PETROGENETIC EVOLUTION OF INTERMEDIATE MAGMAS FROM THE MOGOLLON-DATIL VOLCANIC FIELD DURING ARC-RIFT TRANSITION: INSIGHTS FROM WHOLE-ROCK GEOCHEMISTRY OF THE BEARWALLOW MOUNTAIN ANDESITE

The Mogollon-Datil volcanic field (MDVF) in southern New Mexico is the result of punctuated volcanism and the transition between arc and rift magmatism. The distinct style of arc volcanism in the MDVF contrasts significantly with the “normal” steady-state model, in which volcanism occurred far inland beneath a flat subducting slab that rolled back to a steeper dip. During the mid-Tertiary (~50 Ma), this initiated large-scale MASH zones to develop in the lower to upper crust of western North America, induced by upwelling of basaltic magma. 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.

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 tectonic 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. Linear trace element compositions and ratios 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. Therefore, the source is inferred to come from a moderately deeper source (e.g. lithospheric mantle) than the preceding ignimbrite eruptions, although trace element signatures still indicate a stronger relation to a subduction heritage rather than asthenosphere-derived rift magmatism.