THE DEFORMATION BEHAVIOUR OF HORNBLENDE IN THE FLORENCE SHEAR ZONE,CENTRAL AUSTRALIA

Amphibole minerals usually exhibit brittle behaviour in low temperature deformation. Observation of microstructural features on some mafic mylonites from the Florence Ductile Shear Zone of central Australia has revealed that the hornblende grains within these high temperature mylonites display extensive plastic deformation behaviour characterized by intensely strained, fracture free hornblendes with pronounced intracrystalline strain features and strong crystallographic preferred orientation. Grainsize refinement of hornblende in the mafic mylonites is a result of the local grain boundary migration. The factors responsible for the plastic deformation behaviour of hornblende have been explored and discussed. High temperature during the mylonitization played an important role in enhancing the ductility of hornblendes and geometrical softening also made some contribution.