THE COONEY TUFF: ASH FALL IN A CALK-ALKALINE SUBDUCTION RELATED VOLCANIC FIELD, SOUTHERN NEW MEXICO

Caldera systems are capable of outputting voluminous quantities of volcaniclastic material with a wide range of negative environmental impacts. Determining the behaviors of previously erupted systems may help inform predictive models used to evaluate hazards and assess risks for analogous active volcanic systems. The Mogollon-Datil volcanic field is a 40-20 Ma cluster of caldera activity in southern New Mexico tied to the subduction of oceanic lithosphere beneath the North American continental plate. The calk-alkaline magmatism of the calderas in this field produced several voluminous regionally dispersed ash flow tuffs. This study will specifically examine volcanic rocks output by the Mogollon caldera to further elucidate their petrogenic origins, attempt to determine if they may be of the same magmatic source, and yield data to help model processes that would generate magma of these particular compositions. Subsequent to output of the Cooney formation, the Mogollon caldera was overprinted by the magmatic activity associated with the Bursum caldera, as well as tectonic dismemberment resulting from the Basin and Range extension in the area.

The Cooney Formation is a ~34 Ma compositionally zoned rhyolitic to quartz latite ash flow tuff. The units of the Cooney Tuff include the Cooney Canyon member (multiple tuff cooling units separated by discontinuous volcaniclastic sandstone beds, poorly- to densely-welded, 10-30% phenocrysts of Na-rich plagioclase, biotite, oxides and lithics), the Whitewater Creek member (single densely-welded unit, <1% phenocrysts and sparse lithics) and the South Fork member (≥5 partially- to densely-welded units separated by mafic lava flows, similar in character to the Cooney Canyon). This study will examine compositional variation between the Cooney Tuff members to better understand the magma plumbing system during each eruption. Specifically, we evaluate possible crustal components of these units, along with geochemical signatures of arc or rift related magmatism. The possible relationship between these Cooney members and potential caldera outflow in the Big Lue Mountains of Arizona is also of interest, as no Mogollon caldera outflow sheets have been conclusively identified. Preliminary petrologic results suggest that these tuffs are sourced from the same magmatic system.