INCORPORATION OF METASEDIMENTARY UNITS INTO THE DEEPEST EXPOSED LEVELS OF THE NORTH CASCADES CONTINENTAL ARC: COMPARISON OF THE SKAGIT AND SWAKANE GNEISSES

The incorporation of metasedimentary units into the deep crustal levels of continental magmatic arcs may have important consequences for fueling magmatism and/or localizing weaknesses in thickened arc crust. The 10–12 kbar Swakane Gneiss and 8–10 kbar Skagit Gneiss occupy some of the deepest exposed levels of the Late Cretaceous–Eocene North Cascades continental magmatic arc; these units are investigated to understand the mechanisms, timing, and consequences of sediment burial.

The Skagit and Swakane gneisses preserve similar upper-amphibolite facies P-T conditions but have different lithologies and structural settings. The Swakane Gneiss is > 90% quartz–biotite paragneiss, is not intruded by arc-related plutons, and is separated by faults from all other units. In contrast, metasedimentary units make up < 10% of the Skagit Gneiss, which form lenses within voluminous ca. 76–45 Ma tonalitic orthogneiss.

Zircon U-Pb geochronology and Hf-isotope analyses from 11 Swakane and 8 Skagit samples provide insight into the metamorphic history and protolith of these units. The youngest detrital zircons reveal maximum depositional ages (MDAs) for the Skagit samples mostly between ca. 134–96 Ma, with one ca. 79 Ma outlier, and, for the Swakane, between ca. 93–81 Ma. Skagit and Swakane samples with MDAs > ca. 87 Ma dominantly have Mesozoic age peaks and initial Hf-isotope values between CHUR and depleted mantle compositions. The ca. 79 Ma Skagit and ca. 86–81 Ma Swakane samples also have Proterozoic populations (ca. 1.8–1.6 Ga and 1.38 Ga) and distinctly unradiogenic Hf-isotope compositions for Late Cretaceous detrital zircons. Age and Hf-isotope patterns with MDA for all samples match trends observed in forearc and accretionary wedge units and suggest that protoliths were incorporated by underthrusting or underplating of sediments from west of the arc. The Skagit and Swakane metasedimentary rocks were incorporated into and buried within the arc at similar times, between ca. 74–65 Ma and 79–66 Ma, respectively, coeval with a pulse of magmatism in the North Cascades. These results demonstrate the variability in protolith age of sedimentary material incorporated within the crystalline core, despite the similar protolith source and timing of burial.