What lies beneath the ice? Using the geochemistry and geochronology of modern river cobbles to better decipher the evolution of a glaciated volcanic arc (Wrangell Arc, Alaska, U.S.A.)

The ~30 Ma to Recent Wrangell Arc (WA), Alaska is an ideal location to study subduction and slab-edge magmatism. However, the WA covers a huge area (~15,000 km2) and ~29% of the WA is covered by glaciers with rugged topography, making bedrock sampling challenging. We addressed these barriers with geochemical and geochronologic analyses on igneous cobbles collected from rivers encircling the WA. Results show that magmatism migrated southward and then northwestward, mirroring bedrock studies. Cobble geochemistry overlaps bedrock results, where calc-alkaline and adakitic cobbles are spatially and temporally ubiquitous in the WA, indicating that subduction and slab melting have been dominant processes. Intra-arc extension-related transitional-tholeiitic cobbles are not found in southwestern WA rivers and are limited in both bedrock and cobble data. The novel tandem cobble method closely matches the bedrock record in geochemical, temporal, and spatial contexts and can be used when bedrock access concerns and/or for characterizing watersheds.