Layer by layer: remapping, petrology, and geochronology of the Madison Junction area, Yellowstone National Park

Abstract

Recent reevaluation of the Lava Creek Tuff (LCT; 0.631 Ma) has begun to challenge the two-member subdivision of the LCT, with evidence across the Yellowstone Plateau Volcanic Field for greater diversity and the presence of numerous, discrete packages, rather than two members. During this reassessment, a vitrophyre unit in the Madison Junction area, previously classified as part of the LCT, has now been dated as part of the Mount Jackson Rhyolite series (MJR; 0.78 Ma). As this unit does not fall within known MJR units, this finding raises questions about the stratigraphy, frequency, and volume of MJR eruptions, challenging previous assumptions about the relationship between these units. Remapping, petrologic descriptions, 40Ar/39Ar dating, whole-rock geochemistry, and electron probe microanalysis on sanidine of each LCT package and MJR unit in the Madison Junction area document five LCT packages and three MJR units. The five LCT packages are divided based on field observations, with variations in lithic types and crystallinity counts. Within these five packages, whole-rock trace-element concentrations and Ba in sanidine suggest that two to three magma bodies fueled the LCT in the Madison Junction area. The three MJR units are divided based on ages, petrologic descriptions, and barium concentrations in sanidine, where the Mount Haynes (0.70 Ma) and Gibbon River vitrophyre (0.78 Ma) have similar, lower barium concentrations than the Harlequin Lake flow (0.83 Ma); further, this vitrophyre is found to contain glass shards, suggesting an explosive component to the MJR, never before documented. Additionally, a local stratigraphic section was found with a lower crystal-rich lava flow capped by a basal vitrophyre grading into the overlying LCT. This is the first finding of a clear stratigraphic section from a pre-LCT lava flow to the onset of the LCT eruption. Nearby, another isolated lava outcrop, dated 0.64 Ma has barium concentrations that lie within the low Ba LCT population, suggesting these lower, older lavas may represent the onset of LCT volcanism. This research expands our understanding of the LCT and the MJR eruptions, finding several new components to both the LCT and the MJR eruptive suites.