RHYOLITE RISING: TEXTURAL EVIDENCE OF HIGH-SILICA MAGMA DYNAMICS RECORDED IN DIKES AT SUMMER COON VOLCANO, CO

Rhyolitic eruptions are some of the most explosive on Earth, but because they are relatively rare on human timescales only two have been observed using modern geophysical techniques (Chaitén in 2008 and Cordón Caulle in 2011). The most recent eruptions at South Sister and Medicine Lake volcanoes in the US Cascades Range were all predominantly rhyolitic, dike-fed eruptions occurring several kilometers away from the main volcanic edifice. Understanding how rhyolitic magmas are transported and how the plumbing system evolves during these eruptions is key to forecasting future vent locations and understanding silicic eruption dynamics, such as effusive-explosive transitions. Field observations of exposed rhyolite dikes provide insight into the dynamics of rhyolite transport and the development of conduits that feed high-silica eruptions.

At the Oligocene-aged Summer Coon stratovolcano, located on the eastern edge of the San Juan volcanic field in Colorado, at least 3 rhyolitic dikes are well exposed within the eroded edifice, with at least one inferred to have fed a flank eruption. They are oriented radially about a central intrusive complex, with widths ranging from 5 to greater than 25 m and lengths ranging from 2.5-4 km. The dikes exhibit an array of textures related to shear during magma flow, such as meso- and microscopic flow-banding, elongation lineations, and ductilely stretched obsidian clasts at brecciated margins. Sharp spatial transitions in these textures and the degree of vitrification appear to differentiate episodes of rhyolite emplacement and cooling. We investigate how flow fabrics change between the margin and the interior, which are interpreted to record the initial emplacement and the final phase of intrusion respectively. These dikes display sharp textural and glass boundaries in their interiors, implying complex, multi-stage cooling. We interpret these textures in the context of magma flow dynamics, with the goal of better understanding conduit processes during the transport and eruption of high-silica magmas.