Tectonic Evolution of the Labyrinth Mountain Area, Cascades Crystalline Core, Washington: Unraveling Intertwined Deformation, Intrusion and Metamorphism
Samarium-neodymium garnet geochronology, uranium-lead zircon geochronology and detailed structural and petrologic analyses place new constraints on the relationships between deformation, intrusion, metamorphism and loading in the southern Nason terrane, Cascades Crystalline core. Gamet leaching experiments result in high-precision Sm-Nd isochron ages for peak metamorphism of the Nason Ridge Migmatitic Gneiss synchronous with 90.6 ± 2.4 Ma emplacement the Labyrinth Mountain granodiorite pluton. Garnets in the pluton wall-rock thus likely grew in response to heating during magmatism. Intrusion and deformation in schist rafts and pluton wall-rock indicate that magmatism was active during NE-SW arc-normal contraction and was accommodated by wall-rock disaggregation, stoping, NE-SW shortening in the wall-rock and corresponding NW-SE extension. Peak metamorphic conditions preserved in the host rock at the time of the Labyrinth Mountain pluton emplacement are 6.6-8.4 kbar and 630-670 °C. This suggests that pressure increased at least 2.2 kbar after 93.5-96 Ma emplacement of the nearby Mount Stuart batholith. All data are compatible with SW-directed upper-crustal thrust-loading and mid-crustal folding prior to and synchronous with ca. 91 Ma magmatism and peak metamorphism. Microstructures evidence foliation development resulting from ductile shortening that locally postdates ca. 90-91 Ma garnet growth.