New Mexico Geological Society Annual Spring Meeting — Abstracts

Ar/Ar and U/Th ages of the ring-fracture domes at Valles caldera provides critical information to assess the eruption frequency and related hazards. Although large-volume caldera-forming eruptions are impressive and worthy of investigation, the most likely events at most Quaternary caldera systems, including Valles, are the smaller-volume higher-frequency eruptions that characterize postcaldera volcanism. Geologic mapping, petrology, and geochemistry indicates that following the 1.264 Ma caldera-forming eruption of the Upper Bandelier Tuff as many as 38 lava flows and tuffs erupted from vents on or adjacent to the resurgent dome and along the ring-fracture zone. The limited number of ages for the postcaldera lava flows has lead to a generalized eruptive history and for many of the ring-fracture domes only a single age has been determined. Initial efforts to characterize the eruptive history at Valles caldera have focused on the youngest eruptions, specifically the East Fork Member (i.e., Banco Bonito lava, Battleship Rock ignimbrite, and El Cajete pyroclastic beds), La Jara dome, and South Mountain lavas. Dating of sanidine with and without melt inclusions from La Jara and South Mountain domes illustrates the effects of excess Ar in melt inclusions. Dating of single-crystals without inclusions yields eruptive ages of 533.3 ± 1.2 ka and 533.3 ± 1.3 ka for La Jara dome and the oldest flow exposed at South Mountain, respectively. Detailed geochemistry of the phenocrysts and melt inclusions for these two units will be necessary to assess whether they are coeval or are from two different eruptions with a short repose period. The new Ar/Ar ages for South Mountain, together with previously published ages, suggest that the duration of dome construction was at least ~ 30 ka. Ar/Ar dating of sanidine with melt inclusions from the La Jara and South Mountain lavas yields erroneous eruptions ages of 534.1 ± 1.2 and 539 ± 2 ka, respectively, along with a scattered distribution of older ages. These inaccurate ages are interpreted to reflect excess Ar in melt inclusions. Single-crystal sanidine ages indicate that the Battleship Rock ignimbrite and co-erupted El Cajete pyroclastic beds erupted at 74.4 ± 1.3 ka. The Banco Bonito lava, the youngest eruption at Valles, erupted at 68.3 ± 1.5 ka. Both of these units contain a significant population of xenocrysts (up to 100% of the dated grains) or grains with excess Ar. U/Th SIMS dating of zircon in the East Fork Member yields ages ranging from the eruption age to > 350 ka (i.e., secular equilibrium) indicating that the ~ 450 ka hiatus between the South Mountain and East Fork Member eruptions was not accompanied by magmatic dormancy. Instead, the youngest series of eruptions at Valles caldera were erupted from a magma body with a protracted history, remnants of which may constitute the imaged shallow crustal magma body. Future work will focus on generating high-precision Ar/Ar ages for all the postcaldera units, U/Pb ages for each dome complex, and detailed geochemistry.