STRUCTURE AND AGE OF ORTHOGNEISSES WITHIN THE EAST-CENTRAL SKAGIT GNEISS COMPLEX; INSIGHTS INTO THE MAGMATIC EVOLUTION OF THE NORTH CASCADES ARC

Continental magmatic arcs are a major site for the generation of continental crust. Magma emplacement into arc crust is not constant and undergoes periods of high magma flux. An important tool for understanding the cause of these magmatic flare-ups is to determine their timing and the extent of magmatism. The Late Cretaceous–Eocene North Cascades arc, Washington, is characterized by three flare-ups. The middle, ca. 75–59 Ma flareup is considered the least voluminous but previous estimations of the magmatism did not consider magmatic material within the Skagit Gneiss. The Skagit Gneiss is part of the deeply exhumed mid-crust of the North Cascades arc and is dominated by orthogneiss. This study uses mapping at the 1:24000 and 1:12000 scale and U-Pb zircon geochronology from 21 samples to distinguish among different orthogneiss bodies within the central Skagit Gneiss. Our mapping shows that the central Skagit Gneiss is composed of a series of elongate orogen-parallel (SE-NW) orthogneiss bodies with crystallization ages between 72 and 47 Ma. The more extensive orthogneiss bodies show a general east to west age progression from 72 Ma to 59 Ma. Two SE–NW elongate zones are dominated by 49–47 Ma orthogneiss sheets, which intrude into the older orthogneiss. These areas are near the eastern margin of the Skagit Gneiss and crystalline core boundary, and near the western margin of the Skagit Gneiss. Foliation strikes and lineation trends across the map area are to the northwest. In the central part of the Skagit Gneiss, foliation rotates to the E–W, forming a fold consistent with the regional antiform noted elsewhere in the Skagit Gneiss. Orthogneisses are less strongly deformed, and pegmatite intruding the orthogneiss are abundant within the fold/antiform core. Throughout the map area, orthogneisses as young as 47 Ma are strongly deformed, indicating that ductile deformation occurred late in the tectonic history of this part of the crystalline core. The prevalence of intrusive material from the middle flareup in the Skagit Gneiss indicates that this flareup was likely more voluminous than previously attested and would have added significant heat into the arc crust. This heating may have sufficiently weakened the crust to allow for incorporation of forearc and backarc sediments into the mid-crust of the arc.