GEOCHEMICAL CHARACTERISTICS OF THE PSEUDOTACHYLITES OF MUSGRAVE PROVINCE

The pseudotachylite breccia deposits of Musgrave Province of Australia, are about 5 km wide and run intermittently for 300 km with approximately 10% pseudotachylite veining. The veins are range in width from a few centimeters up to 4 m and can be traced for up to 10 m. Pseudotachylites occur only in the granulite facies rocks and some pseudotachylyte veins have been plastically deformed, indicating nearly contemporaneous ductile and brittle behavior.

The matrix of the pseudotachylyte veins is less siliceous than the host rocks, probably 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. Pseudotachylite veins are relatively depleted in Ca, Mg, Mn and H₂O than the host rock, but the veins are relatively enriched in Ca, Mg and Na along with Fe, Ti, and Cr perhaps due to instantaneous non-equilibrium melting and very high oxygen fugacity.

XRD analysis by using energy discrimination settings to reduce fluorescence due high concentration Fe, of separated pseudotachylite vein confirmed the presence of about 17% of pargasite [NaCa₂(Mg₄Al)(Si₆Al₂)O₂₂(OH)₂], that ruled out the anhydrous origin of the Musgrave pseudotachylites.

The results suggest that the Musgrave pseudotachylite is impact generated and that a portion of the melt is from the bolide. This is a significant discovery that should lead to a reexamination of the origins of Musgrave pseudotachylite breccia. The seismic origin was probably a misinterpretation owing to the proximity of pseudotachylite veins to the Woodroffe Thrust Fault.