MID-EOCENE ARC VOLCANISM IN CENTRAL WASHINGTON CASCADES: PETROLOGY OF THE TANEUM FORMATION AND ITS RELATION TO FARALLON PLATE SUBDUCTION AND BREAKOFF

During the Eocene, the Pacific Northwest underwent major geologic reorganization accompanied by widespread magmatism and extension. Subduction of the Farallon Plate was ongoing, but its relationship to magmatism and the location of the arc are poorly understood. A roughly north-south trending belt of ~52-49 Ma volcanic rocks exposed in the central Washington Cascades contains some formations with arc chemical traits and others without. This study focuses on one of the former, the Taneum Formation (TF), with the goals of characterizing its petrology, geochronology, and tectonic setting.

The 40 km² TF is an elongate NW-trending unit containing >1000 m of subaerial basaltic andesite lavas and rhyolitic tuffs (Tabor et al., 1984). Recent U-Pb zircon dating yielded an age of 51.309 Ma (Eddy et al., 2016). Plagioclase and pyroxene are the main phenocrysts; amphibole is present is some samples. Compositionally, the TF is bimodal with silica contents of 51.8-61.5 wt% in the lavas and 70.3-78.5 wt% in the tuffs. All samples are calc-alkaline and on spidergrams display Ta-Nb depletions characteristic of arc magmas. REE patterns are similar (La/Yb=3.0-8.6) across the compositional spectrum with no evidence of significant amphibole fractionation. In addition, many of the TF lavas have adakite traits including low Yb (<1.8 ppm), low Y (<18 ppm), high Al2O3 (>15 wt.%), high Sr (>400 ppm), and Sr/Y >20.

The TF is similar in age and composition to the nearby Silver Pass Volcanics (SPV), both of which consist of lava flows and tuffs with intermediate to felsic compositions. These units are 1-2 Ma older than the adjacent Basalt of Frost Mountain and Naches Formation that are distinctly different in composition and include mafic lavas that lack arc signatures. We suggest the TF and SPV represent mid-Eocene arc magmatism that was superseded at ~50 Ma by the eruption of diverse lavas, some with MORB affinities, that formed in response to breakoff of the Farallon slab following Siletzia accretion. TF adakites may have formed by melting at the edges of the rupture in the slab or of deep arc crust. Ongoing Sr-Nd isotopic analysis and LA-ICP-MS U-Pb zircon dating will help better constrain the duration and origin of TF magmatism.