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Ejected fragments of magma mush in the Fish Canyon Tuff (Colorado, USA) and the processes within magma mush that feed crystal-rich supereruptions

Morgan Harris , Guil Gualda

Abstract

The Fish Canyon Tuff (Colorado, USA) erupted following the collapse of the La Garita caldera around 28 Ma and resulted in one of the largest-known ignimbrite deposits. This ~5000 km3 ignimbrite of dacitic composition has a high phenocryst content and near solidus mineral assemblage. During initial field observations, coarse-grained, crystal-rich fiamme within a pink, interstitial matrix were found within the Fish Canyon Tuff. The fiamme are abundant in some intracaldera horizons, varying in size from a couple of centimeters to decimeters. The largest crystals are feldspars, up to 1 -- 2 centimeters in length, with other crystals that are several millimeters in length. The composition of these fiamme appear to be similar to the rest of the ignimbrite, but they have coarser-grained, crystal-rich textures. Initial textural observations lead us to conclude that these fiamme are ejected fragments of magma mush. Further work will include optical and scanning electron microscopy analysis of thin sections, which will allow us to study the relationship between the fiamme matrix and the crystals within it. We will also use energy-dispersive spectrometry to analyze the composition of matrix and crystals in these fiamme. The results will help us understand the origin of these crystal-rich fiamme and understand processes within magma mush for systems that feed crystal-rich supereruptions.