Lithology, Structure, Geochronology, and Tectonic Implications of the Spider Glacier Unit and Holden Assemblage, North Cascades, Washington

To better understand the effects of deeply emplaced sediments on continental magmatic arcs, researchers study crustal cross sections where meta-sediments are exposed at the surface, such as the crystalline core of the North Cascades of Washington. The 8–12 kbar Swakane Biotite Gneiss has been considered the deepest meta-sedimentary unit in the section, but my mapping shows that the adjacent Spider Glacier unit is probably structurally lower. The Spider Glacier unit consists of biotite and hornblende gneisses, quartzites and rare meta-peridotites, all deformed by highly variably oriented folds with a mean moderate west plunge. My U-Pb dating of detrital zircon indicates that the maximum depositional age (MDA) of the Spider Glacier unit is ~118–110 Ma, which is older than the 93–81 Ma MDA of the Swakane Gneiss. The only rare folding in the Gneiss implies that a tectonic contact separates it from the Spider Glacier unit. Based on rock types and detrital zircon age patterns, the Spider Glacier unit was likely deposited outboard of the Cordilleran arc at 110 Ma or later, incorporated into the arc and folded and metamorphosed between ~110–74 Ma, and imbricated with the Swakane Gneiss between 74–65 Ma. The age data imply that the Spider Glacier unit is a Cretaceous component of the Cascade River-Holden assemblage. U-Pb ages of zircons in the Holden Village area support recent work suggesting that the assemblage includes metamorphosed Late Permian intrusive rocks. Permo–Triassic arc rocks of the assemblage are likely separated from the Cretaceous component by an unrecognized tectonic boundary or unconformity