Crystallinities of the Lovejoy Basalt: A Comparison of Proximal and Distal Flow Units

The Lovejoy Basalt is a series of basaltic lavas that erupted from vents located at Thompson Peak near Susanville, California. Lavas of the Lovejoy Basalt flowed nearly 250 km southwest to Oroville, Chico, across the Sacramento Valley to Orland, and to Putnam Peak near Vacaville. Lavas have 4% MgO and calcium rich plagioclase crystals (up to 16% CaO, An 82). Recently published 40Ar/39Ar ages cluster at approximately 15.4 Ma (1). To understand how the Lovejoy Basalt flows were able to stay hot enough to flow long distances across northern California, we investigate the groundmass crystallinities to extract physical parameters of lava flow emplacement. SEM- Back Scatter Electron images are used to map and quantify groundmass crystallinities of Lovejoy Basalt flows at four locations, Red Clover Creek, Table Mountain, near the area of Little Grass Valley Reservoir, and Putnam Peak. Red Clover Creek is located approximately 46 km from the source and has a sequence of 7 flows. Quantification of crystallinities helps constrain the localized cooling regime at flow boundaries. Observations of quenched crystals at the base of one flow unit suggest that the lava was emplaced on a cold surface, which signifies a gap in time between the emplacement of flow units. Samples from Table Mountain, located approximately 119 km from the source, were collected from a single, thick flow unit approximately 30 m thick. The complex patterns of crystallinities from the base to the top of the flow suggest that pulses of new lava were injected into the inflating flow as groundmass crystallization occurred. Crystal sizes in the Table Mountain samples are five times larger than groundmass crystals in the more proximal Red Clover Creek flow units. This reflects that the flow emplacement temperature of the Table Mountain flow unit was low enough for plagioclase crystallization to continue but high enough for the flow to advance 70 km greater than that traveled by the Red Clover Creek sequence of flows. Additional sampling sites compare groundmass crystallinities of flows across northern California. (1) Garrison, et al., 2008