GEOCHEMISTRY OF THE LATE EOCENE GRAYS RIVER VOLCANICS, SOUTHWESTERN WASHINGTON AND NORTHWESTERN OREGON: EVIDENCE FOR A SLAB WINDOW IN THE CASCADIA FOREARC

The Grays River Volcanics (GRV) is one of several Late Eocene to Early Oligocene volcanic units that erupted within the Cascadia forearc basin. Consisting of >2000 m of submarine and subaerial flows and volcaniclastics, the GRV (~42-37 Ma) extend from NW Oregon into SW Washington and show a younger-to-the-north age progression. These lavas are chemically similar to, but younger than, the 44 Ma Tillamook Volcanics (TV) of the Oregon Coast Range. Previous genetic explanations for the GRV and TV include forearc rifting, a hot spot, or passage of a slab window. The goals of our investigation are to characterize the chemical and Sr-Nd isotope signature of the GRV and better constrain the tectonic setting in which they formed.

The GRV postdate the more voluminous 53-45 Ma Crescent Basalts (CB) and show greater enrichment in Ti, K, and P. Although this could be attributed to smaller degrees of melting, differences in incompatible trace element ratios indicate different mantle sources for the GRV and CB. For example, the GRV have higher average La/Yb_N (7.4 vs. 2.7) and Zr/Hf (40.4 vs. 35.6) and lower Zr/Nb (6.95 vs. 11.5) than the CB. Ongoing Sr-Nd isotope analyses will further characterize these mantle sources. Based on the lack of an arc signature and the likelihood that mantle rocks above the subducting plate would be too cool to yield GRV basalts, we suggest that the GRV source was below the subducting plate. This evidence for an asthenospheric source, combined with northward age progression of volcanism, support a slab-window setting. A similar setting may also explain younger forearc basaltic centers in Oregon if left-stepping offsets in the subducting Kula-Farallon ridge caused southward jumps of the ridge-trench triple junction.