Rb/Sr isotopic and geochemical evolution of a recrystallized shear (mylonite) zone at Broken Hill

Relatively narrow (1 to 1,000 metres) planar zones of intense shear deformation and retrograde metamorphism (retrograde shist zones, RSZ) are common in the Willyama Complex around Broken Hill. A Rb/Sr isotopic study of one of these zones has revealed an unexpected isotopic pattern. All of the analysed points lie to the left of and above the 1665 Ma isochron of the Potosi Gneiss host. This result indicates a different isotopic composition of the zone as a whole rather than a chemical redistribution of Potosi Gneiss within a closed system. The isochron plot scatter is similar to that of ubiquitous metamorphic pegmatites in the basement of the Broken Hill region. The shear zone observation, however, is shown to be unrelated to the pegmatite segregation process and requires the introduction, by elemental exchange, of a more radiogenic strontium, whilst maintaining relatively fixed Rb/Sr ratios. Significant proportions of SiO₂, K₂O and Ba were removed at the same time. Na₂O, Al₂O₃, TiO₂, MgO, P₂O₅, Zr, Ni, Sc, Y, Nb, Rb remain essentially constant after allowing for volume loss of the removed elements. CaO and Sr increase slightly overall. This pattern of elemental exchange is consistent with the breakdown of K-feldspar to muscovite and the essentially isochemical recrystallization of the other major phases (quartz, plagioclase and biotite) that are observed petrographically. Silica produced by the K-feldspar breakdown and a significant amount of the free quartz must also have been removed. Large scale fluid transport is required to achieve these results. The total rock isochron gives no indication of the age of the retrograde events. However two biotite-total rock joins indicate that the last internal isotopic redistribution occurred 458 Ma ago, shortly after the time of a regional low grade metamorphism. ⁸⁷Sr/⁸⁶Sr v 100/⁸⁶Sr plots confirm that a mixing process took place at about this time and not at 1,490 Ma or the present day. The introduced fluid had an ⁸⁷Sr/⁸⁶Sr ratio of about 0.794. If this figure still represents the original source value then it has come from what was long established mature crust at 458 Ma and not the mantle. The large fluid volumes required for the silica dissolution preclude a source in the local rock porosity, and suggest a meteoric fluid which has reacted extensively with the Willyama Complex metamorphics.