Magmatic Evolution and Eruptive History of the Granitic Bumping Lake Pluton, Washington: Source of the Bumping River and Cash Prairie Tuffs

• Main Author: King, John Frederick
• Format: Others
• Published: PDXScholar 1994
• Subjects: Intrusions (Geology) > Washington (State) > Bumping Lake Region; Geology > Bumping Lake Region; Geology
• Online Access: https://pdxscholar.library.pdx.edu/open_access_etds/4765; https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=5837&context=open_access_etds

The 25 Ma Bumping Lake pluton ranges in composition from quartz diorite to granite with the granitic facies comprising approximately 90% of the pluton's surface area. The granite may be classified as calcalkaline, peraluminous and I-type with some Stype characteristics. A late-stage, mafic-poor facies fills cooling related extensional fractures. The pluton was passively emplaced into the Ohanapecosh Formation at a shallow level in the crust. Contact relationships vary from sharp where the contact is vertical to gradational at the roof of the pluton. Where gradational, stoped xenoliths from the roof of the pluton increase in size, angularity and retain more of their primary textures as the contact is approached. Spacial trends in major and trace elements support the interpretation that xenoliths were stoped and assimilated into the melt The predicted Rayleigh number for the pluton when molten is 107 and the predicted Reynolds number is approximately 10-9. Based on these values, the magma of the pluton probably did not convect, and if it did, convection was weak and not a significant process. Based on variations in Eu/Eu* and Sr values, plagioclase fractionation was an important process in the petrogenesis of the pluton. Additionally, fractionation of accessory minerals rich in light rare-earth elements (LREE) resulted in successive depletion of LREE with progressive differentiation. Two separate regions of the pluton are highly differentiated as indicated by high Si02 values, high Rb/Zr ratios, and low Zr and Ti02 values. Mapping by the author indicates that the pluton projects beneath the Mount Aix caldera. Dates of three tuffs derived from the caldera are equivalent to the pluton, and two of these tuffs are chemically indistinguishable from the granite facies of the pluton. This implies that the Bumping Lake pluton represents the chilled remains of the magma chamber that fed the Mount Aix caldera.