Provenance and metamorphism of the Swakane Gneiss; implications for incorporation of sediment into the deep levels of the North Cascades continental magmatic arc, Washington

The Swakane Gneiss, interpreted to represent sedimentary strata metamorphosed at 8-12 kbar, is the deepest exposed crustal levels within the exhumed North Cascades continental magmatic arc, yet the nature and age of its protolith and the mechanism by which it was transported to deep-crustal levels remains unclear. Zircons from 11 paragneiss and schist samples were analyzed for U-Pb age and Hf-isotope composition in order to investigate the tectonic history of the Swakane Gneiss from protolith deposition to metamorphism within the North Cascades arc. Zircons interpreted to have crystallized in situ during metamorphism and/or melt-crystallization within the Swakane Gneiss at depth have ca. 74-66 Ma ages. Detrital-zircon age and Hf-isotope characteristics demonstrate provenance shifts that correlate with maximum depositional ages of ca. 93-81 Ma. Samples deposited between ca. 93 and 88 Ma have dominantly Mesozoic age peaks with initial εHf values between depleted mantle and chondritic uniform reservoir (CHUR), whereas ca. 86-81 Ma sample show the addition of distinct Proterozoic populations (ca. 1380 and 1800-1600 Ma) and Late Cretaceous zircons with unradiogenic Hf-isotope compositions. Similar detrital-zircon age and Hf-isotope patterns are observed in several Upper Cretaceous forearc and accretionary wedge units between southern California and Alaska along the North American continental margin. The connection between the Swakane Gneiss and these potential protoliths located outboard of Cordilleran arc systems indicate burial by either underplating of accretionary-wedge sediments or underthrusting of forearc sediments. Therefore, the protolith and incorporation history for the Swakane Gneiss is likely similar to those of deep crustal metasedimentary units elsewhere in the North Cascades (i.e., the Skagit Gneiss Complex) and to the south along the continental margin (i.e., the Pelona-Orocopia-Rand schists and Schist of Sierra de Salinas). These observations suggest that burial of sediment outboard of continental magmatic arc systems may be a major mechanism for the transfer of sediment to the deep levels of continental arcs.