GARNETS AND ZIRCONS AGREE: GEOCHRONOLOGIC CONSTRAINTS ON LATE CRETACEOUS LOADING AND METAMORPHISM IN THE CASCADES CORE, WA

New 90-91 Ma Sm-Nd garnet ages in the Nason Ridge Migmatitic Gneiss (NRMG) are synchronous with a U-Pb zircon age from an adjacent granodiorite pluton northeast of the Mount Stuart batholith, suggesting that peak metamorphism was locally related to heating during intrusion. High molarity sulfuric and hydrochloric acid garnet leaching employed to remove potential LREE enriched inclusions yields high precision (+/- 1.1 Ma) garnet-whole rock and garnet-matrix isochron ages. Metamorphic textures throughout the Labyrinth Mountain to Jove Peak study area support thermobarometric data and indicate that significant loading had occurred prior to 91 Ma. This conclusion is compatible with the results from previous studies, which reveal nearly isobaric 88-86 Ma heating and garnet growth in the Chiwaukum Schist and NRMG northwest of the study area. The absence of structural discontinuity between the 91 Ma Labyrinth Mountain pluton and the nearby 96 Ma (Matzel, 2004) Mount Stuart batholith make the study area particularly well suited for constraining the timing of loading. Conservative estimates for pressure increase result in a loading rate of at least 1.7 km/m.y. Structural fabrics evidence continued NE-SW contraction and ductile shortening during and after 91 Ma peak metamorphism, an observation which may offer insight into terrane-wide mid-crustal processes responsible for creating an apparently steep isobaric gradient.