MINERAL GEOCHEMISTRY AND GEOTHERMOBAROMETRY OF THE EOCENE GRANITE FALLS STOCK, BALD MOUNTAIN PLUTON, AND MOUNT PILCHUCK STOCK, WASHINGTON STATE

The Bald Mountain pluton (BMP) and Mount Pilchuck stock (MPS) are located in the Cascade Mountains, Washington State. The Granite Falls stock (GFS) is located to the west of the Cascades in the Puget lowlands. All three igneous bodies have similar Eocene ages (44–50 Ma), intrude the western mélange belt, and are in contact with faults (Tabor et al. 2002; Dragovich et al. 2016). The BMP and MPS are S-type peraluminous granites whereas the GFS is metaluminous granodiorite. Mineral geochemistry was determined by electron probe micro-analyzer, Florida Center for Analytical Electron Microscopy. on amphibole, biotite, muscovite, and feldspar. Amphiboles (n=7), found only in the GFS, are calcic and primarily edenite. Micas occur only in BMP and MPS. Biotite in the BMP (n=22) and MPS (n=18) are and have peraluminous compositions. Muscovite in the BMP (n=10) and MPS (n=9) have primary compositions. GFS plagioclase (n=8) range from oligoclase to labradorite; MPS (n=12) from albite to andesine; BMP (n=4) are albite. Potassium feldspar (Or₆₅–Or₉₆) are restricted to the BMP (n=8) and MPS (n=5). Utilizing mineral geochemistry, pressure and temperature estimates were determined. Luhr et al.’s (1984) biotite thermometer for the BMP and MPS yield temperatures of 629°C ±23 and 652.8°C ±17, respectively. Massone and Schreyer’s (1987) phengite barometer for the BMP and MPS yield pressures of 4.5 kb± 0.3 and 5.4 kb± 0.8, respectively. Ridolfi et al. (2010) amphibole geothermobarometer for the GFS yields a temperature of 820°C ± 10 and a pressure of 0.69 kbar ± 0.07. These P and T support the interpretations the BMP and MPS crystalized at depth from melted continental crust; whereas, the GFS formed much shallower. This suggests a large amount of vertical displacement for the BMP and MPS occurred along their border fault, the Pilchuck River Fault, a probable regional transtensional fault, prior to 45 Ma. This depth information is consistent with mapping by Dragovich et al. (2016) that suggest transtensional structures controlled the emplacement of the shallow GFS continental arc and its coeval extrusive rhyolitic magmas. Mid-Eocene extension, uplift, and crustal melting support Farallon Slab breakoff following Siletzia accretion, all being responses to hot asthenosphere upwelling through a gap in the slab.