Paper No. 39-2
Presentation Time: 9:00 AM-3:30 PM
GEOCHEMISTRY AND TEXTURAL CHARACTERISTICS OF THE BUMPING LAKE PLUTON: A RESURGENT BATHOLITH OF THE MOUNT AIX CALDERA, CENTRAL WASHINGTON
PATRIDGE, Karyn A., 2725 SE Salmon St, Portland, OR 97214 and HAMMOND, Paul E., 1305 SW Upland Dr, Portland, OR 97221
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.