GEOCHEMISTRY AND TEXTURAL CHARACTERISTICS OF THE BUMPING LAKE PLUTON: A RESURGENT BATHOLITH OF THE MOUNT AIX CALDERA, CENTRAL WASHINGTON

The post-caldera Bumping Lake pluton (BLP, 23.75 ± 0.14 Ma) formed during resurgence of the Mount Aix caldera (24.71 ± 0.23 Ma), a syn-eruptive structure located ~20 miles east of Mount Rainier in the Southern Washington Cascades. It is one of the largest plutons in region, and covers an area of 117 km², extending NW along a shear zone on the southwest margin of the Olympic-Wallowa lineament (OWL). Compositions range from diorite to granite, and two facies have been defined. The majority (70%) of the pluton is composed of a salt-n-pepper biotite-rich granite characterized by mafic inclusions and mixing/mingling textures, while the remainder is composed of more weathered coarser grained biotite-poor granite lacking mafic inclusions. Transitions across the facies are abrupt, suggesting there are thermal/compositional boundaries that developed as a result of multiple intrusions contributing to the overall construction of the pluton. The mafic inclusions range from 53%-66% SiO₂ and vary in shape, size and degree of resorption. It's unclear whether they are enclaves, xenoliths, or a mixture of both, as their shapes vary from rounded to more angular in different parts of the pluton.

Major element diagrams indicate granite compositions overlap in chemistry with the caldera-forming Bumping River tuff (24.71 ± 0.23 Ma), supporting a co-genetic relationship. REE diagrams demonstrate minor trace element differences attributable to separate but similar fractionation histories and mineral assemblages. The pluton also shares compositional affinity and age-relations with nearby Sleep Lake and White River plutons, suggesting they may be related. The mafic inclusions plot at lower SiO₂ along the same compositional trend as the pluton, suggesting they represent less evolved counterparts of the same or similar source. They also share chemical affinity with pre-caldera Nile Creek andesite lava flows and dikes associated with the North Fork Rattlesnake Creek (NFRC) volcanic center, inferring a long-lived regional source fed volcanism to both systems, and perhaps the region.