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

The Mogollon-Datil volcanic field (MDVF) of Southern New Mexico erupted over 30 tuff units related to at least 15 caldera-forming volcanic centers. Most of these tuffs are difficult to distinguish in the field. This study looks to increase the understanding of the whole rock geochemistry and age of the volcanic tuffs in the Water Canyon quadrangle near Socorro, NM. The tuffs included in this study are the La Jencia, Vick’s Peak, Caballo Blanco, the Turkey Spring tuff, and the South Canyon tuffs. The La Jencia, Vick’s Peak, Cabballo Blanco and Turkey Springs tuffs are all phenocryst-poor. While the South Canyon Tuff is more phenocryst-rich. Zircons were analyzed with laser ablation ICPMS for U-Pb geochronology to determine the magmatic age of four tuffs. Petrographic thin sections were made for all of the samples in order to determine crystallinity. Whole rock samples were analyzed by XRF and ICPMS for major and trace element composition and variation within each tuff.

The Caballo Blanco Tuff is a crystal-poor rhyolite lithic tuff with quartz, sanidine, and K-feldspar phenocrysts and lithic with some welding. The La Jencia Tuff is a rhyolite crystal-poor ash flow tuff with K-feldspar as the primary phenocryst. Welding was not observed in the field or in thin section. The Vick’s Peak Tuff is an extremely crystal-poor rhyolite ash flow tuff with only quartz phenocrysts. The South Canyon Tuff is a rhyolite is a lithic tuff with a red-brown matrix, a high volume of vesicles, and is crystal-poor. The sample mainly consists of highly weathered, subhedral to anhedral quartz phenocrysts with trace amounts of Fe-Ti oxides and vesicles. Lithic fragments litter the sample, most of the phenocrysts are extremely weathered, some of which are completely deteriorated. The Turkey Springs Tuff is a rhyolite ash flow tuff that is very crystal-poor with a light grey matrix. There are sparse Fe-Ti oxides and euhedral quartz phenocrysts along with vesicles and pumice, without welding, throughout the sample.