The research contained in this project involves deciphering what chemical components are added into magmatic systems that form the roots of volcanoes at great depths during their evolution and incorporation into the Earth's crust. In some volcanic systems, magma derived from the Earth's mantle incorporates deeply buried sedimentary rocks, which has important implications for understanding the chemical composition of volcanic systems and the potential hazards associated with their eruption. The goal of this study is to evaluate how and when sediments rocks are transferred to deep levels beneath volcanic systems and how they affect the chemical evolution of the crust. The North Cascades of Washington represents an ancient magmatic system that has undergone collapse and significant erosion, which has allowed deep levels of the system to be exposed at the present-day Earth's surface. Using a variety of different radiogenic isotopic techniques, combined with field studies, the principal investigators will fingerprint the characteristics of source of the sedimentary rocks that were located in the deep levels of the ancient volcanic system. Possibilities for the origin of this material include sediments that were accumulated in front of a down going oceanic plate and then buried and melted as the plate was subducted. This information will provide crucial evidence for testing different structural and chemical models for the incorporation of sedimentary crust into magmas and will allow the PIs to fingerprint the material that feeds volcanic systems and possibly leads to large-scale volcanic eruptions, which has important societal impacts. Ultimately, this study is providing a more thorough understanding of the processes that control the nature and timescales of magmatism in modern and ancient continental volcanic arcs, which has implications for how new continental crust is formed. In addition to the scientific goals of this research, this project is supporting the training of graduate and undergraduate students in an STEM discipline, is supporting the research efforts of an early career scientist, and is contributing to research infrastructure at both collaborating institutions. Outreach efforts include the development of geologic brochures aimed at helping to educate visitors regarding the geologic evolution of North Cascades National Park and a website that summarizes research results. The principal investigators will also lead a field trip for professional geoscientists and students as part of an international meeting that will occur in Seattle in Fall 2017.
Only a few field-based studies have examined how metasedimentary rocks become incorporated into the mid- to deep crust of continental magmatic arcs, even though their presence has significant mechanical and geochemical consequences for the arc system. Metamorphosed sedimentary rocks and their melts, produced if these rocks cross the solidus, are weak and thus strongly affect the overall rheology of the system. Emplacement of sediment into an arc via underplating may also control the timescales of magmatism and thus the architecture of the arc by driving the high-magma flux episodes noted in multiple ancient arc systems (e.g., Sierra Nevada, Coast Mountains-North Cascades). The principal investigators are evaluating how and when sediment is transferred into the mid to deep levels of arcs by investigating the importance of a range of processes, including 1) emplacement of subducted crustal material that rises buoyantly off of the downgoing slab (relamination); 2) burial by underthrusting/imbrication of forearc or backarc sediments; 3) gradual burial of sediments and volcanoclastic rocks during arc magmatism; and/or 4) construction of an arc on thickened crust that is already composed of voluminous metasedimentary rocks. The principal investigators will carry out an integrated field and laboratory study of the Late Cretaceous-Eocene crystalline core of the North Cascades. The principal investigators will determine the origin of the metasedimentary rocks via field mapping, bulk-rock Nd analyses, U-Pb and Hf-isotope study of detrital zircons, and thermobarometry to determine the tectonic context of these rocks, maximum-achieved pressures and source signatures. Analytical studies will also be conducted on detrital zircons from units in the forearc and backarc, which represent potential protoliths of the Skagit and Swakane metasedimentary rocks. The results from this study can be extrapolated to active arc systems (e.g., the Andes) to better understand the context of metasedimentary rocks: how they are emplaced within the arc system, and what the consequences are of their presence for the evolution of the arc and potentially for the overall production of andesitic continental crust. Another important outcome will be a better understanding of other ancient, potentially analogous arc systems, such as Fiordland, New Zealand and the Sierra Nevada.
