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How to emplace your rhyolite: Mechanisms for rhyolitic dike emplacement from field observations and microtextural analysis at Summer Coon volcano, Colorado, USA

Gui Aksit 1,2, Meredith Townsend3,2, Thomas Giachetti2, Kathy Cashman2, Katie Springer2, Andrew Harp4

  • Affiliations:  1 Geology Department, Whitman College, Walla Walla, USA; 2 Earth Sciences Department, University of Oregon, Eugene, USA; 3 Department of Earth & Environmental Sciences, Lehigh University, Bethlehem, USA; 4 Earth & Environmental Sciences Department, California State University, Chico, Chico, USA

  • Presentation type: Talk

  • Presentation time: Thursday 16:00 - 16:15, Room S150

  • Programme No: 1.9.6

  • Theme 1 > Session 9

Abstract

Rhyolitic eruptions exhibit complex behavior, involving hybrid explosive-effusive activity. These eruption dynamics are regulated by processes occurring in the deeper conduit. The nature of rhyolitic material at depth, whether a coherent, viscous magma, or a fragmented gas-pyroclast mixture, influences the interactions between magmatic material and host rock, as well as degassing paths for exsolved volatiles. Conduit morphology, and the accretion of material, may influence the progression of intrusion and eruption, but cannot be directly observed at modern systems. Data from ancient, exposed conduits can illuminate the dynamics and evolution of rhyolite intrusions at depth, and inform our interpretation of the processes occurring at modern volcanic centers. At Summer Coon, an eroded Oligocene stratovolcano in the San Juan volcanic field, Colorado, USA, three radial rhyolitic dikes are well-exposed within the edifice. We present data for one ~1.5-km long, segmented dike with paleo-depths during intrusion estimated to be ~1.1 km proximal to the edifice up to 0.7 km distally. Like the other two, it displays three distinctive zones from margin to interior: an obsidian rich, brecciated margin zone, a glassy flow-banded zone, and a platy interior zone. Brecciated margins are analyzed for clast size distribution and modal abundance (glass vs. lithics). This analysis is combined with SEM-BSE microtextural analysis of interior zones to determine the method of brecciation (shear or explosive fragmentation), and whether the interior represents re-sintered fragments or unfragmented magma. Comparison of these zones along the dike will allow us to characterize emplacement conditions over the course of intrusion.