Evidence of Impact Shock Metamorphism in Basement Granitoids, Cooper Basin, South Australia

Abstract

The original observation of parallel closely spaced (microns to tens of microns) planar features in quartz grains from basement granitoid samples of Cooper Basin drill holes by I.T. Uysal, followed by Universal stage, scanning electron microscope (SEM) and energy dispersive spectrometry (EDS) tests by A.Y. Glikson, identify intracrystalline planar deformation features (PDF) in quartz which correspond to unique Miller indices diagnostic of shock metamorphism. U-Stage measurements of angles between the C-optic axis of quartz (Cqz) and poles to planar deformation features (Ppdf) in the same quartz grains (87 planar sets in quartz 54 grains) define intracrystalline planar orientations dominated by {10-13} and {10-12} Miller indices, correlated with shock pressures above 10 Gpa and above 20 Gpa, respectively (Engelhard and Stoffler, 1968; French, 1968; Stoffler, 1974; Stoffler and Langenhorst, 1994; Grieve et al., 1996; French, 1998). These Miller indices are characteristic of large impact structures, including Chesapeake Bay (D~85 km), Woodleigh (D~120 km) and other impact structures. SEM/EDS studies of core samples indicate features consistent with shock metamorphism, including recrystallized pseudotachylite veins and microbreccia veins injected into resorbed quartz grains. Extensive alteration of feldspar to sericite and illite represent hydrothermal effects.