DUAL MAGMATIC EVENTS FORMING THE BALD ROCK PLUTON IN OROVILLE, CA

The Bald Rock Pluton (BRP) is a zoned pluton located in Butte County, California, and is part of the Sierra Nevada Batholith that was previously interpreted by Compton as a single zoned pluton that was contaminated by wall rock. Larsen looked at zircons throughout the pluton dating the BRP from 142 to 133 Ma. and noticed a difference in crystal habit between the core trondhjemite and the outer granodiorite and tonalite, indicating the possibility of two different magmatic events. In 2019, 29 samples were collected approximately along a transect from rim to core and sent out for whole-rock major and trace element geochemical analyses. The BRP contains rocks with a silicon content of approximately 66% for tonalite and granodiorite, and then jump to 72% for trondhjemite, which indicates that two different plutons of different compositions could be intruded into the area. Petrographic analysis shows zoned plagioclase, indicating that magma recharge possibly played a role in the formation of the pluton, and xenoliths at the edge of the pluton indicate assimilation. Hornblende crystals can be constrained to a specific temperature and pressure condition using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Using the pressure and temperature conditions with the whole rock geochemistry, the Magma Chamber Simulator (MCS) can be used to help identify whether the pluton was influenced by magma recharge, fractional crystallization, assimilation, a combination, or is indeed composed of two different types of magma. Preliminary fractional crystallization simulations using the most mafic sample collected and an intermediate sample both yield concentrations of silica lower than collected samples of trondhjemite. Preliminary magma recharge simulations were run using both the most mafic sample recharged with the same composition and another run using trondhjemite as a recharge composition, also yielded silica concentrations too low. This indicates that the trondhjemite core formed from a completely different magma and was a secondary pluton unrelated to the rest. This can help understand not only how the BRP formed but also the surrounding plutons in the area.