IMPACT ORIGIN OF MUSGRAVE PSEUDOTACHYLITES: THE PETROGRAPHIC EVIDENCES

Musgrave province of Central Australia illustrates a wide zone (~5 km) of pseudotachylite breccia intermittently for 300 km with up to 10% veining. The hanging wall of Woodroffe Thrust is characterized with over a hundred meters sheared pseudotachylites and overprinted by undeformed injection veins. The veins are measured on the ground in width from a few centimeters up to 4 m and can even be traced for up to 10 m. Though the petrographic analysis with optical and electron microscopy revealed very thin veins in scale of microns and spread through the host rocks. As the wide distribution, volume and thickness (micron to meter) pose a dilemma on the genesis of pseudotachylites, a more detailed petrographic analysis was conducted with optical, Raman, electron microscopy and XRD.

The mineralogical compositions of the separated pseudotachylites veins were determined with Empyrean 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 rule out the anhydrous origin of the Musgrave pseudotachylites.

Furthermore, petrographic analysis also 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.

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 prominent and well known thrust fault.