Abstract
Reactions in mafic amphibolite marking the transition from amphibolite to granulite facies were investigated by hydrostatically heating and deforming solid cylinders of An30 plagioclase + tschermakitic hornblende + minor quartz for up to 795 h at temperatures of 700–1000 °C and mean stresses of 0.7–2.1 GPa. Melting of quartz + plagioclase + amphibole began at 775 °C in the presence of H2O and at 900 °C under H2O-undersaturated conditions. The amphibole crystals underwent Mg-Tschermak substitution and the plagioclase crystals became more anorthitic. At 875–1000 °C, the rock became a partially melted granulite with hornblende, anorthitic plagioclase, augite, and hypersthene. The augite and hypersthene nucleated on amphibole crystals in liquid films adjacent to quartz crystals. During subsequent growth (and decreasing surface/volume ratio) the pyroxene crystals abandoned their coherent interfaces with their host amphibole crystal for energetically more favorable and more mobile incoherent interfaces and grew outward into the enveloping liquid that was produced by the breakdown of plagioclase + amphibole + quartz. There is a measurable effect of stress and deformation on these reactions. Some phases grown in deformed samples have slightly, but systematically, different compositions from phases grown in hydrostatically heated samples. Unfortunately, the system is too complex to assess the implications for natural metamorphism. Comparison with equilibrium petrology experiments indicates that the reactions were incomplete and the compositions and proportions of the phases produced during the experiments are metastable. This suggests that amphibolite-facies-to-granulite-facies equilibrium textures and phase compositions cannot be experimentally investigated in unpowdered mafic rock because reaction rates are too slow, unless experiments can be conducted for longer than one month.
