Shear zones in the southern Eyre Peninsula, South Australia: Quartz c-axis fabrics in granulite facies mylonitic orthogneisses and relationship to mafic dykes

Abstract

The south eastern portion of the Eyre Peninsula, South Australia, is dominated by a suite of granites and orthogneisses deformed in steep-dipping shear zones during the Kimban Orogeny (1730–1690 Ma). The largest is the transpressional Kalinjala Shear Zone with shortening strains of ≥90% and shear strains of γ = ≥20, with other near-parallel secondary shear zones displaying similar kinematics. These ductile shear zones are dominated by mylonitic orthogneisses, with shearing commonly nucleating on, or adjacent to, precursor mafic dykes. The mafic dykes are highly sheared, folded and boudinaged with a newly formed matrix assemblage (orthopyroxene + clinopyroxene + garnet + magnetite + quartz + K-feldspar) that indicates the peak P/T metamorphic conditions of 0.7–1.0 GPa and 750–850 °C. Partial melting, developed as leucosomes, is associated with this metamorphism. Microstructural relationships between quartz crystallographic preferred orientation (CPO) and shape preferred orientations (SPO) analyses of these polymineralic rocks are best interpreted in terms of a granulite facies isothermal temperature regime of ~750 °C. The quartz c-axis fabrics, obtained using an optical fabric analyser, reveal a range of CPOs from weak girdles normal to the shear plane, to maxima parallel to the Y-strain axis. There is an overall symmetric orthorhombic pattern, with c-axis fabrics oriented with respect to the XY plane of flattening. There is a minor component of non-coaxial deformation in some shear zones. These fabrics indicate the increased activation of predominantly the prism-<a > slip system in quartz within the shear zones, which is characteristic for these high-temperature and high-strain deformation conditions.