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Glaciovolcanism at the Mount Cayley volcanic field, Garibaldi volcanic belt, Southwestern British Columbia Kelman, Melanie Catherine

Abstract

This thesis investigates glaciovolcanism in the Mount Cayley volcanic field (MCVF) of southwestern British Columbia's Garibaldi Volcanic Belt (GVB). The MCVF is dominated by intermediate magma compositions, has been intermittently glaciated, and has extreme topography. No study to date has focused on intermediate composition glaciovolcanic deposits in a similar setting. The core of this thesis is a 1:20,000 volcanological map, which was used in conjunction with field observations, geochemical data, and petrographic examinations to investigate the volcanological history of the MCVF, to investigate the eruptive processes that produce specific glaciovolcanic landforms, and to make predictions about paleo-ice thicknesses and distributions. The MCVF (and the entire GVB) has three dominant intermediate composition glaciovolcanic landform types: subglacial domes, flow-dominated tuyas, and impoundment features. Subglacial domes are irregularly-shaped piles of lava flows representing subglacial eruptions that commenced beneath 100-650 m of ice and did not breach the surface; in many cases, the subglacial domes grew to within 150 m of the ice surface. Flow-dominated tuyas are steep-sided, flat-topped stacks of lava flows representing subglacial eruptions that ultimately breached the ice surface. Subglacial domes and flow-dominated tuyas represent the same eruptive process, the primary difference being whether or not the ice was breached. Impoundment features are subaerial lava flows with steep flanks or termini representing channeling or ponding of lava by ice. All three landforms have intense fine-scale jointing that indicates cooling surfaces inconsistent with apparent paleotopography. Additionally, all lack features recording eruption into water (pillows and hyaloclastite), indicating efficient and continuous meltwater drainage during eruptions. This is different from glaciovolcanic deposits in most other settings. It also makes these deposits distinct from basaltic glaciovolcanic deposits in the GVB. A combination of ice geometry, topography, and magma composition is responsible; thin ice promotes the formation of drainage pathways around subglacial vents, while thin ice coupled with steep bedrock topography creates hydraulic gradients away from vents. Magma composition probably also plays a role, due to its effect on quantities of ice melted and the pressure in subglacial vent cavities and drainage conduits.

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