EVOLUTION OF MAGMA PROPERTIES AND FLOW DIRECTIONS DURING DIKE EMPLACEMENT AT SUMMER COON VOLCANO, CO

Dikes are the primary conduit for magma transport at arc stratovolcanoes. They can propagate directly to the summit or be diverted to the flanks and erupt, or alternatively arrest in the subsurface. Determining what drives dike propagation to the surface versus arrest is important for assessing hazards and potential eruptive locations at modern arc volcanoes. Dike emplacement may be internally limited by the buoyancy or viscosity of the magma, which are controlled by magmatic properties such as composition, crystallinity, and volatile content, and dynamic processes like crystallization and exsolution. Propagation trajectories may also be controlled by external stresses, such as those arising from the load of a volcanic edifice. The interplay of these factors, and their correlation to emplacement directions and eruption or arrest is poorly constrained.

Dikes exposed at eroded arc stratovolcanoes provide an opportunity to analyze and compare magma transport directions and magmatic conditions over the life of a volcanic system. On the eastern edge of the San Juan Volcanic Field, Colorado, USA, Oligocene-aged Summer Coon volcano has been eroded to reveal approximately 20 radially emplaced silicic dikes within the edifice. Here we present petrofabric data collected along the length of two of the largest silicic dikes, and compare this to magma bulk density, porosity, and crystallinity to determine how magma properties evolve during intrusion and whether these variations in magma properties are correlated to transport direction. Flow fabrics for the dikes are found to be dominantly lateral along the exposed lengths. Bulk rock compositions for these dikes are trachydacite to rhyolite, and bulk densities range from 2350 to 2590 kg/m³. Dike margins have total porosities from 0.6-10.4%, and interiors have total porosities of 3.2-12.7%. Crystal area fractions of the interiors and margins are relatively similar, ranging from 27.8 to 35.7%. Preliminary analysis does not indicate any systematic relationship between flow direction and variations in magmatic properties. To establish why these seemingly buoyant magmas were driven laterally, we will incorporate mechanical modeling of edifice stresses to determine the combined role of magmatic properties and external controls on magma pathways beneath stratovolcanoes.