PETROLOGY AND U-PB ZIRCON GEOCHRONOLOGY OF OLIGOCENE AND MIOCENE RHYOLITE TUFFS FROM THE NORTHERN MOGOLLON-DATIL VOLCANIC FIELD

The Mogollon-Datil volcanic field of Southern New Mexico erupted over 30 tuffs from at least 15 caldera-forming volcanic centers. Most of these tuffs are difficult to distinguish in the field. This study looks to better understand the ages of tuffs near Las Cruces and Socorro, NM, and includes the Turkey Springs, South Canyon, Box Canyon, Bell Top 2, and the Hell’s Mesa tuffs.

The Turkey Springs Tuff and Bell Top 2 tuff are crystal-poor, while the Hell’s Mesa, South Canyon and Box Canyon tuffs are more crystal-rich. Bell Top 2 Tuff is a rhyolitic ash flow tuff with pumice, quartz and plagioclase, while the Box Canyon, also known as Bell Top 6, is more crystal-rich, but is similar in appearance and mineralogy. The upper-caldera facies member of the Hell’s Mesa Tuff is a rhyolite lithic tuff with quartz, plagioclase and sanidine phenocrysts. Lithic fragments are red crystal-rich rhyolites. South Canyon Tuff is a rhyolite lithic tuff with a red-brown matrix, with quartz phenocrysts. Most of the phenocrysts are extremely weathered, some of which are completely deteriorated. The Turkey Springs is a non-welded, rhyolite ash flow tuff with a light grey matrix, euhedral quartz phenocrysts, and pumice throughout the sample.

Zircons were analyzed with laser ablation ICPMS for U/Pb geochronology to determine the magmatic age of six of the tuffs. The Bell Top 2 erupted at 36 Ma with a U/Pb magmatic age of 36.73 ±0.41 Ma. The Bell Top 4 erupted at 34.96 ±0.04 Ma with a U/Pb magmatic age of 34.5 ±0.5 Ma. The Box Canyon Tuff erupted at 33.51 ±0.13 Ma and a U/Pb age of 33.41 ±0.22 Ma. It is suggested for both tuffs that magma residence was short and do not overlap between tuffs. The Hells Mesa tuff erupted at 31.94 ±0.17 Ma with U/Pb age of 32.63 ±0.85 Ma. An unknown tuff from the Socorro area has a U/Pb age of 35.10 ±1.2 Ma, which is similar to eruption ages of both the tuffs of Farr Ranch and Rock House Canyon, which contain ³⁹Ar/⁴⁰Ar ages of 34.42 ±0.12 Ma or 34.57 ±0.13 Ma, respectively. The South Canyon Tuff erupted at 27.36 ±0.07 Ma and U/Pb magmatic age of 27.8 ±2 Ma; and the last erupted tuff in the MDVF, the tuff of Turkey Springs erupted at age of 24.33 ±0.3 Ma. This tuff produced a single zircon crystal with an age of 25.6 ±1.7 Ma. Similar to the Las Cruces area tuffs, these tuff are suggested have short residence times with zircon crystallization that dos not overlap between tuffs.