Master of Science (MS)
Construction, Geochemistry, Geology, North Cascades, Pluton, Structure
This thesis provides an analysis of magmatic structures and geochemistry from a portion of the mid-crustal, ~92-90 Ma Seven-Fingered Jack pluton, North Cascades, Washington. The study area was divided into three separate domains. Rocks in the northern domain (~9 km2) are characterized by N-NNW- trending, moderately to steeply plunging magmatic folds that reflect regional shortening. The central (~3 km2) and southern (~3.5 km2) domains are structurally homogeneous and contain a dominant NW-striking magmatic foliation. Schlieren are locally developed, several meters long, thin (cm-scale), and strike NW and dip steeply to the NE. Field data indicate the pluton is constructed of numerous, ≤300 m-wide, ≤1 km-long, sheet-like bodies separated by sharp and gradational contacts that are identified by textural and/or compositional differences. During construction, earlier sheets acted as host rocks to younger increments of melt. Detailed modal analyses (n=39) by point counting, coupled with XRF and ICP-MS geochemical analyses (n=8), indicate that the hornblende-biotite tonalite that comprises the study area is overall chemically and texturally homogeneous. Geochemical data further indicate that parental magmas may have come from a relatively shallow source, compared to the deeper, coeval Tenpeak pluton, and incorporated evolved crustal material.
Elkins, Scott William, "Structure, Construction, and Geochemistry of the Highly Elongate, Cretaceous Seven-Fingered Jack Pluton in the Devil's Smokestack Area, North Cascades, Washington" (2015). Master's Theses. 4537.