Affiliation: College of Arts and Sciences, Department of Geological Sciences
Geochemical analyses of leucosomes in deep crustal migmatites and gneisses provide a better understanding of the connection between the deep crust and large-volume, high-silica rhyolites. Similarities between the leucosomes and rhyolites suggest some rhyolites may be sourced from the deep crust. This provides an alternative to models calling for upper crustal rhyolite sources. Despite these similarities, K2O, MnO, CaO, Rb/Sr, Y, and U values are lower in leucosomes compared to rhyolites and must be explained. Fractional crystallization models using leucosome compositions demonstrate that a larger segment of the rhyolite data is obtainable but the most enriched portions of large silicic ignimbrites remain unattainable. The degree of partial melting during incongruent melt reactions likely plays a key role in controlling the release of these elements. Modeling of these partial melt reactions suggest that the most enriched ignimbrite chemistries can be achieved through low degrees of partial melting in the deep crust.