TIMING AND DURATION OF MIGMATITE CRYSTALLIZATION IN THE POTTERS POND MIGMATITE DOMAIN, WESTERN IDAHO SHEAR ZONE: RELATIONSHIPS BETWEEN PARTIAL MELTING AND DEFORMATION DURING TRANSPRESSION

The Potters Pond migmatite domain (PPMD) is a zone of structurally and compositionally heterogeneous migmatites located ~10 km to the southwest of Cascade, Idaho, within the Western Idaho shear zone (WISZ). The PPMD is the only known exposure of migmatites within the WISZ over its ~300 km length, occurring where the shear zone orientation changes from 020° south to 000° north of the migmatite domain. The exclusive occurrence of migmatites within this dextral transpressional restraining bend offers a unique opportunity to evaluate the interplay of partial melting and deformation processes during the evolution of a major crustal shear zone. Structural mapping within the migmatite domain has identified multiple generations of migmatite (e.g., metatexite, melanocratic and garnet bearing diatexite) and varied structural fabrics. Melt-present deformation features (e.g., leucosome in dilatant structural sites) and solid-state fabrics (e.g., shear bands, stretched quartz) are ubiquitous throughout the PPMD. Leucosomes were sampled from structurally and compositionally distinct migmatite localities in order to characterize the timing and duration of partial melting and its relationship to deformation in the WISZ. U-Pb age determinations of zircon by means of LA-ICP-MS document an extended period of migmatite crystallization through the Early Cretaceous and into the Late Cretaceous. Analyses of migmatite samples suggests that the Potters Pond migmatite domain records two main protracted periods of crystallization; ²⁰⁶Pb/²³⁸U ages dominantly range from ca. 145 to 128 Ma and ca. 104 to 90 Ma for rim overgrowths and new zircon growth attributed to migmatite crystallization. These analyses, coupled with field observations of melt-present deformational structures, indicate coeval migmatite crystallization and deformation in the western Idaho shear zone. Additionally, these analyses suggest some migmatite crystallization occurred prior to western Idaho shear zone deformation, and is likely associated with the docking of the Blue Mountains terrane with ancestral Laurentia. Migmatization during WISZ deformation indicates that, at least locally, a partially molten lower crust was an influential factor affecting the deformational processes responsible for the evolution of the WISZ.