CONTRIBUTION OF METASEDIMENTARY ROCKS TO SMALL SCALE MELTS WITHIN THE DEEP LEVELS OF CONTINENTAL ARCS: INSIGHT FROM THE SWAKANE AND SKAGIT METASEDIMENTARY ROCKS AND ASSOCIATED MELT GENERATIONS

The incorporation of metasedimentary rocks into the mid to deep crust of continental magmatic arcs provides fertile material for the production of small, outcrop-scale partial melts to orogen-scale, magmatic flare-up events. To better understand the partial-melting history of metasedimentary rocks within the crystalline core of the Late Cretaceous–Eocene North Cascades arc, Washington, seven ~5–35 cm thick, layer-parallel leucosomes and 6 dikes and other felsic intrusive material were collected from the most deeply exhumed components of the crystalline core, the ~9–12 kbar Swakane and ~8–10 kbar Skagit gneisses, for split-stream, LA-ICPMS U-Pb zircon geochronology and trace-element chemistry. Hf-isotope compositions were then obtained from a subset of zircons. The studied layer-parallel leucosomes were all strongly deformed and, based on their textural appearance and shared deformation and interlayering with the host metasedimentary rocks, were interpreted as likely in situ melts. Crystallization ages of layer-parallel leucosome samples of the Skagit Gneiss range from ca. 69 to 48 Ma. In comparison, similar samples from the Swakane Gneiss have ages clustered tightly from ca. 73–66 Ma. The dikes and larger bodies of late felsic material that intrude the Swakane Gneiss crystallized from ca. 74–68 Ma. Two of the Skagit layer-parallel leucosomes yielded a very limited range of radiogenic εHf_i values ranging from +11.9–+8.8, with two outliers at +6.5 and -2.4. Zircons from one of the Swakane leucosomes also yielded a very limited range of εHf_i values of +6.1–+3.4. Conversely, zircons from non-deformed felsic material that intrudes the Swakane had a wide range of εHf_i values from +12.3 to -15.6. The majority of the leucosomes did not contain apatite or monazite and overall had low P₂O₅ amounts (≤ 0.01 wt. %) despite the abundant apatite in Swakane and Skagit metasedimentary rocks. These results suggest that many of the strongly deformed leucosomes within the gneisses were not in situ melts from the host metasedimentary rocks, but likely were derived from a deeper, more radiogenic source. The majority of melting of the Skagit Gneiss may have instead contributed to pluton and orthogneiss bodies found throughout the crystalline core, whereas the more silicic Swakane Gneiss likely only underwent very localized melting.