PETROGENESIS OF THE EL RECHUELOS RHYOLITE, JEMEZ MOUNTAINS VOLCANIC FIELD, NEW MEXICO, USA

The El Rechuelos Rhyolite represents volcanic activity preceding the multiple caldera collapse events of the Jemez Mountains volcanic field (JMVF) in northern New Mexico. Six spatially associated rhyolite units are being studied in order to interpret their relationships to one another and the eruptive history of the JMVF. Limited previous research includes five K/Ar dates and one ⁴⁰Ar/³⁹Ar date, in addition to whole rock geochemistry, petrography, and radiogenic isotope analyses. The research on these units is being expanded using ⁴⁰Ar/³⁹Ar geochronology, and more in-depth whole rock geochemistry, detailed petrography, and electron microprobe analyses. Four distinct eruptive units have been defined. Each of the four eruptive episodes are characterized by distinctive petrography and geochemistry, which when combined with their ages, suggests that these units are the products of separate, independent magma batches. The oldest unit, the “Early Rhyolite,” is comprised of phenocrysts of plagioclase, sanidine, biotite, and quartz in a devitrified groundmass containing spherulitic plagioclase, sanidine, and quartz. The “Intermediate Rhyolite” unit consists of quartz and sanidine phenocrysts in a devitrified groundmass containing plagioclase and sparse biotite. The “Pumice Ring” unit is composed of phenocrysts of plagioclase, quartz, biotite, and hornblende in a groundmass comprised of glass and microlites of the same minerals. The three youngest units appear almost identical to each other; these units are nearly aphyric, comprised primarily of rhyolitic glass and small phenocrysts of quartz, plagioclase, and sanidine, with sparse biotite. All six of these units plot as rhyolite on the Le Bas Classification Diagram, however there are four distinct suites based on major element comparisons, such as SiO₂ vs. K₂O, Al₂O₃, and CaO. Additionally, trace element geochemical analyses, including Nb vs. Sr, Rb, and Th, indicate four separate magma batches, with only the three youngest units being related to one another. While previous studies have disputed the relationships of these units to one another, and the namesake of the El Rechuelos Rhyolite, we suggest that these six units represent four separate eruptive episodes, and that only the three youngest units should retain the name of El Rechuelos Rhyolite.