PLAGIOCLASE TEXTURES REVEAL A COMPLEX MAGMATIC HISTORY FOR BASALTS IN THE POISON LAKE CHAIN, LASSEN REGION, CALIFORNIA

We present new electron microprobe spot and transect data for plagioclase phenocrysts in two geochemical groups of calc-alkaline basalts erupted in the Poison Lake chain (PLC). The PLC, a small area (<30 km²) located east of Lassen Peak in the southernmost Cascade Arc, includes 39 units that have been subdivided into nine, contemporaneous (<100ka) geochemically distinct groups. Two of these groups, the basalts of Bogard Buttes (bb) and Pine Creek (bpc) have geochemical compositions that are consistent with crustal processing (e.g., fractionation, mixing, assimilation). Plagioclase phenocrysts found in bb and bpc units display multiple textures that, when analyzed petrographically, reveal the complex magmatic history of the basalts erupted in the PLC. The basalts of Bogard Buttes and Pine Creek are porphyritic; phenocrysts are dominated by plagioclase (<1-8%), with sparse olivine and/or pyroxene (<2%). Plagioclase phenocrysts exhibit 5 types of textures: 1) simple zoning, 2) oscillatory zoning, 3) sieving, 4) sieving and zoning, and 5) glomerocrysts. These types of textures are commonly attributed to multiple episodes of mixing (thermal and/or compositional) in the magmatic system.

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.