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