The role of fluids in retrograde shearing: Broken Hill, Australia

Shear zones at Broken Hill, Australia, are highly deformed structures that have provided a focus for migrating aqueous fluids, resulting in the widesp read retrogression of rocks within and adjacent to their boundaries. Sheari ng probably occurred soon after regional metamorphism at greenschist to low er amphibolite facies. Lowering of delta(18)O values in the range of 10-12 parts per thousand in metasedimentary host rocks to 7-8 parts per thousand in sheared metasediments suggests that isotopic exchange occurred due to pe rvasive fluid flow. Time integrated fluid fluxes of 1-7 x 10(6) m(3)/m(2) a nd calculated metasomatism suggest that retrograde shear zones were enriche d in silica and were subjected to a high degree of fluid howl This is consi stent with petrographic observations showing the crystallisation of abundan t quartz and muscovite. (C) 2000 Elsevier Science B.V. All rights reserved.