TEXTURAL AND GEOCHEMICAL VARIATION OF THE OLIGOCENE KNEELING NUN TUFF, MOGOLLON-DATIL VOLCANIC FIELD, NEW MEXICO

The Kneeling Nun Tuff (KNT) is a rhyolitic ash flow tuff, erupted at 34.89±0.05 Ma from the Emory Caldera in the Mogollon-Datil Volcanic Field (MDVF) of southern New Mexico. The KNT consists of over 900 km³ of erupted material, and represents one of the largest single eruptions in the MDVF. Previous work on the KNT has focused on stratigraphic and field descriptions, ⁴⁰Ar/³⁹Ar dating, and zircon dating.

This undergraduate research project seeks to determine, using petrographic and geochemical evidence, whether the KNT represents a single or a compound cooling unit, whether the Kneeling Nun Tuff and Bell Top 5, considered to be the same lithostratigraphic unit, are chemically distinct, and suggest a simple model of the pre-eruptive conditions of the KNT that contributes to the growing body of geological research in the MDVF. Twenty samples were collected, and analyzed by handheld XRF for trace element chemistry, and by petrography. Based on these preliminary analyses, four samples were selected for further investigation by XRF and ICP-MS to analyze major element and trace element contents, and EPMA for feldspar compositions. These data are compared to other similar composition MDVF ignimbrites including the Bell Top Tuff and tuffs of the Datil Group.

Dominant phenocryst phases in all KNT samples are K-feldspar, quartz, plagioclase, and biotite, with modal percentages varying widely between samples. K-feldspar phenocrysts are non-twined sanidines, ranging from Or_58-71. Plagioclase feldspar phenocrysts are ablite to oligoclase, ranging from An_9-25. Lithic fragments and pumice are variable in volume with a correlation of greater volume closer to the hypothesized source caldera. Pumices range from welded to non-welded textures with in stratigraphic section.

Trace element data suggests that the KNT is chemically distinct from the Bell Top 5. KNT Nb concentrations range from 15-21 ppm and Sr ranges from 180-280 ppm while Bell Top 5 samples range from 69-76 ppm and 16-60 ppm, respectively. Trace element contents that suggest significant degrees of chemical differentiation. Negative Eu anomalies of 0.57-0.62 were noted in KNT samples. Major element variation show a linear relationship between the samples analyzed, suggesting magma mixing following fractional crystallization, consistent with petrographic observations.