INSIGHT INTO THE DEFORMATION AND MAGMATIC HISTORY OF A LONG-LIVED ARC: THE RECORD PRESERVED IN METASEDIMENTARY ROCKS OF THE NORTH CASCADES ARC, WA

Incorporation of metasedimentary rocks into continental magmatic arcs may have a significant effect on how strain is partitioned within arc crust and may lead to the production of potentially large volumes of felsic melt. Abundant metasedimentary rocks are exposed throughout the Cretaceous–Eocene North Cascades continental magmatic arc, Washington, between two major strike-slip faults. The Skagit Gneiss Complex (SGC) is one of the most deeply exhumed (8–10 kbar) bodies within the arc crystalline core and consists primarily of orthogneiss and in the northern SGC, metasedimentary rocks. Highly deformed paragneiss rocks indicate that the SGC accommodated much of the strain in the arc during and after the protolith sediment was incorporated. Geochemical analyses and U-Pb and Hf isotopic composition of detrital zircons indicate that the northern SGC consists of two groups of metasedimentary rocks, both of which were derived from either an accretionary wedge or forearc source and incorporated into the arc from ca. 74–65 Ma. Sediment of back-arc affinity was also metamorphosed to similar 8–10 kbar conditions within a step-over in the eastern bounding fault zone, representing a third group of metasediments. The metasedimentary rocks within the fault system tend to have limited to no metamorphic zircon growth, with a few zircons suggesting these bodies were buried to mid-crustal depths by 74 Ma. Overall, the three metasedimentary groups were likely incorporated into the deep levels of the arc during the second of three high magma addition events. New epsilon Nd combined with previously published data reveal that plutons associated with this second ‘flare-up’ from ca. 77–60 Ma yield juvenile εNdi values of 3.1–5.2. These are similar in isotopic composition to the earlier ‘flare-up’, 2.0­–5.4 for ca. 96–84 Ma plutons, but more juvenile in comparison to the majority of SGC metasedimentary rocks whose εNdi values range from -7.9 to 6.0. Overall, these results suggest that the incorporation of sediment from both sides of the arc affected deformation within the arc but did not drive the production of a large volume of magma. Instead, the ca. 74–65 Ma pulse of magmatism probably weakened the crust and combined with likely changes in the highly oblique nature of the plate boundary, facilitated sediment incorporation into the arc.