THE PLUTONIC-VOLCANIC CONNECTION IN THE CASCADE ARC: INSIGHTS FROM THE MOUNT BAKER - CHILLIWACK AREA, WA

The Mount Baker Volcanic Field (MBVF) is the youngest and southernmost element in a linear magmatic belt that extends from Mount Baker northeastward ~50 km to Mount Redoubt, North Cascades. Comprising this Baker-Redoubt belt (BRB) are nine epizonal plutons of the Chilliwack batholith (12 to ~1 Ma) and four volcanic centers (~12 Ma to <10 ka) that display a systematic younger-to-the-southwest age progression. Although the cause of this magmatic migration is unknown, the BRB provides an excellent opportunity to examine the relationship(s) between plutonism and volcanism in an arc setting. Specific questions to be addressed are: (1) What is the genetic relationship between the plutonic and volcanic rocks? (2) To what extent have the compositions of the plutonic rocks been modified by late magmatic and/or subsolidus processes? (3) Is it possible to distinguish plutons that fed volcanic systems from those that did not?

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.