HOMOGENEITY IN TIME AND SPACE: A MONOGENETIC BASALTIC ERUPTION IN THE POISON LAKE CHAIN, SOUTHERN CASCADES

The Poison Lake Chain (PLC), located just east of the Lassen Volcanic Center in the southern Cascades, is a series of 38 small-volume basaltic cones and associated flows, subdivided into 8 groups by geochemistry and mineralogy. Although dominated by primitive basalts, the Bogard Buttes group in the PLC includes 12 units with major and trace element compositions that suggest modification relative to their primitive neighbors. In an effort to understand the processes that modify primitive basalts in the shallow crust, we examined the longest contiguous flow erupted in the PLC, unit 5 of the basalts of Bogard Buttes (bb5). This flow measures 5km from vent to toe, and the cone and associated flow have been described as a single unit (Muffler et al., 2011). Previous geochemical work examining samples collected along the length of the flow confirmed the homogeneity of the unit; however, little has been done to understand any temporal changes in geochemistry during the eruption. We present new petrographic and major and trace element data from a vertical sequence of ten levee overflows near the cone and compare them to 10 previously analyzed samples taken at regular intervals both laterally across and longitudinally along the flow. Major and trace element analyses from the overflows reveal no systematic variation, suggesting that bb5 is temporally homogenous. Furthermore, phenocryst populations within the vertical series of overflows are uniform (and similar to those along the length of the flow) - generally less than 10% total and dominated by plagioclase with minor amounts of olivine (~1%). However, there is a slight increase in olivine concentration (up to 3%) in the upper two overflow units, consistent with elevated Ni and Cr concentrations. Our new geochemical data support the interpretation of bb5 as a single unit, erupting a homogeneous composition from a shallow magma chamber. Geochemical homogeneity and uniformity in phenocryst populations support the conclusion that bb5 may have spent too little time in the shallow crust to experience significant modification.