PLAGIOCLASE TEXTURES REVEAL A COMPLEX MAGMATIC HISTORY FOR BASALTS IN THE POISON LAKE CHAIN, LASSEN REGION, CALIFORNIA
To determine whether magma influx or thermal mixing dominated, we collected core and rim data from multiple phenocrysts in each of the 12 bb and 3 bpc units; for samples with large and complex phenocrysts, we also collected data in transects across individual crystals. For all units analyzed for this study, core compositions are dominated by An80-An89, low FeO (0.4-0.65 wt %), and low MgO (0.06-0.10 wt %); groundmass and rim compositions overlap and have lower An (An66-An79), and higher FeO (0.55-1.0 wt %) and MgO (0.08-0.15 wt %). Transects across large and complex phenocrysts and trends in trace elements with respect to An suggest that this magmatic system experienced a complicated history. Trends in trace elements and An content suggest an evolution from magma influxes during early eruptions to multiple episodes of magma influx and thermal mixing in later units. Using trace element compositions of the phenocrysts combined with textures observed in thin section, we present a model that reflects the complex crustal processing associated with basalts in the PLC and perhaps elsewhere in basaltic systems of the Cascades.