GEOLOGY OF THE MT HAY REGION, ARUNTA BLOCK, CENTRAL AUSTRALIA

We have investigated the granulites of the Mt Hay region, southeastern Arunta block, central Australia, in order to constrain deformation styles, microstructures, and fabrics in lower continental crust. The Mt Hay region, part of the Strangways Metamorphic Complex, consists of mafic-composition Proterozoic granulites that deformed at conditions of 700-800°C and 8-9 kbar. The Mt Hay region consists of three main NW-SE-trending ridges: Mt Hay ridge, Capricorn ridge, and Ceidlh Hill ridge.

The Mt Hay ridge is a kilometer-scale sheath-like fold, presently synformal, whose nose plunges steeply to the northeast. Preliminary AMS (Anisotropy of Magnetic Susceptibility) measurements indicate that magnetic fabrics do not directly correlate with observable field fabric. Capricorn ridge, separated from Mt Hay ridge by a shear zone, contains an approximately 0.5 kilometer thick anorthositic granulite that is the principle focus of our studies. The anorthositic granulite consists of medium-grained plagioclase (~An80), with up to 15% pyroxene, hornblende, and quartz. Mafic minerals delineate the foliation and lineation, defined by compositional banding and shape-preferred orientation. Foliation is isoclinally folded at the kilometer and decimeter scale. Preliminary Electron Back-Scatter Diffraction pattern analyses conducted at the University of Liverpool indicate a weak plagioclase CPO (Crystallographic Preferred Orientation). Capricorn Ridge and the anorthositic granulite are cut by a late-stage shear zone in which plagioclase anorthite content decreases (An~50), overall quartz content increases, and quartz domains define a foliation. Within this shear zone, quartz grains exhibit a CPO and plagioclase CPO appears random.