4-D Reconstruction of Magmatism at a long-lived Andesite Locus: Goat Rocks Volcanic Complex, Cascade Arc

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The large composite volcanoes that are distinctive of subduction zones are the surface expressions of the magma factory that distills mantle material to create and change the Earth’s crust. While the volcanoes themselves pose hazards to human health and property, the magma systems that drive them result in the circulation of hot water used for power and recreation, and also lead to the formation of many types of ore deposits at depth. For these reasons, the nature and development of subduction volcanoes and the magmatic underpinnings that feed them are critical. This project will investigate the long-term (~3 million year) development of the Goat Rocks Volcanic Complex, a cluster of eroded composite volcanoes in the Washington Cascade Range, to understand the origin and storage conditions of magmas erupted there and how they changed over time. Broader impacts of this study include the training of a female PhD student and undergraduate students in diverse field, laboratory, publication, and outreach skills; collaboration with the Washington Department of Natural Resources on projects and publications; and an animation of the development of the Cascade Volcanic Arc.The Goat Rocks Volcanic Complex makes a particularly good natural laboratory because it is well exposed by glacial erosion, it is relatively young, and it spans a large part of the global compositional range of subduction zone stratovolcanoes. Preliminary work at the Goat Rocks Volcanic Complex indicates that the magma system changed substantially in time, from potassium-rich magmas stored at modest depth in the crust, to low-potassium and more water-rich magmas stored at greater depth. Detailed stratigraphy and geochronology (mainly 40Ar/39Ar dating) will provide a more robust time framework for the geochemical analyses of rocks and minerals to be used in reconstructing the magmatic history (e.g., pressure, temperature and water conditions), which will be quantified using computer models that couple mass and thermal balance. The reconstruction of this volcanic complex will then be compared to the magmatic histories of other subduction volcanoes in the Cascades and in other arcs. A particular aim is to evaluate conflicting models of magma generation and storage, i.e. (1) a model wherein deep crustal magma generation is prevalent, with only transient shallow storage before eruption; vs. (2) extensive interaction between crust and magma at all levels, as suggested by exposed sections of volcanic arc crust. We posit that seen through the lens of development in time, such models may be reconciled.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.