Abstract
Magma transport at stratovolcanoes dominantly occurs through dikes, which may erupt at the summit or flanks, or completely arrest within the subsurface. The variable pathways of dikes are controlled not only by external forces, like edifice loading, but also by internal magmatic processes during propagation, such as volatile exsolution, degassing, and crystallization. These processes cannot be directly observed at active volcanoes, but evidence of these processes may be preserved in dikes exposed at ancient volcanoes. We investigate two well-exposed silicic dikes at the eroded Oligocene Summer Coon volcano in Colorado, USA. Through image analysis, we assess how flow fabrics at dike margins and interiors, which record magma emplacement directions during intrusion, correlate to changes in porosity, density, and composition along dike length, which fundamentally influence magma buoyancy. One dike (A) displays dominantly low-angle fabrics at the margins along its entire 5.25-km length, while the other dike (S) shows a shift from low-angle fabrics in the proximal segment to inclined and high-angle fabrics in the medial and distal segments. To explore the causes of these different propagation styles, we compared bulk density to fabric angle, finding no correlation for Dike A and a negative correlation for Dike S. Calculated magma densities at the time of intrusion indicate that an edifice-related buoyancy barrier likely influenced lateral propagation near the center of the volcano for both dikes. However, the transition in Dike S from low-angle to high-angle propagation may reflect evolving buoyancy or a change in confining stress, ultimately allowing the dike to ascend and feed a distal eruption.