MUCH ADO ABOUT RHYOLITE: MECHANISMS FOR RHYOLITIC DIKE EMPLACEMENT FROM FIELD OBSERVATIONS AND MICROTEXTURAL ANALYSIS AT SUMMER COON VOLCANO, CO

Rhyolitic eruptions exhibit complex behavior characterized by alternating phases of predominantly explosive or effusive activity, along with extended periods of hybrid activity, such as what was observed at Cordón Caulle in 2011-2012. These eruption dynamics are influenced by processes occurring at depth within the conduit, which are difficult to observe directly during eruptions, or to infer from surface deposits alone. Field observations of fossil rhyolite conduits provide crucial context regarding the mechanisms of magma emplacement, including the depth and relative timing of processes like viscous magma flow, magma fragmentation, post-fragmentation ash sintering, and interactions with wall-rock materials.

Summer Coon volcano, Colorado, is an Oligocene arc stratovolcano which experienced a phase of rhyolitic magmatism. Three large rhyolite dikes are exposed in the edifice with paleodepths up to 1.2 km, with one showing evidence of being a feeder dike. All three dikes exhibit similar textural zones from margin to interior: a brecciated margin with variable host rock and obsidian clast contents; a resistant, glassy, flow-banded interior zone; and a recessive, platy interior zone. We present a detailed map of zone thicknesses and kinematic indicators for the best exposed rhyolite dike. We also provide textural data collected from the three morphologic zones to determine: 1) the mechanism for emplacement, either as a viscous magma or as a fragmented gas-pyroclast mix, and 2) the thermal evolution of the dike. This dike is composed of at least 4 segments along its 1.5 km exposure and has widths ranging from 3.8 m to 8 m. Macro- and microscopic evidence, such as flow-banding, elongation lineations, and ductilely deformed obsidian, suggests it was laterally emplaced, and there is no indication that it was a feeder dike. We use microtextural analysis of clast size distribution to determine the style of fragmentation (e.g., shear or explosive) responsible for the margin zone, and how this varies along the length of the dike. We also infer the emplacement history of the dike by comparing margin to interior, which records the evolution of dike material from initial emplacement to final cooling. With this work, we aim to determine the mechanism of intrusion and implications for effusive/explosive behavior of rhyolites.