195-13 Quantifying the Contribution of Orthogneisses to Flare-up Magmatism in a Deforming Arc: North Cascades Arc, Washington

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism (Posters)

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Continental magmatic arcs are characterized by periods of high flux magmatism (flare-ups). The presence of widespread melt and elevated geotherms during these periods reduce crustal strength and can change how deformation is accommodated in the crust. Generally magmatic flux is calculated using plutons within exhumed arc sections. However, arc systems can also contain large amounts of deformed orthogneiss, but their contribution to magmatic flux has largely been ignored. The North Cascades continental magmatic arc, Washington, experienced three periods of high flux magmatism from ca. 96–87 Ma, ca. 78–60 Ma, and ca. 50–45 Ma based on estimates from plutons within the exhumed arc. The Skagit Gneiss represents one of the most deeply exhumed (~8–10 kbar) units of the arc. It consists of ~1200 km² of orthogneiss and lesser metasedimentary rocks. Here we combine new U-Pb zircon geochronology from 18 orthogneiss samples and whole rock geochemistry from 9 samples, along with previous geochronology work and field mapping (Shae et al., 2008; Michels et al., 2008; Wegener et al., 2024; Hoinville et al., pers comm), to define magmatic addition rates during the second and third flare-up events. These data suggest that the Skagit Gneiss is composed of several orogen parallel (NW–SE) sheeted orthogneiss units that crystallized during the middle flare-up between ca. 78–60 Ma. These large (~250 km²) orthogneiss units are intruded by younger, relatively smaller (~20 km²) sheeted units that crystallized during the last flare-up event from ca. 50–45 Ma. Our new data indicate that during the ca. 78 –60 Ma high flux event, >1000 km² magmatic area was added. Subsequently, ca. 50–45 Ma high flux event represents only a minor (~50 km²) magmatic area addition. Interestingly, the magmatic area addition from the plutons during the ca. 78–60 Ma high flux event is comparable to the orthogneisses but is significantly larger for the younger ca. 50–45 Ma period. In addition, pegmatites and dikes that crosscut the Skagit Gneiss indicate that ductile deformation was active during ca. 66–47 Ma, suggesting that high flux magmatism may have played an important role in how deformation was partitioned within the arc. More importantly, our data indicates that orthogneisses also represent significant magmatic contributions during continental arc flare-ups.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10182

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