PETROLOGY AND TECTONIC SETTING OF THE SILVER PASS VOLCANICS, CENTRAL CASCADES, WA: EARLY EVIDENCE OF FARALLON SLAB BREAKOFF

In the Pacific Northwest, the middle Eocene (52-45 Ma) was characterized by widespread igneous activity related to the accretion of Siletzia and subsequent rollback and breakoff of the Farallon slab. This breakoff is record by a NNW trending belt of 51.6 – 48.1 Ma volcanic and plutonic rocks that extends for >150 km in western Washington (Kant et al., 2018). The Silver Pass Volcanics (SPV) are some of the oldest rocks in this belt. Goals of this study are to: (1) characterize the petrology and geochemistry of the SPV, (2) compare the SPV with the adjacent and similarly-aged Taneum Formation (TF), and (3) use these data to gain insight into the early stages of slab breakoff.

The SPV (51.3 Ma; Eddy et al., 2016) consist of ~3600 feet of subaerial lavas and tuffs now folded into a syncline (Lofgren, 1973). They include tholeiitic basaltic andesites and andesites (SiO₂ = 52.8-63.7 wt.%, La/Yb = 2.3–7.4) and calc-alkaline basaltic andesites through rhyolites (SiO₂ = 54.0-71.0, La/Yb = 4.4-7.2). Tholeiitic rocks occur mainly low in the section but some are interbedded with calc-alkaline rocks higher up. It has been suggested the SPV are correlative with the TF and the units are geochemically similar, but the SPV are on average more iron-rich (avg. Fe₂O₃ = 7.8 vs. 5.9 wt.%), have a weaker arc signature (avg Ba/Nb = 62 vs. 96), and lack adakites. Both the SPV and TF differ from other units in the breakoff belt in that they are not bimodal.

The diversity of magma compositions present in the SPV and TF likely reflect both spatial and temporal changes during slab rupture. We suggest the early tholeiitic rocks formed when hot mantle rose through the tear in the slab and underwent decompression melting, whereas the calc-alkaline rocks originated within a mantle wedge affected by earlier subduction. TF adakites may represent melting at the edges of the slab tear.