BETWEEN THE CRESCENT AND THE ARC: PETROLOGY OF EARLY-TO-MIDDLE EOCENE IGNEOUS UNITS IN WESTERN WASHINGTON

Numerous minor, and in some cases unnamed, igneous units crop out in western Washington where they occur temporally and geographically between the oceanic basalts of the early Eocene Crescent Formation and subduction-related rocks of the mid-Eocene to present Cascade arc. These units formed near the end of the poorly-understood Challis Event (~54-45 Ma), when rapid, shallow subduction of the Farallon slab was terminated due to slab breakoff and volcanic activity shifted from eastern Washington and Idaho to the modern Cascade arc as subduction renewed at a steeper angle (c. 43 Ma). Slab breakoff and tectonic reorganization were caused by accretion of the Siletzia terrane (Schmandt and Humphreys, 2011).

To better understand the magmatic and tectonic transition from the Challis episode to the modern arc we have measured major and trace element data from 20 early-to-middle Eocene units extending approximately 180 km northwest-southeast. Studied units include: Oso volcanics (46 Ma), Mt. Persis volcanics (45 Ma), Puget Group (42 Ma), Naches Fm. (44 Ma), Frost Mountain volcanics (47 Ma), and the Mt. Fuller plug (47 Ma). These units are lithologically diverse (47-79% wt SiO2); some are bimodal, others have adakite or S-type granite traits. The Lily Pond Lake rhyolite, Mt. Persis volcanics, Mt. Fuller plug, Puget Group, Mt. Pilchuck stock, Oso Volcanics, and Granite Falls stock contain arc signatures, while the Milk Creek basalt, Bald Mountain stock, and Hanson Lake rhyolite do not. Our results indicate the Early-to-Middle Eocene was a time of basaltic magmatism and crustal melting in a forearc setting, perhaps in response to the breakoff of the Farallon slab. Further geochronological and geochemical studies of additional early-to-middle Eocene igneous units in western Washington are needed to help clarify and constrain our understanding of the transition from the Challis Event to the modern Cascade arc.