Cretaceous partial melting, deformation, and exhumation of the Potters Pond migmatite domain, west-central Idaho

Abstract

The Potters Pond migmatite domain (PPMD) is a heterogeneous zone of migmatites located ∼10 km 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 024° south to 005° north of the migmatite domain. Structural mapping within the PPMD has identified multiple generations of migmatite with varied structural fabrics. Leucosome layers were sampled from distinct migmatite localities and morphologies (e.g., metatexite and diatexite) to determine the timing and duration of partial melting in the PPMD. U-Pb age determinations of zircon by means of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) document two periods of protracted migmatite crystallization during the Early and Late Cretaceous. Early Cretaceous (ca. 145–128 Ma) migmatite crystallization ages are coeval with the collision and suturing of oceanic terranes of the Blue Mountains province with North America and the formation of the Salmon River suture zone (SRSZ). Migmatite crystallization ages from ca. 104–90 Ma are associated with Late Cretaceous dextral transpression in the WISZ. Field observations and geochronology of crosscutting leucosome relationships are interpreted to record deep crustal deformation and anatexis associated with formation of the SRSZ, subsequently overprinted by solid-state deformation and renewed anatexis during the evolution of the WISZ. These data are the first direct evidence of the synmetamorphic fabric related to the SRSZ east of the initial Sr 0.706 isopleth and indicate that the WISZ is a temporally distinct overprinting structure.