The Skagit Gneiss (North Cascades, Washington) Revisited: New P-T Results from Recently Discovered Al2SiO5-Bearing Gneiss

The crust of continental arc systems is typically thick and characterized by high heat flow, but few continental arcs have been exhumed to sufficiently deep levels to allow investigation of the metamorphic and magmatic processes that occur at depth. The Skagit Gneiss, a migmatitic unit of the North Cascades, exposes abundant migmatitic meta-igneous and metasedimentary rocks that formed at deep levels of a continental arc. Initial petrologic studies of the Skagit Gneiss suggested that the crystalline core recorded low-pressure metamorphism (Misch, 1968). The application of thermobarometric methods to metapelitic rocks of the Skagit dramatically changed the interpretation of the metamorphic history, revealing pressures of 8-10 kbar at 650-725 ºC followed by nearly isothermal decompression to 3-5 kbar (Whitney, 1992). These P-T studies were based on the 4 metapelitic samples (2 sillimanite-bearing; 2 kyanite-bearing) known at the time. During recent investigation to connect the metamorphic history to the processes and timing of deformation, magmatism, and partial melting, additional metapelite samples were discovered. On Ruby Mountain, near the eastern margin of the Skagit Gneiss, a garnet-sillimanite schist contains cordierite rims on garnet and records a maximum pressure of 9-10 kbar at 750-800 ºC. Garnet-kyanite-cordierite schist yields similar results: 10 kbar at 700 ºC. Both metapelites reveal ca. 60 Ma metamorphic dates, coeval with magmatism and partial melting; however, magmatism and melting continued into the Eocene. Ruby Mountain is key for tectonic models of the North Cascades because a boundary between high-grade rocks of the Skagit Gneiss and a lower-grade oceanic unit is located near the summit. The contact between these two units is a low-angle structure. The presence of syn-kinematic Al2SiO5 in the metapelites suggests that they were deformed during high-temperature metamorphism and decompression, possibly during exhumation of the high-grade rocks in the footwall of the low-angle structure.