DISTAL EXPOSURES OF THE MIOCENE PEACH SPRING TUFF IN THE MARBLE AND SHIP MOUNTAINS OF SOUTHERN CALIFORNIA

The Peach Springs Tuff (PST) is a uniquely identifiable ignimbrite deposit formed by a large caldera-forming volcanic eruption during the early Miocene. Previous investigations have suggested that distal ignimbrite deposits located within the central Mojave region of southern California correlate to the PST eruptive event. To evaluate this hypothesis, this study investigates ignimbrite samples collected from outcrops within the Marble Mountains (n = 6) and Ship Mountains (n = 8) in southern California. In outcrop, the ignimbrite deposits of the Marble and Ship Mountains are moderately to strongly welded, relatively crystal-poor (5-20%), and contain variable amounts of pumice and lithic fragments. Although discontinuous, a black vitrophyre layer can occasionally be observed near the base of the ignimbrite deposits. In hand sample, sanidine phenocrysts are the most recognizable mineral phase, displaying a distinctive bluish adularescence. Petrographic analysis reveals abundant phenocrysts of sanidine (~50-60%) with less abundant plagioclase (~20-25%), biotite (~15-20%), hornblende (~5-10%), and Fe-Ti oxides (~2-5%). Quartz is also present, but is typically a very minor phase (≤ 1%). Accessory minerals include titanite and zircon. Our modal mineralogy agrees well with petrographic analyses of the PST’s proximal deposits reported by previous studies. EPMA analyses were collected on biotite, hornblende, and sanidine phenocrysts. Our analyses closely match the mineral compositions observed within proximal outcrops of the PST. Collectively, our field, petrographic, and EPMA data support that the ignimbrite outcrops within the Marble and Ship Mountains are consistent with being distal deposits of the PST.