PETROLOGY OF MAFIC PLUTONS IN THE SNOQUALMIE BATHOLITH, NORTH CASCADES, WASHINGTON

The Oligocene-Miocene Snoqualmie batholith is exposed over an area of ~600 km2 in the Washington Cascades north of Mt. Rainier. Although comprised predominantly of granodiorite and quartz monzonite, the batholith also contains ~8% mafic plutons, which Erikson (1969) concluded are representative of the parent magma(s) from which the Snoqualmie batholith formed. However, subsequent study by the USGS (Tabor et al., 2000) suggests that several of these gabbro and diorite bodies may be older than previously thought and not genetically related to the granitic rocks. The present study seeks to better understand the relationship of these mafic rocks to the rest of the batholith, and also to compare them chemically with mafic plutonic and volcanic rocks elsewhere in the Cascades.

Four mafic stocks are included in this study: the Money Creek Gabbro (MCG), the Harris Creek Diorite (HCD), the Granite Lakes Gabbro (GLG), and the Quartz Creek Diorite (QCD). Only the QCD is demonstrably contemporaneous with the granitoid rocks of the batholith (based on field relations); intrusive relations or contact metamorphic effects suggest the HCD and GLG are older than nearby granitoid rocks, whereas the age of the MCG relative to the batholith is unknown. The most basic plutons are the MCG (49.7-54.5 wt.% SiO2; Mg#=56-48), which is associated with primitive basaltic dikes (Mg#=65; 191 ppm Ni), and the olivine-bearing GLG (47.5-53.2 wt.% SiO2; Mg#=68-53). The HCD (49.6 – 56.5 wt.% SiO2; Mg#=60-38) and the QCD (55.4-63.5 wt.% SiO2; Mg#=52-37) are quartz- and hornblende-bearing. All four plutons display LILE enrichments and Nb depletions characteristic of arc magmas. Ongoing work that compares these plutons with mafic rocks elsewhere in the Cascades will help to assess whether the nature of mantle-derived magmas has varied over time and/or space in this region of the arc.