SEDIMENT INCORPORATION INTO CONTINENTAL MAGMATIC ARCS: A CASE STUDY OF THE NORTH CASCADES RANGE, WASHINGTON

Sediment incorporated into the deep levels of continental magmatic arcs can have profound effects on the rheology of the arc and may contribute to magmatic fluxes. The North Cascades Range, Washington, arguably exposes the largest percentage of metasedimentary rock of any exhumed continental arc. The metasediments are located within two crustal domains that represent the deepest portions of the arc and have the youngest cooling ages and ductile deformation, the Skagit Gneiss and the Swakane Gneiss. Both the Skagit and Swakane Gneisses are strongly deformed and likely record a significant portion of the Cretaceous to Eocene transpressional to transtensional deformation history of the arc. Structural relationships, U-Pb zircon dates, and Hf-isotope zircon compositions from metasedimentary rocks from the two Gneiss units suggest that three different bodies of sediment were emplaced at likely mid-crustal (~8–12 kbar) depths within the arc. The U-Pb and Hf analyses indicate that two of the sediment groups were probably sourced from the forearc to the North Cascades arc, whereas mapping on the eastern side of the Skagit Gneiss suggests that backarc sediment was also incorporated into the arc. The sediment was emplaced by ca. 75–65 Ma, coeval with major magmatism within the North Cascades arc. Crystallization of thin (≤ 30 cm thick)layer-parallel leucosomes and dikes within the metasedimentary rocks indicates melting also overlaps this time interval; however, Hf-isotopic compositions of zircon and bulk-rock Nd analyses from the small crystallized melt bodies indicate that they were not sourced from the metasedimentary rocks but rather injected melts from a likely deeper source with a near depleted mantle composition. Therefore, the melts derived from incorporated sediment bodies may not have fueled the coeval magmatic flare-up within the North Cascades arc. The exhumed roots of the North Cascades arc thus reveal how sediment incorporation into an arc may change how deformation is partitioned within arc systems and that sediment incorporation may not play a significant role in magmatic flare-ups.