MINERAL INSIGHTS INTO THE MAGMATIC PROCESSES OF PRE-CALDERA ANDESITES AT BONANZA CALDERA, SAN JUAN VOLCANIC FIELD, COLORADO

The textures and composition of mineral phases and populations provides a window into the petrogenesis and evolution of some of the first magmas erupted during the mid-Cenozoic San Juan Volcanic Field (SJVF) in Colorado. At Bonanza Caldera in the east SJVF, andesite lavas of the Conejos Formation were erupted from multiple volcanic centers between 35 and 33 Ma, and precede the explosive felsic caldera-forming eruptions. The Conejos in this area is understudied, primarily at the mineral-scale which could enhance models for pre-caldera magmatic evolution.

Textures and chemical composition were examined in minerals from 3 different samples. Petrographic analysis of thin sections was used to characterize mineral assemblages, abundances, and textures such mineral zonation. Samples contain a phase assemblage dominated by plagioclase, clinopyroxene, and orthopyroxene with minor hornblende and olivine. All samples also contain abundant glomerocrysts with variable but similar phases to the phenocrysts, and with disequilibrium textures indicative of different origins. Petrography and SEM imaging were used map the thin sections, and pick specific crystals for in-situ chemical analysis via EPMA. Major elements were measured in ~220 rim and interior spots in pyroxene and hornblende present as phenocrysts and in glomerocrysts. Combined petrography and compositions supports the presence of different mineral populations amongst the phenocrysts and the glomerocrysts. Chemical proxies may indicate origin from more mafic, and potentially deeper, magmas for the glomerocryst populations.

Ongoing work will further delineate the populations, especially those with possible deeper-crustal origin. Thermobarometry will be used to gain better constraints on the magma storage conditions of the various mineral populations. Comparisons between these populations, and other samples will facilitate modeling of open-system magmatic processes such as fractionation, mixing, and crustal assimilation.