Experimental Melting of Biotite Plus Plagioclase Plus Quartz Plus Or Minus Muscovite Assemblages and Implications for Crustal Melting

Abstract

In order to understand the role of mica-rich rocks as a source of granite magmas, a series of melting experiments was performed on two different starting materials. The first composition is a model biotite gneiss consisting of 30 wt % biotite, 30 wt % plagioclase, and 40 wt % quartz. The second composition is a model two-mica pelites consisting of 15 wt % biotite, 15 wt % muscovite, 30 wt % plagioclase, and 40 wt % quartz. Experiments were performed under vapor-absent conditions at 1.0 GPa and between 750 degrees and 950 degrees C. With only biotite in the starting material the volume of melt is always less than 15 vol % below 900 degrees C and reaches 25 vol % at 950 degrees C. In experiments that involve both biotite and muscovite in the starting material, the melt proportion increases up to 28 vol % at 825 degrees C and reaches 60 vol % at 950 degrees C. For the biotite-plagio clase-quartz (BPQ) assemblage, the solidus is located at 800 degrees C at 1.0 GPa. The melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet + biotite + orthopyroxene + plagioclase + quartz. In addition, the experiments show that at 1.0 GPa biotite can be stable above 950 degrees C. With both micas in the starting material (BPQM), the solidus at 1.0 GPa is located at 750 degrees C. The melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet + sillimanite + biotite + quartz + plagioclase + Kfeldspar in experiments below 900 degrees C. At 950 degrees C the residuum consists of garnet + orthopyroxene + biotite + plagioclase. The melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz. Mineral modes of the source rocks, particularly the amount of quartz, are at least as important as the amount of available H2O in controlling the melt fraction generated during crustal anatexis.