Master of Science (MS)
Robert B. Miller
Construction, Cretaceous, Geochemistry, North Cascades, Washington, Seven-Fingered-Jack Intrusive Suite
The highly elongate Seven-Fingered-Jack intrusive complex (SFJIC) of the North Cascades, Washington provides an excellent opportunity to study the construction, geochemistry, and structure of a well-exposed mid-crustal pluton. The interior ~92 Ma Main Body tonalite has markedly heterogeneous and mafic domains that may represent interactions between multiple batches of melt. The ~78 Ma marginal Kelly Mountain suite includes a mafic complex containing abundant hornblendite, diorites of variable heterogeneity, and tonalite. Field mapping and structural, petrographic, and geochemical analyses are interpreted to indicate that this intrusive complex was built incrementally by many batches of melt. The dominantly NW-striking and NE-dipping magmatic foliation is overprinted by weak solid-state fabrics. The magmatic foliation is folded in both units and has N-NW hinge lines. These time-transgressive folds are consistent with regional contraction during and shortly after emplacement. Modal analysis by point-counting indicates that the dominant mafic mineral switches from biotite in the older tonalite to hornblende in the younger tonalite. This modal analysis coupled with XRF and ICP-MS geochemical analyses shows that although similar, the younger and older tonalites plot distinctly enough to indicate that they may be derived from different magma sources. Geochemical data also indicates that the Main Body tonalite likely has a shallower magma source than the coeval, but structurally deeper Tenpeak pluton.
Dustin, Kelly N., "Structure, Construction, and Geochemistry of the Cretaceous Seven-Fingered-Jack Intrusive Complex in the Klone Peak Area, North Cascades, Washington" (2015). Master's Theses. 4632.