Linking zircon U–Pb and garnet Sm–Nd ages to date loading and metamorphism in the lower crust of a Cretaceous magmatic arc, Swakane Gneiss, WA, USA

Abstract

Synorogenic basin development and subsequent collapse and loading provide important constraints on the Late Cretaceous crustal structure of the North America Cordillera. High grade metasedimentary rocks of the Swakane Gneiss in the North Cascades have been interpreted to include ca. 73Ma detrital zircon that requires a protolith age considerably younger than metamorphic ages of adjacent terranes. This detrital interpretation requires rapid loading rates of 7–8mm/yr, heating to 750°C in <5myr, and partial exhumation prior to ca. 68Ma. We present new U–Pb zircon and Sm–Nd garnet dates for the Swakane Gneiss that directly date metamorphism and provide a basis for evaluating this model of Late Cretaceous deposition, loading, and crustal structure.Garnet preserves compositional evidence for growth during high pressure amphibolite-facies metamorphism and little subsequent modification. New garnet–rock Sm–Nd isochrons of 73.5±1.2, 71.3±2.8, and 65.8±0.7Ma, tightly constrain the timing of garnet growth. Swakane Gneiss zircons preserve complex magmatic, detrital, and metamorphic histories. Most zircons have distinct cores and homogeneous overgrowths. Detrital zircon cores include significant Proterozoic and Mesozoic components with low U/Th ratios. Homogeneous metamorphic zircon rims have high U/Th (>5) and define an array of concordant U–Pb dates from 75 to 63Ma. The new zircon U–Pb and garnet Sm–Nd ages establish that the youngest regional metamorphism was diachronous and occurred between 75 and 63Ma. Our dataset precludes post 73Ma deposition and instead suggests that sedimentation occurred after ca. 91Ma and ended before ca. 75Ma. Using these age constraints for deposition and metamorphism, we propose new loading rates of 1–3mm/yr and conclude that the Swakane Gneiss protolith was likely deposited in a small, short-lived, intra-arc basin that subsided, filled in, and collapsed quickly due to overthrusting of arc crust.