EVIDENCE FOR CRUSTAL PROCESSING BASALTS IN THE POISON LAKE CHAIN, LASSEN REGION, CALIFORNIA

We present major- and trace element whole rock data and mineral chemistry for basalts from the Poison Lake Chain (PLC) in the southernmost Cascades. The PLC is located east of Lassen Peak and encompasses 39 units that have been subdivided into 9 groups based on chemistry and mineralogy (Muffler et al., 2011). This study focuses on the basalts of the Stephens Campground group (BOSC), a set of 5 cones and flows, designated units 1-5, in the SW portion of the PLC. Units 1-3 in the BOSC are clustered together; units 4 & 5 are located about 2 km north of the other units. Major and trace elements from the BOSC reveal primitive basalt compositions and evidence of some crustal processing. Unlike other processed samples in the PLC, major elements are highly varied. Although major element compositions overlap for all BOSC units, minor-, trace elements and their ratios show two distinct trends. Units 1-3 show flat trends with little trace element variability, whereas trace elements in units 4 & 5 show a wide range of trace and minor element compositions. Trace element ratios that distinguish sources confirm two trends: little variation in units 1-3 and wider variation in units 4 & 5. Similar patterns are observed in mineral textures among the samples in this study. Those units with little trace element variation, units 1-3, contain phenocrysts of olivine, plagioclase, and clinopyroxene and are dominated by small monomineralic clusters of plagioclase glomerocrysts. Units 4 & 5 have fewer single phenocrysts of olivine, plagioclase, and clinopyroxene and are dominated by large polymineralic glomerocrysts of olivine, plagioclase, and/or clinopyroxene. We interpret these differences to reflect multiple and variable crustal processes, including crystallization, possible mixing, and remobilization of previously crystallized basaltic minerals, for the units identified as the basalts of Stephens Campground. Furthermore, comparisons of basalts in this study with other primitive basalts in the PLC suggest that two trends in the BOSC may indicate different petrogenetic relationships for units 1-3 than for units 4 & 5. This study helps to characterize the crustal processes and possible sources of BOSC group and has bearing on our understanding of ways that basalt is modified as it moves through the crust in the southern Cascades.