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

• Published in: Lithos 2012-08, Vol.146-147, p.128-142
• Main Authors: Gatewood, Matthew P., Stowell, Harold H.
• Format: Article
• Language: English
• Subjects: Garnet Sm–Nd; North Cascades; Orogenic loading rates; Swakane Gneiss; Synorogenic basin evolution; Zircon U–Pb
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2012.04.030

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 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. ► We present new depositional and metamorphic ages for the Swakane Gneiss. ► Garnet ages of 73.5±1.2Ma, 71.3±2.8Ma, and 65.8±0.7Ma date metamorphism. ► Zircon rim ages are mixed between a ca. 91Ma detrital and a ca. 71Ma metamorphic component. ► The Swakane Gneiss protolith was deposited in an intra-arc basin after 91Ma and before 75Ma. ► The Swakane Gneiss was loaded at rates of 1–3mm/yr.