Large volcanic eruptions are fascinating displays of our Earth's internal energy. They involve a series of complex processes from magma generation in the mantle and deep crust to the catastrophic outpourings of hot ash and gas on the surface of our planet. The processes by which silicic magma behave in shallow crustal reservoirs are critical to our understanding of volcanism and crustal construction. In that respect, large silicic ignimbrites (i.e., pyroclastic flow deposits) provide an exceptional record of how such magmas evolve in interaction with their environment. An ideal region in which to study large-volume silicic magmatism (especially from perspectives of crustal evolution and petrological processes occurring in large silicic magma reservoirs) is the San Juan volcanic field (Colorado, USA). Highly productive Tertiary magmatism (at least 17 large-volume ash-flow sheets erupted between ~29.5 to 26.9 Ma) through continental crust dominated by Precambrian lithologies allows a number of possible studies, some of which will be explored in this proposal.
It is proposed to use large ignimbrite sheets from the San Juan volcanic field as natural laboratories for silicic magma evolution processes. The team will combine results from geochemical micro-analytical techniques (such as electron microprobe, LA-ICP-MS and microdrilling-TIMS analyses) in conjunction with numerical simulations of magma dynamics to better understand the mechanisms leading to compositional zoning in large ash-flow tuffs; the dynamics of crustal assimilation in upper crustal conditions; and examine all necessary conditions leading to eruption of large reservoirs. New analytical data in addition to published geochronological and geochemical data on San Juan magmas will be used to provide the ideal stepping stone for scrutinizing key rock units and merging physical and geochemical models of magmatic processes.
