Chronology and geochemistry of the Boot Heel volcanic field, New Mexico
— William C. McIntosh and Charles Bryan

High-precision ⁴⁰Ar/³⁹Ar geochronology, paleomagnetic analyses, and geochemical studies allow reliable correlations of regional ignimbrites (ash-flow tuffs) that define a time-stratigraphic framework for the late Eocene-Oligocene Boot Heel volcanic field of southwestern New Mexico. Previous studies identified and locally correlated many of the field's large-volume ignimbrites, but were unable to establish sufficient regional correlations to develop an integrated stratigraphy. New ⁴⁰Ar/³⁹Ar dating results from single-crystal and multigrain (bulk) sanidine provide precise (±0.25-0.5%) ages for sanidine-bearing rhyolite to rhyodacite ignimbrites and lavas. Paleomagnetic polarity and direction data, together with geochemical analyses, augment regional correlations based on ⁴⁰Ar/³⁹Ar data. Nine large-volume ignimbrites in the Boot Heel volcanic field erupted in two distinct pulses (35.2-32.7 Ma and 27.6-26.8 Ma) separated by a 5.1-m.y. hiatus in ignimbrite activity, Source calderas are recognized for eight of the ignimbrites, and seven of the ignimbrites include widespread regional outflow facies. Caldera activity shifted from east to west during the life span of the volcanic field. Local volcanic units intercalated with the regional ignimbrites include basaltic, andesitic, dacitic, and rhyolitic lava flows and pyroclastic rocks. Some of these units are associated with regional ignimbrite calderas. The early and late pulses of ignimbrite volcanism are geochemically distinct. The 35.2-32.7-Ma ignimbrites are in general less evolved, contain more hydrous minerals, have lower concentrations of incompatible trace elements, and more shallowly dipping trace element enrichment/depletion patterns than the three 27.6-26.8-Ma ignimbrites. The younger ignimbrites were apparently derived from less-volatile-rich magmas as a consequence of progressive change from subduction to extensional tectonic environments between 32.7 Ma and 27.6 Ma. Because of the lower volatile contents, higher degrees of fractionation occurred prior to cauldron-forming eruptions.