New Mexico Geological Society Annual Spring Meeting — Abstracts

Preliminary ⁴⁰Ar/³⁹Ar dates from rhyolite lavas on and near the resurgent dome of the Valles Caldera imply that lavas previously considered to be associated with resurgence actually predate resurgence. The timing of resurgence is constrained by the Upper Bandelier Tuff (UBT), erupted at 1.22 Ma, and the eruption of the oldest ring fracture dome, Del Medio, between approximately 1.13 and 1.21 Ma (Spell et al., 1996; Izett and Obradovich, 1994). According to prior mapping and interpretations, the Deer Canyon (DC) member of the Valles Rhyolite Formation was erupted after caldera collapse but prior to resurgence and the Redondo Creek (RC) member was erupted during resurgence
of the caldera (Smith and Bailey, 1968). In contrast to this interpretation, ⁴⁰Ar/³⁹Ar ages on sanidine phenocrysts range from 1.70 ± 0.02 to 1.45 ± 0.05 Ma for the DC member, and 1.63 ± 0.11 to 1.28 ± 0.03 Ma for the RC member. These data indicate that the mapped DC and RC members represent multiple geologically and temporally distinct rhyolite eruptions, most or all of which are older than the UBT, and therefore were erupted before caldera collapse and not during resurgence. The exposure of these older rhyolite lavas on the resurgent dome implies that the magnitude of resurgence relative to caldera fill thickness is considerably greater than previously thought, and was sufficient to expose rhyolites that were originally part of the pre-collapse caldera floor.