Experimental Studies of Igneous Rock Series: The Farrington Complex, North Carolina, and the Star Mountain Rhyolite, Texas

Abstract

The phase relationships of four specimens from the Farrington Complex ranging in composition from granite to tonalite, and of four specimens from the Star Mountain Rhyolite, have been determined in the presence of 15-20 weight per cent $$H_{2}O$$ in the pressure range from 1 to 3 kb. PT curves have been located for the beginning of melting and for the complete melting of quartz, alkali feldspar, and plagioclase. Glass percentages have been determined for representative samples by point counting in polished thin sections. The melting relationships for these two widely different series of rocks aid in defining the primary phase volumes in the "Granodiorite Tetrahedron." It has not been possible experimentally to reproduce the unusual natural phe-nocryst-groundmass relationships of the Star Mountain Rhyolite; this failure is attributed at least partly to the experimental condition of excess water, and confirms the conclusion, drawn from petrographic evidence, that these magmas were erupted with very low water contents.