USING GARNET INCLUSIONS TO TEST SHARED GLACIAL PROVENANCE IN COASTAL AND PUGET SOUND BEACH SANDS

We test whether or not garnets on Puget Sound beaches have similar provenance to those on outer coast beaches. The difference could reveal information about glacial and fluvial transport. Several beaches along the northern Washington Coast contain garnet-bearing sands, but the source of these garnets is enigmatic. Garnet inclusions could be used as a tracer to help identify the source and transport mechanism of these sands. As garnets form they can trap small inclusions of other minerals that record information about the tectonic setting under which the garnet was formed, such as subduction zones or areas of regional metamorphism. The North/British Columbia Cascades have experienced significant regional metamorphism and are a potential source. If that is the source, how did the garnets get to the coast? Here, we test the hypothesis that garnet inclusions in sands found on Puget Sound beaches have the same composition as garnet inclusions—and thus come from the same original source rock—as the garnets on the outer coast. If this is the case, the Puget Sound garnet sands may help to trace the path of the garnets, via a combination of glacial and fluvial transport, from the Northern Cascades, through the Puget Sound, and out to the coast. Garnet samples from two coastal and three Puget Sound beaches have been separated from their matrix and mounted on slides. The slides have been polished to access the interior of the grains, allowing the garnet inclusions to be analyzed with scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDS). Preliminary results show that inclusions from garnets collected on the outer coast of Washington demonstrate pressure-temperature conditions associated with regional metamorphism, indicating that those garnets were not produced by Cascadia subduction processes. Ongoing work will test whether Puget Sound garnet beach sands show the same inclusion composition as the coastal garnet beach sands. Beyond investigating the provenance of Washington garnet sands, this study will further develop the use of garnet inclusions as a tracer for detrital sediment provenance.