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 Musgr ave Ranges, central Australia. The thrust dips gently to the south and is interpreted as an extension of the Woodroffe Thrust, which was for med about 550 Ma ago. Mylonites at the base of the thrust grade upward s into ultramylonites, which pass abruptly into a pseudotachylyte-bear ing zone approximately 1 km thick, containing approximately 4% of pseu dotachylyte 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 blo cks of ultramylonite are present in some of the pseudotachylytes, and some pseudotachylyte veins have been plastically deformed, suggesting nearly contemporaneous semi-ductile and brittle behaviour. The matrix of the pseudotachylyte shows spectacular examples of igneous quench mi crostructures, especially skeletal and dendritic crystals of plagiocla se and feathery pyroxene dendrites. Also present are glass devitrifica tion microstructures (spherulites), evidence of liquid flow, and partl y melted residual grains with former glassy rims showing different opt ical properties from those of the surrounding isotropic material. Thes e features confirm that the pseudotachylyte formed by melting in anhyd rous conditions. The matrix of the pseudotachylyte veins is less silic eous 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 f rom very high-temperature (>1000 degrees C). Melting and quenching is probably due to very local, short-lived rises in temperature accompani ed by dilation.