U-PB AND HF ISOTOPE SIGNATURE OF DETRITAL ZIRCONS FROM THE SWAKANE GNEISS OF THE NORTH CASCADES, WASHINGTON: IMPLICATIONS FOR RAPID BURIAL AND MARGIN-PARALLEL TRANSLATION

The Swakane Gneiss is the deepest exhumed crust within the North Cascades continental magmatic arc, WA. It consists predominantly of quartz–biotite paragneiss that has a Late Cretaceous protolith of unknown provenance and anomalous tectonic history. Arc-related plutons do not intrude the Swakane and it is interpreted to have reached ca. 40 km depth within the arc system by ~73 Ma. One kyanite metapelite, one garnet–biotite gneiss, and three biotite gneisses were collected for detrital zircon and Hf analyses along a foliation-normal (W–E) transect across the northern Wenatchee block to characterize the protolith and understand the tectonic significance of the Swakane Gneiss. Detrital zircon results reveal maximum depositional ages of ~76–80 Ma. All samples are characterized by Late Cretaceous (77­–90 Ma) and Jurassic (150–165 Ma) peaks. Proterozoic (1.4 & 1.7 Ga) peaks and scattered Paleozoic dates are present in 4 of 5 samples. The abundance of Proterozoic zircons increases from W to E, with the westernmost sample lacking dates > 350 Ma. The Hf isotope compositions of the Jurassic to Early Cretaceous zircons are dominantly juvenile (εHf_i = +15 to +5), likely recording an arc-signature. Less radiogenic Hf isotope compositions, indicative of re-working of cratonic rocks, are abundant in 110–70 Ma zircons (εHf_i = +3 to -19). Zircons from both Proterozoic populations (1.4 and 1.7 Ga) yield εHf_i of +10 to -5.

Detrital zircon ages from the Swakane Gneiss are similar to the forearc Nanaimo Basin of SW British Columbia and the Pelona-Orocopia-Rand schists and forearc units of southern CA; however, no Hf data exists from these rocks for comparison. The presence of Late Cretaceous detrital zircons in the Swakane supports rapid incorporation forearc sediments into the arc via east-directed underthrusting shortly after deposition. The Proterozoic zircons in the Swakane have Hf-isotope compositions that are similar to the Mojave province but not to comparable northern sources, such as the Belt Supergroup. This links the protolith of the Swakane Gneiss to SW US sources and supports early Cenozoic margin-parallel transport (the Baja-BC hypothesis). Thus, the U-Pb and Hf signature of detrital zircons from the Swakane Gneiss records both rapid burial and northward translation.