CRUSTAL PROCESSING RECORDED BY PLAGIOCLASE IN EVOLVED BASALTS OF BOGARD BUTTES IN THE POISON LAKE CHAIN, LASSEN REGION, CALIFORNIA

We present results of a petrographic and compositional study of plagioclase in the basalts of Bogard Buttes (BoBB) in the Poison Lake Chain (PLC), east of Lassen Peak in the Southern Cascades. The small area (less than 50 km²) of the PLC encompasses a series of contemporaneous (100ka), mantle-derived calc-alkaline basalts. Dominated by primitive compositions, the PLC is composed of 39 basaltic cones and flows that have been subdivided into nine groups based on similar major-element geochemistry and mineralogy. Here, we focus on two groups, the basalts of Bogard Buttes (BoBB) and Pine Creek (BoPC), located in the southeastern portion of the PLC. The BoBB and BoPC are geochemically evolved, with low MgO (4.5-5.6%), Ni (26-45 ppm) and Cr (18-48%) concentrations. Evolved basalts in the PLC (BoBB and BoPC) are porphyritic and dominated by plagioclase (2-4%) with sparse olivine (<2%) phenocrysts. A recent study of trace element compositions in a single flow of the BoBB suggests that mapped units represent individual batches of magma with consistent compositions. Throughout the nine units analyzed for this study, core compositions are relatively constant (An₈₄-An₈₈) in plagioclase phenocrysts. Rim An-contents (An₇₃-An₇₅) are consistent in samples throughout both groups and correlate with compositions of groundmass plagioclase. However, BSE imagery and detailed petrography of plagioclase phenocrysts reveal complex textures and variable compositions between core and rim. Thus, although core compositions are consistent, the combination of simple zoning, oscillatory zoning, sieved-texture, and glomerocrysts found in BoBB and BoPC samples record distinct processing during subsequent crystal growth. We recognize multiple populations of plagioclase textures in each sample and discuss the implications for crustal processing. The homogeneity of plagioclase core compositions suggests that evolved basalt flows in the PLC are likely cogenetic and may reflect the presence of a single large batch of magma that periodically erupted on the surface. The wide range of mineral textures in the evolved basalts of the PLC suggests that the crystallization and eruptive histories of basalts in this area are complex, and provide insight into the wide range of crustal modification processes arc basalts may experience.