DIFFERENTIATING METASEDIMENTARY BODIES OF THE SKAGIT GNEISS IN THE CRETACEOUS-EOCENE NORTH CASCADES ARC TO DETERMINE PROCESSES OF SEDIMENT INCORPORATION IN CONTINENTAL MAGMATIC ARCS

­­­­Sediment incorporated into the mid- to deep-levels of continental magmatic arcs significantly modifies the composition, rheology, and petrology of arc crust. Metasedimentary rocks are exposed in the Skagit Gneiss within the exhumed mid to lower crust of the Cretaceous–Eocene North Cascades arc. Previous studies suggest that multiple sediment packages were incorporated by underthrusting material from both the western (forearc and accretionary wedge sediments) and eastern (backarc sediments) sides of the arc. Furthermore, metasediments interpreted to be sourced from the forearc and/or accretionary wedge show detrital zircon ages with two distinct patterns: Group 1 metasediments have Proterozoic–Paleocene ages with large Late Cretaceous–Paleocene peaks, whereas Group 2 have Late Triassic–Paleocene ages but lack Proterozoic ages. These sediments were incorporated from ca. 74–65 Ma during a period of high magmatic addition rates, suggesting a link between these processes.

This investigation further differentiates bodies of metasedimentary rocks throughout the Skagit Gneiss using field mapping, geochronology, pressure-temperature (P-T) data, and geochemistry. Preliminary pressure data, using the trapping pressure of quartz in garnet (QuiG), suggest pressures in the range of 4-8 kbar for three samples from Group 2 metasediments. Garnet-biotite, titanium-in-biotite, and THERMOCALC average P-T thermometry indicate temperatures ranging from 650–800°C for six samples (one from Group 1 and five from Group 2). The preliminary P-T data does not provide clear distinctions between the groups. However, bulk rock major- and trace-element geochemistry show Group 1 samples have higher Si and lower Fe, Mg, and Mn, and are slightly enriched in light rare earth elements and depleted in heavy rare earth elements compared to Group 2 samples. Thus, Group 1 metasediments appear to be metamorphosed from more mature sediments than Group 2 metasediments. Overall, a clearer understanding of the dominant processes for sediment incorporation in the Skagit Gneiss and how these different metasediments bodies affected the evolution of the North Cascades will be gained by identifying the differences in the metasediment packages and using this data to interpret their potential protoliths and incorporation history.