EOCENE PYROCLASTIC DEPOSITS AT CHIMACUM, WASHINGTON: ADAKITE MAGMATISM IN THE CASCADIA FOREARC

Pyroclastic deposits that record subduction-related volcanism in the Cascadia forearc are exposed at Chimacum Rock, located on the northeast corner of the Olympic Peninsula near Port Townsend. Although not yet dated, stratigraphic constraints indicate the rocks are Eocene in age. The exposed section is about 90 m thick suggesting that it was deposited within topographic constraints such as a paleovalley. Most of the section is massive and nearly monolithologic, characterized by angular clasts of hornblende dacite up to one meter in diameter in a fine-grained matrix. However, at the base of the exposed section is a stratified sandy unit. The lithology, clast character, lack of stratification, and presence of gas segregation pipes mark the deposit as a block and ash flow, which is unlikely to have traveled more than ~10 km from its source.

Twelve clasts collected from five locations within the study area were analyzed by XRF and nine samples were analyzed petrographically. The clasts contain phenocrysts of plagioclase, hornblende, and opaque oxides and have SiO2 contents ranging between 62-64 wt. %, which characterizes the samples as dacites. They have adakite traits as defined by Defant & Drummond (1993) including Sr/Y > 20-40, Y < 18 ppm, and Al₂O₃ > 15 wt. %. Based on petrographic evidence and trace element modeling, magma mixing appears to be responsible for much of the chemical variation among the samples.

Dikes of similar adakite composition occur ~60 km to the south in the Bremerton Hills. These dikes lie on the opposite side of the Bremerton fault from Chimacum, raising the possibility that the two localities are related by offset along the fault. However, although both deposits are adakites with similar SiO₂ contents, they are sufficiently different in other chemical traits (e.g., Ca/Na) that we cannot use their location to document offset along the fault. The Chimacum Rock and Bremerton Hills adakites may be related to subduction of the Kula-Farallon Ridge, and thus help to constrain the position of this trench-ridge junction during the Eocene.