PETROLOGY AND GEOCHEMISTRY OF RHYOLITE TUFFS OF THE BELL TOP FORMATION, DONA ANA COUNTY, NEW MEXICO

The Mogollon-Datil Volcanic Field (MDVF) in Southern New Mexico has many exposed ignimbrites of Eocene-Oligocene age. During this time, subduction of the Farallon Plate beneath the North American plate transitioned to extension of the Rio Grande Rift, leading to an ignimbrite flare-up in southern New Mexico. The caldera forming eruptions produced numerous ash flow tuffs including the Bell Top Formation tuffs, which are divided into four members. The Bell Top Tuffs have been dated by ⁴⁰Ar/³⁹Ar with ages ranging from 31.4 Ma to 42.1 Ma. These tuffs are suggested to have come from two calderas during the first episode of volcanism of the MDVF ignimbrites. Here we present new thin section petrography and major and trace element geochemistry from 16 samples representing Tuff 2-4 and compare these data with samples from the Kneeling Nun Tuff (Bell Top 5) and similar age ignimbrites from southern New Mexico. Samples were collected from two stratigraphic sections in the Sierra De Las Uvas Mountains, consisting of six ridge defined outcrops. The Bell Top Tuffs 2-5 are defined as small-volume ignimbrites (<100 km³). Geochemically, the Bell Top Tuff samples are trachyte to high silica rhyolite (67-75 wt% SiO₂). Tuffs are crystal poor with phenocryst phases consistent through the members to include quartz, K-spar, plagioclase, biotite and trace hornblende, apatite, and zircon. Tuff 2 is a vitric ash flow tuff with well-developed eutaxitic texture and locally contains spherulitic zones. The hand sample contains 30% ash 25% quartz, 20% biotite, 10% lithics, 10% pumice, and 5% plagioclase. Tuff 3 is distinct with a large volume of pale-red pumice making up ~20% of the total volume; it contains 80% pumice, 15% quartz, and 5% biotite with a pink matrix that is about 72% of the hand sample. The outcrop has a gradual incline that leads up to a steep face. Tuff 3 looks pinkish-orange on the weathered face. Tuff 4 contains a lower volume of pumice, but the pumice have been flattened and are larger, up to 30 cm along the long axis. The hand sample contains 35% quartz, 20% glass, 20% biotite, 10% lithics, 10% pumice, and 5% plagioclase. We suggest that the three Bell Top Tuffs (2-4) represent eruptions from at least two different sources. The Bell Top 2 and 3 are sourced from the Organ Caldera and the Bell Top 4 tuff sourced from a similar magma chamber to the Kneeling Nun Tuff.