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

Progressive localisation of strain within Proterozoic high-grade rocks in t he Southern Cross area of the Willyama Supergroup, Broken Hill, has resulte d in the formation of shear zones of highly strained D-2 fabrics separated by less strained D-2 zones that preserve pre-tectonic relationships, comple x fold interference patterns and abundant pegmatites related to a D-1/M-1 d eformation/metamorphic event. The metamorphic peak (M-1) is characterised b y biotite-sillimanite-quartz-alkali feldsparplagioclase with garnet or cord ierite in metapelites, formed at about 750 degreesC and 400-500 MPa, presum ably relatively late in M-1 as garnet has overgrown the S-1 biotite-sillima nite fabric. The D-2 strain is concentrated in high-strain zones that form sub-planar and anastomosing networks of apparently simple S-L tectonite fab rics associated with subhorizontal shearing and are locally overprinted by D-2 leucosomes in the more aluminous metasediments. M-2 involved identical metamorphic conditions as Mi. Late D-2 muscovite-bearing pegmatites intrude d 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 acro ss a muscovite fabric at about 600 degreesC at 400-600 MPa, resulting from the addition of H2O to the original upper amphibolite facies assemblages. Y ounger phases of retrogression, generally involving thrusting, are usually localised in sites of pre-existing high strain. They are characterised by c hlorite-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 pe riods of repose at the ambient geotherm. (C) 2001 Elsevier Science B.V. All rights reserved.