Preliminary Studies of the Emplacement of Trachytic Lava Flows and Domes in an Ice- Contact Environment: Mount Edziza, British Columbia, Canada

Mount Edziza Volcanic Complex (MEVC) lies within the Northern Cordilleran Volcanic Province (NCVP), in northwest British Columbia, Canada. The eruption products include basalt, trachyte, and rhyolite that have been emplaced in a variety of subaerial, sub-ice and subaqueous environments from about 8Ma to less than 2000 y.b.p. The Ice Peak Formation (IPF) is approximately 1Ma (Souther, 1992) and includes basaltic and trachytic effusive and explosive products. Two IPF bluffs on the western side of Edziza, Ornostay Bluff (OB) and Koosick Bluff (KB), and another nearby construct (Triangle Dome, TD) were interpreted by Souther (1992) as sequences of trachytic lavas flows (OB, KB) and a lava dome (TD) formed in an ice-contact environment. This interpretation was confirmed through detailed field examination of joint patterns, visible textures, and other structures. Both OB and KB consist of several lava flows, which vary from several to 75 or more meters in thickness, emplaced on a steep surface. Basal flow breccias (locally hydrothermally-altered), are overlain by massive conchoidally-fractured lava with large-sized (greater than 1m in width) poorly-developed columns, local flow banding, and folded incorporated flow breccia. The tops of the flows display better-developed and smaller- scale columns with well-developed perpendicular planar joints. Textures are comparable to those in rhyolite domes, implying a similar viscosity on emplacement. Column orientations and joint measurements at OB and KB indicate local cooling against a steep surface, which is interpreted as ice. Spectacular fanning columns at TD likely indicate emplacement of several very steep-sided domes within a sub-ice cavity or cavities. Geochronology and volatile analysis of glass is currently ongoing. Detailed study of the products will help constrain paleo-ice thicknesses at the MEVC at the time of lava emplacement. Preliminary interpretation based upon the thicknesses of TD and OB suggests a minimum ice thickness of at least 225 meters at both centers.