LOADING AND METAMORPHISM WITHIN THE SALMON RIVER SUTURE ZONE, WEST-CENTRAL IDAHO

Medium- to high-grade schist and amphibolite of the Salmon River suture zone (SRSZ) in west-central Idaho record mid-crustal deformation during Jurassic to Cretaceous assembly of the Blue Mountains province and amalgamation to cratonic North America. These rocks provide important constraints on the rates, processes, and timing of terrane collision during crustal thickening. Pressure-temperature-time (P-T-t) paths from pseudosections, garnet compositional zoning, and thermobarometry suggest that tectonic loading was followed by near isobaric cooling. Counter-clockwise P-T-t paths are inferred from garnet core compositions, peak mineral assemblages, and mineral-rim compositions. Garnet Sm-Nd indicates that peak metamorphic ages vary across two major thrust plates: the Rapid River (RR) and Pollock Mountain (PM). High-grade amphibolite from the PM plate records 141-124 Ma garnet zone metamorphism (peak =675ºC, 8 kbar) and is thrust on top of dominantly medium-grade schist from the RR plate that underwent 112.5 Ma garnet zone metamorphism (peak ≤655ºC, 8.5 kbar). The discrepancy in age suggests that juxtaposition occurred after high-grade metamorphism in the PM plate and prior to medium-grade metamorphism in the RR plate. Thrusting and thermal relaxation or conductive heat flow from the Idaho batholith could be responsible for the younger metamorphism in the RR plate. However, the spatial distribution and the known ages of plutons are not compatible with contact metamorphism; therefore, a thrust origin is most likely. The medium-grade RR rocks were thrust over low-grade volcaniclastic rocks of the Seven Devils Group in the Wallowa arc terrane, which were likely metamorphosed after ca. 112 Ma. This stacking of increasingly higher temperature, older metamorphic rocks upon younger, lower temperature rocks suggests that the youngest metamorphism may also be related to progressive tectonic burial by thrusting. Integration of garnet geochronology and P-T-t paths across the SRSZ produces a detailed model for metamorphism, thrusting, and magmatism which is compatible with sequential NW directed thrusting from SE to NW.