THE PETROLOGY AND GEOCHEMISTRY OF THE KNEELING NUN TUFF, MOGOLLON-DATIL VOLCANIC FIELD, NEW MEXICO

The Kneeling Nun Tuff is an ash flow in the Mogollon-Datil Volcanic field (MDVF) of southern New Mexico dated by ⁴⁰Ar/³⁹Ar at 34.89 ±0.05 Ma. Ignimbrites of the 40,000 km² MDVF were erupted episodically from 36 Ma to 24 Ma with the Kneeling Nun erupting in the first of four identified volcanic episodes. The eruption source, the Emory Caldera, is located about 30 km southwest of the town of Winston, New Mexico. The Kneeling Nun consists of more than 900 km³ of volcanic material with disagreement on whether the tuff is a single cooling unit or multiple flows. Previous work on the tuff has focused on field descriptions and stratigraphy around the source caldera. Here we focus on stratigraphic and lateral chemical and mineralogical variation throughout the Kneeling Nun Tuff. We present new thin section petrographic analysis and major element geochemistry of 20 whole rock samples representing six stratigraphic sections. Stratigraphic sections were measured along two traverses, the first a north-south traverse covering the maximum extent of the tuff. The second traverse is east-west through the caldera. Our overall goal is to: 1) determine if the Kneeling Nun was deposited as a result of a single eruptive cooling unit or multiple eruptive units. 2) Contribute to the “big picture” understanding of the complex volcanic history of the Mogollon-Datil volcanic field through a description of the petrology, stratigraphy, and geochemistry of the Kneeling Nun Tuff.

Stratigraphic description of the six sections suggest only minor phenocryst variation, but extreme variation in matrix color. The color of the matrix ranges from whitish grey to dark red, with the white colored matrix being more common. In hand sample, only slight variations in the phenocryst assemblage is observed with the dominant phases being quartz, sanidine, and plagioclase. Lithic and pumice fragments are common in throughout each section. Petrographic analysis suggests that the most distal reach of the flow had a greater modal percent matrix, while the most proximal samples had a greater modal percent of phenocrysts. The most proximal samples contain more phenocryst-sized quartz, while the most distal samples had more groundmass-sized quartz. These relationships were observed regardless of lateral or stratigraphic position suggesting a single cooling unit.