ONSET AND TEMPO OF IGNIMBRITE FLARE-UP VOLCANISM IN THE EASTERN AND CENTRAL MOGOLLON-DATIL VOLCANIC FIELD, SOUTHERN NEW MEXICO

The Cenozoic ignimbrite flare-up produced numerous, voluminous volcanic fields across the western U.S. and Mexico between ~40 – 18 Ma, including the Mogollon-Datil volcanic field (MDVF) in southern New Mexico (~36 – 24 Ma). The tempo of volcanism and temporal-spatial variations in activity can provide information about upper crustal processes, yet the initial phase of volcanic activity in the MDVF is not well constrained. Numerous tuffs lack correlation with other units and/or source calderas, and thus accurate volume estimates are difficult. Here, we present a suite of new ⁴⁰Ar/³⁹Ar geochronology and whole rock and feldspar geochemistry to elucidate the tempo of volcanism, variations in magma geochemistry and storage histories, and estimates of eruption volumes via outflow sheet correlation during the initial phase of activity in the eastern-central MDVF.

New ages constrain the onset of caldera volcanism in the MDVF at 36.5 Ma, and eruption of rhyolitic tuffs and lava flows/domes continued until 33.8 Ma. Sample mineralogy, whole-rock and feldspar geochemistry, and melt temperature estimates are consistent with eruption of increasingly crystal-rich and fractionated magma over time. The oldest tuffs (36.5-35.7 Ma) are crystal-poor (<5%), whereas most younger tuffs have intermediate (15-25%) crystallinity, except the crystal-rich (30-60%), voluminous (900 km³) Kneeling Nun Tuff (KNT) at 35.3 Ma. Sanidine Or#, Pb, and Rb concentrations also increase over time. Crystal rich, highly fractionated tuffs have source calderas in the central MDVF, just west of a proposed boundary between the Basin and Range (B&R) and Rio Grande rift provinces. Extension in the B&R may have helped facilitate eruption of predominantly younger, more crystal-rich tuffs with longer pre-eruption magma storage.

Geochronology combined with sanidine and whole-rock geochemistry was used to correlate or differentiate tuffs in the MDVF. The average eruption recurrence interval (RI) is ~200 ka (n = 13) over the initial stage from ~36.5–33.8 Ma. However, the RI is shorter (~135 ka) from the onset of activity until the voluminous KNT eruption at ~35.3 Ma, after which the RI increases to 350 ka before activity pauses at ~33.8 Ma. During this initial phase, the total volume of rhyolite erupted in the eastern-central MDVF was ~2000 km³.