26-9 Investigating arc crust evolution: the link between metamorphism and magmatism during a high-flux magmatic event

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

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Abstract:

The middle to deep crust of continental magmatic arcs continually evolves as magmatic flare-ups introduce heat and potentially consume supracrustal rocks, and fluids and partial melts are released from the host rocks during metamorphism. The Late Cretaceous–Eocene North Cascades continental magmatic arc, Washington, exposes a crustal section, with abundant metasedimentary rocks, orthogneisses, metavolcanics, and plutons exhumed from up to >40 km depth. Geochronology data from plutons indicate that there were three high-flux magmatic events throughout the arc history. Here we investigate the metamorphic and partial melting record preserved within the metasedimentary rocks found among the igneous rocks via U-Pb zircon and Sm-Nd garnet geochronology. Zircon from samples of two deep-crustal metasedimentary units from within the central to northeastern part of the exhumed crystalline core of the arc reveal that the units were metamorphosed at 6–10 kbar and 10–12 kbar from ca. 75–65 Ma. These units contain sediment that was sourced from the forearc and underthrusted into the mid crust of the arc. In comparison, metasedimentary rocks within the eastern bounding fault system record younger Sm-Nd garnet metamorphic ages of ca. 60 Ma. The first fault system sample represents sediment sourced from the backarc that was buried to pressures of 9–11 kbar within a step-over zone in the fault system. The other sample was collected farther south and is from one of the host-rock units; it achieved 7 kbar pressures. All the metasedimentary units reveal metamorphism during the middle flare-up, with sediment incorporation from the forearc occurring during the early to middle stages of the flare-up and sediment in the fault system recording metamorphism during the final stages. Subsequently, the rocks underwent near-isothermal (~600–800 °C) decompression, partial melting and intrusion by melts starting at ca. 68 Ma and ending at ca. 45 Ma. Magmatism associated with the middle flare up likely weakened the crust allowing for incorporation of sediment and burial of the host rocks during regional shortening associated with the convergent margin. The record in the metasedimentary rocks indicates that the thickened arc crust remained hot until the final flare-up from ca. 50–45 Ma, when slab break off caused orogenic collapse and exhumation of the deeper crustal rocks.

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