Strain localisation and high-grade metamorphism at Broken Hill, Australia: a view from the Southern Cross area

Abstract

Progressive localisation of strain within Proterozoic high-grade rocks in the Southern Cross area of the Willyama Supergroup, Broken Hill, has resulted in the formation of shear zones of highly strained D 2 fabrics separated by less strained D 2 zones that preserve pre-tectonic relationships, complex fold interference patterns and abundant pegmatites related to a D 1/M 1 deformation/metamorphic event. The metamorphic peak (M 1) is characterised by biotite–sillimanite–quartz–alkali feldspar-plagioclase with garnet or cordierite in metapelites, formed at about 750°C and 400–500 MPa, presumably relatively late in M 1, as garnet has overgrown the S 1 biotite–sillimanite fabric. The D 2 strain is concentrated in high-strain zones that form sub-planar and anastomosing networks of apparently simple S–L tectonite fabrics associated with subhorizontal shearing and are locally overprinted by D 2 leucosomes in the more aluminous metasediments. M 2 involved identical metamorphic conditions as M 1. Late D 2 muscovite-bearing pegmatites intruded the area. A further Proterozoic D 3/M 3 event localised in the sites of D 2 high strain is characterised by the growth of staurolite and garnet across a muscovite fabric at about 600°C at 400–600 MPa, resulting from the addition of H 2O to the original upper amphibolite facies assemblages. Younger phases of retrogression, generally involving thrusting, are usually localised in sites of pre-existing high strain. They are characterised by chlorite-muscovite bearing assemblages. It is proposed that the events D 1/M 1,D 2/M 2 and D 3/M 3 occur within separate heating events separated by periods of repose at the ambient geotherm.