PETROGRAPHIC CHARACTERISTICS OF MUSGRAVE PSEUDOTACHYLITE INDICATING IMPACT ORIGIN

Pseudotachylite breccias of the Musgrave Province central Australia are found in granulite facies rocks in zones up to 5 km wide running intermittently for 300 km with up to 10% pseudotachylite veining, presumed to be formed by melting in anhydrous conditions. The veins are measured on the ground in width from a few centimeters up to 4 m and can be traced for up to 10 m. Observation with optical and electron microscopy revealed very thin veins in scale of microns and spread through the host rocks. As the extent, volume and width (micron to meter) pose a dilemma on the genesis of pseudotachylites, detailed petrographic analysis was conducted with optical and electron microscopy, X-ray Diffraction (XRD). Mineral composition of the separate veins of pseudotachylites were determined with Empryean Multipurpose X-Ray Diffractometer as Anorthite (48.01%), Augite (27.69%), Pargasite (17.72%), Quartz (3.22%), Biotite (2.45%) and Magnetite (0.91%).

The presence of significant amount of Pargasite [NaCa₂ (Mg₄Al)(Si₆ Al₂)O₂₂(OH)₂] in the veins ruled out the anhydrous origin of the Musgrave pseudotachylites.

Petrographic analysis identified diaplectic glass and multiple sets of planar deformation features (PDFs) and partial isotropization of quartz in samples from Mount Frazer, along with shocked feldspar and thetomorphs, diaplectic glass that retain their original shape, at even higher pressures, crystals may undergo atomic-structural displacements that convert them to glasses without passing through a melt stage.

The results suggest an impact origin for the Musgrave pseudotachylite breccias, thus replacing the seismic origin hypothesis which appears to be a misinterpretation due to the proximity of the pseudotachylite breccias to a known thrust fault.