PHYSICAL VOLCANOLOGY OF THE MULE CREEK RHYOLITE, ARIZONA AND NEW MEXICO

Tertiary delamination of the subducted Farallon plate led to asthenospheric upwelling and large-scale rhyolitic volcanism in Arizona and New Mexico. Most of this volcanism occurred as highly explosive, caldera-forming eruptions. However, several rhyolitic eruptions occurred on a smaller scale and involved effusive, dome-forming lava. One such event was the formation of the Mule Creek Rhyolite, exposed just west of the town of Mule Creek, New Mexico.

In this study, pyroclastic and lava facies were mapped and a model was developed to explain the eruption of the Mule Creek Rhyolite, including the number of vents, the types of eruptions, and the style of dome growth. Trace elements of different lavas were compared using inductively-coupled plasma mass spectrometry. These analyses were used in addition to field relations to determine the number of vents. Pyroclast morphologies were described using scanning electron microscopy to determine the extent of phreatic involvement in the explosive phases. The developed model was compared to existing models of rhyolite domes and dome fields, including those of Glass Mountain, Obsidian Dome, and the nearby Taylor Creek Rhyolite and Enebro Mountain Rhyolite.

The model for the Mule Creek Rhyolite includes an initial explosive phase of pyroclastic flow, fallout and minor surge. Phreatic contribution to the explosive eruption was minimal. A second phase consisted of effusion of lava, which formed an autobrecciated carapace, a layer of quenched vitrophyre, and a massive or flow-foliated core that devitrified. Much of the vitrophyre was later hydrated and perlitized. The high aspect ratio of the domes suggests exogenous growth, and although multiple flow lobes are associated with a single dome, each vent was the source of a single edifice. This model differs from others in that there is no evidence of major vesiculation of lava and there was no sharing of vents between domes.