PETROLOGY AND GEOCHEMISTRY OF THE BEARWALLOW MOUNTAIN ANDESITE, MOGOLLON-DATIL VOLCANIC FIELD, NEW MEXICO

The Mogollon-Datil volcanic field (MDVF) in southern New Mexico is the result of punctuated volcanism and the transition between arc and rift magmatism. During the mid-Tertiary (~50 Ma), large-scale MASH zones developed in the lower to upper crust of western North America, induced by upwelling of basaltic magma from subduction-related arc magmatism. This fueled what is known as the “ignimbrite flare-up” at ~36 Ma in the MDVF. During mid- to late-Oligocene, the tectonic stress regime relaxed, initiating intercontinental extension, resulting in a rapid shift in the style and composition of volcanism to a more basaltic composition effusive volcanic field. Previous studies of MDVF volcanic rocks give significant attention to the large-volume ignimbrites and rhyolite flows as well as the minor basalt flows that occurred between 36 and 20 Ma, thus establishing an inherent bimodal trend. However, the majority of these studies fail to acknowledge the widespread intermediate composition rocks, and they have yet to be fully integrated into the volcanic history of the MDVF. There exists a full compositional range, not solely bimodal, with a complete record from rhyolite to basalt, with a significant volume of intermediate compositions.

Here, we focus on the largest intermediate composition formation in the MDVF, the Bearwallow Mountain Andesite (BWA), in an effort to understand the petrogenesis of intermediate composition lava flows in the MDVF, and the petrologic conditions which allowed the transition from rhyolite to basaltic andesite between 28 Ma and 23 Ma. BWA lava flows are calc-alkaline phenocryst-poor basalts to trachyandesites. Whole-rock major element compositions range from 48-63 wt% SiO₂ with FeO and MgO contents ranging from 5.2-10.5 wt% and 2.0-6.9 wt%, respectively. REE patterns are enriched in LREEs compared to HREEs with La/Yb ratios between 6.8-14.2 and Eu anomalies are moderately depleted. Linear trace element compositions and ratios within and between eruptive centers suggest magma mixing as a dominant process in the petrogenesis of the magmatic system, though phenocryst textures are more suggestive of fractional crystallization. Fractional crystallization of augite pyroxene and plagioclase from the sole basalt composition closely reproduce the variation observed in the andesite compositions.