PETROLOGY OF EOCENE-MIOCENE VOLCANIC UNITS IN CENTRAL WASHINGTON AND THEIR TECTONIC SETTINGS

Across the Pacific Northwest, the middle Eocene (52-45 Ma) was a time of extensive volcanic activity related to the subduction of the Farallon plate and its subsequent rollback and breakoff after accretion of the Siletzia terrane. This period of volcanism, known as the Challis Event, was followed by renewed subduction of the Juan de Fuca plate and establishment of the modern Cascade arc by ~43 Ma. We have studied four geologic units from around Wenatchee, WA, that span this tectonic transition and lie to the east of the modern arc: (1) the Eocene Wenatchee Pinnacles and nearby gabbros, (2) the Oligocene Horse Lake Mountain Intrusive Complex (HL) (Gresens, 1983), (3) the Miocene Andesite of Burch Mountain (BM) (Tabor et al, 1987), and (4) undated rocks from Peoh Point (PP). We compare these units to modern Cascade samples to better understand the tectonic setting(s) of their formation and to gain insight into the tectonic changes that occurred during the Eocene.

Samples from the Wenatchee Pinnacles are calc-alkaline dacites and rhyolites (SiO₂ = 67.9-76.1 wt.%), while the Eocene gabbros occur as tholeiitic dikes (SiO₂ = 52.7-53.2 wt.%). Both units, which are similar in age and occur in close proximity, display arc geochemical traits (LILE enrichment and HFSE depletion) and together are bimodal in composition. HL and PP samples, which are andesites to rhyolites, also have traits that are consistent with a subduction setting, and some have high silica adakite affinities (La/Yb=7-9, Sr/Y=25-28.3). BM samples are low silica adakites (Sr/Y=130-132, Sr 1125-1425 ppm, Yb=0.79-1.0 ppm).

Based on their age and location, the Eocene age rocks could either be products of the last pulse of slab rollback volcanism, or could represent some of the earliest Cascade arc rocks. Compositional similarities between HL, PP, and the modern Cascades suggest that these units are likely related to ongoing subduction of the Juan de Fuca plate. The HL rocks are also arranged as radiating dikes (Gresens, 1983) which suggests they may be the roots of an ancient eroded volcano. Finally, the BM low-silica adakites likely acquired their slab-melt components from melting at the edges of a slab tear or slab window, although the timing of that event could have been prior to the Miocene .