Large volumes of anhydrous pseudotachylyte in the Woodroffe Thrust, eastern Musgrave Ranges, Australia

A mylonitic thrust zone, at least 1.5 km thick, forms a sharp contact between granulite and amphibolite facies gneisses in the eastern Musgrave Ranges, central Australia. The thrust dips gently to the south and is interpreted as an extension of the Woodroffe Thrust, which was formed about 550 Ma ago. Mylonites at the base of the thrust grade upwards into ultramylonites, which pass abruptly into a pseudotachylyte-bearing zone approximately 1 km thick, containing approximately 4% of pseudotachylyte veining. The orientation of the veins appears to be random. Pseudotachylytes occur only in the granulite facies rocks, and their precursors are felsic pyroxene and/or garnet granofelses. Rotated blocks of ultramylonite are present in some of the pseudotachylytes, and some pseudotachylyte veins have been plastically deformed, suggesting nearly contemporaneous semiductile and brittle behaviour. The matrix of the pseudotachylyte shows spectacular examples of igneous quench microstructures, especially skeletal and dendritic crystals of plagioclase and feathery pyroxene dendrites. Also present are glass devitrification microstructures (spherulites), evidence of liquid flow, and partly melted residual grains with former glassy rims showing different optical properties from those of the surrounding isotropic material. These features confirm that the pseudotachylyte formed by melting in anhydrous conditions. The matrix of the pseudotachylyte veins is less siliceous than the host rocks, owing to non-equilibrium melting of pyroxene, garnet and plagioclase. The igneous assemblages of the melt, notably the crystallization of pigeonite, are consistent with rapid cooling from very high-temperature (>1000°C). Melting and quenching is probably due to very local, short-lived rises in temperature accompanied by dilation.