting relationships within these systems imposes important age constraints. This igneous intrusion, originally thought to be of Cretaceous age, is now believed to be of Tertiary age. U-Pb dating of zircons was utilized to resolve this issue. The mineral cordierite occurs within the Beaver Creek Stock ranging in size up to one inch, as reported by Anna Hietenan in 1963. This is the only occurrence of cordierite associated with either the Cretaceous Idaho Batholith or other Tertiary intrusions. The formation of cordierite is one of the main petrologic questions addressed during the course of this project. Cordierite is most commonly formed through assimilation of sedimentary material, although it can originate through magmatic crystallization. The presence of cordierite also imposes distinct pressure and temperature constraints on the system. Aluminum-in-Hornblende Geobarometry was used to determine the pressure and depth of intrusion of the stock. In addition to the unusual occurrence of cordierite, this intrusion exhibits a complex petrologic history and very diverse lithologies within its suite of rocks. The lithologies include cordierite-bearing and non-cordierite-bearing granite, diorite, olivine gabbro, pyroxene gabbro, and hornblende gabbro.