CORRELATING REGIONAL IGNIMBRITES AND ESTABLISHING A CONNECTION BETWEEN ANDESITE AND RHYOLITE VOLCANISM, SCHOOLHOUSE MOUNTAIN CALDERA, SOUTHWESTERN MOGOLLON-DATIL VOLCANIC FIELD, NEW MEXICO

Ignimbrite volcanism in the Mogollon-Datil Volcanic Field (MDVF) occurred in 3 discrete episodes between ~36.5–24 Ma. The Schoolhouse Mountain Caldera (SMC) was active during the first episode and ⁴⁰Ar/³⁹Ar ages for the entire sequence record eruptions from ~34.9–33.8 Ma. The earliest phase of volcanism consisted of dacitic – trachydacitic (58–68% SiO₂) lava flows and intrusions. Syn-caldera volcanism is recorded by the Kerr Canyon sequence: rhyolitic ash-flow tuffs, flows, and collapse megabreccias.

Overlying the Kerr Canyon sequence are alternately emplaced trachyandesite lavas and high-silica (74% SiO₂) rhyolitic ash-flow tuffs comprising the intracaldera Mangas Creek sequence. The andesite flows are an uncommon feature of Mogollon-Datil volcanism and predate most of the Southern Cordilleran Basaltic Andesite (SCORBA) flows by at least 4 m.y. in NM. The McCauley Ranch tuff is the next highest unit and is a densely-welded high-silica (77% SiO₂) rhyolitic ash-flow tuff. The source of this tuff is not known, but it is older than the regional Box Canyon Tuff. Above the McCauley Ranch are crystal-rich rhyolitic porphyry flows and ash-flow tuffs of the Cherokee Canyon sequence. Ash-flow tuffs are variably welded with fiamme in lower densely welded sections. Black vitrophyres are present within ash-flow tuffs in upper sections.

The connection between post-caldera-collapse eruptive products within the SMC system remains unclear. The Cherokee Canyon was postulated to have been sourced from the SMC and correlated with other ignimbrites forming the “Box Canyon Tuff”. Sanidines from SMC ignimbrites have distinct BaO and orthoclase concentrations reflecting derivation from sources distinct from the Box Canyon. Mangas Creek, McCauley Ranch, and Cherokee Canyon sequences have ⁸⁷Sr/⁸⁶Sr ratios (0.7077-0.7080, 0.7135, 0.7101, respectively) that reflect variable crustal input during magma genesis. No distinct trends of ⁸⁷Sr/⁸⁶Sr ratios with time are observed in this system. Additional ⁴⁰Ar/³⁹Ar ages, major/trace element and Sr and Pb isotope analyses will be used to further investigate the origin of SMC volcanic rocks and to develop a robust spatial and temporal record for this system that will facilitate correlation of outflow sheets to source calderas and improve our understanding of volcanism in the MDVF.