Abstract:
The upper amphibolite facies Koolen gneiss dome was subjected to 600–800 MPa pressures and >700 °C temperatures at deep structural levels, and 400–500 MPa and ∼600 °C temperatures at shallow levels during peak metamorphism in the Cretaceous. High strains accompanied the pre-104 Ma metamorphism and formed a penetrative L-S tectonite fabric that deμnes the Koolen dome. The complex was exhumed in three stages between 104 Ma and 84 Ma based on 40Ar/39Ar (hornblende, mica, and K-feldspar) thermochronology. Decompression of the complex was between 104 and 94 Ma and was followed by emplacement of epizonal plutons. Rapid cooling to <200 °C, likely associated with doming of the complex, occurred between 94 and 88 Ma. Final cooling and exhumation of the complex was coeval with motion along a low-angle fault between 88 and 84 Ma. Volcanic and subvolcanic rhyolites were emplaced across the area ca. 84 Ma. Decompression and doming of the complex was coeval with plutonism and is interpreted to be due to internal buoyancy. Final exhumation was coeval with extensional (?) low-angle faulting.