THE PLUTONIC-VOLCANIC CONNECTION IN THE CASCADE ARC: INSIGHTS FROM THE MOUNT BAKER - CHILLIWACK AREA, WA
MBVF andesites (avg.=59.8 ± 2.7 wt% SiO2) are broadly similar in major element composition to quartz diorite-tonalite-granodiorite plutons (avg.=62.4 ± 2.1 wt.% SiO2) and have similar REE patterns, but the plutons have higher contents of some incompatible elements (Rb, Cs, Th, Zr). MBVF rhyodacites (avg.=70.2 ± 3.6 wt% SiO2) are distinct from the andesites in space, time, and composition and share many chemical traits of BRB leuco-granodiorites (avg.=71.7 ± 4.3 wt% SiO2). However, the plutons differ in mafic mineral assemblage (bio ± amph in plutons vs. opx + amph + bio in volcanics) and their REE patterns show greater influence of amphibole (Dy/YbN < 1). In contrast to the majority of older (slightly deeper?) plutons elsewhere in the Chilliwack, all BRB plutons display field and/or petrographic evidence of undercooling (e.g., micrographic textures, aplites), which suggests they vented to the surface. However, because BRB plutons are not depleted in incompatible elements relative to the volcanics, we infer that any eruptions they fed must have been small in volume relative to the intrusive bodies left behind and/or that the eruptions must have involved material that was not significantly differentiated relative to the bulk pluton composition.