The Textural Analysis of Evolved Magmatic Systems; a Case Study on the Fish Canyon Tuff.

Petrographic inspection of many evolved volcanic and high level plutonic rocks, reveals qualitatively complex crystal populations. The detailed textural analysis of such populations is important as it can be used as a proxy to guide macro and micro-geochemical sampling strategy, and can be employed to gain insight into the history of the magmatic system. A comparison of existing Crystal Size Distribution (CSD) data shows a variety of different scenarios for evolved magmatic systems. Many examples show kinked CSDs which are confirmed by micro-geochemical analysis to consist of mixed populations of crystals. We present new CSD data produced for the Fish Canyon magmatic system, which exhibit mainly kinked-curved trends dominated by two crystal populations, one large and one small. Given simple assumptions for growth rates, residence times may be estimated at between 80-150yrs for the small crystal population and 150-400yrs for the large. The CSD plots for the different units of the Fish Canyon magmatic system have been used to show the evolution of the magma chamber over time. Chamber systematics are shown to change from crystal settling process in the pre-caldera dacitic magmas to magma mixing processes in the Fish Canyon eruptives. In some cases crystal fragmentation due to decompression during eruption may also be an important process in modifying the observed CSDs.