Characterizing the earliest stages of partial melting: A study of the pyrometamorphic aureole of Miocene Mt. Perkins pluton, northwest Arizona

Partial melting is an important process in the evolution of continental crust. Due to its progressive nature, evidence of the early stages of partial melting is often overprinted by later, advanced stages of partial melting. Experimental work has provided a model for the early stages of partial melting, but is limited to timescales of hours to weeks. Pyrometamorphic environments within the contact aureole of mafic intrusions provide natural laboratories for the study of the earliest stages of partial melt production on geologic time scales of 102 collected along two traverses of the tilted pyrometamorphic aureole of the Mt. Perkins pluton provide an opportunity to quantify partial melt volume percent and chemistry at 2 kbar. Using titanium-in-quartz geothermometry, EPMA techniques to obtain mineral chemistry, and a point-counting method using imaging software, melt volume percent, whole-rock, melt, and residual chemistry and percent minerals in each category were quantified. Melt volume percent and temperature decrease with distance from the pluton. Melt geochemistries plot near the cotectic in the quartz-(Ab+Or)-An system. Plaigoclase anorthite components were quantified and compared with eachother based on textures. The earliest stages of partial melting are recorded in samples that experienced a lower heat budget distal to the pluton. These data confirm hypotheses that earliest melts are silica rich and become progressively enriched in other cations as melting continues.