PETROLOGY AND TECTONIC SETTING OF SILVER PASS VOLCANICS, CENTRAL CASCADES, WA: MIDDLE EOCENE SLAB BREAKOFF

In the Pacific Northwest, the Middle Eocene (52-45 Ma) was characterized by widespread igneous activity caused by the accretion of Siletzia and subsequent roll-back and break-off of the Farallon slab. Known as the Challis Event, this period of volcanism preceded establishment of the modern Cascade arc. Slab breakoff is recorded in a NNW trending belt that extends for >150 km in Western Washington (Kant et al., 2018). The Silver Pass Volcanics (SP), emplaced near Snoqualmie Pass at 51.3 Ma (Eddy et al., 2016), are similar in age and location to other units in this belt. The goal of this study is to characterize the petrology and the tectonic setting of the SP and place them within the larger context of Eocene magmatism in Washington.

The SP consist of tholeiitic basaltic andesites and calc-alkaline basaltic andesites through rhyolites (SiO₂ wt. %=52.8-69.5), all with the LILE enrichments and HFSE depletions characteristic of arc rocks (Ba/Nb = 27-177). The presence of both tholeiitic and calc alkaline rocks suggests two distinct magma series, also shown by their different REE patterns (La/Yb = 2.3 vs. 4.6-7.2). Pearce element ratios indicate fractionation of plag + pyrox +/- ol is responsible for chemical diversity among the calc-alkaline rocks, and further supports the presence of two magma series. Some samples have adakite affinities (Sr > 400 ppm, Sr/Y = >20), indicating slab melting. The nearby Taneum Formation erupted around the same time as the SP and also contains adakites (Wallenbrock et al., 2017).

We suggest the SP are part of the Eocene breakoff belt, with the tholeiitic rocks forming by melting of upwelling asthenospheric mantle that rose through the tear in the Farallon slab, and the calc-alkaline rocks originating within a mantle wedge affected by earlier subduction. Slab melting at the edges of the tear is a likely explanation for the adakites; SP rhyolites could have been crustal melts. Because the Taneum Formation displays similar characteristics to SP, these two units appear to be related to the same breakoff event. Ongoing work at SP is designed to better characterize the tholeiitic rocks, and to compare the SP with the Taneum Formation in more detail.