Metasomatic alteration associated with regional metamorphism: an example from the Willyama Supergroup, South Australia

Abstract

The Olary Domain, part of the Curnamona Province, a major Proterozoic terrane located within eastern South Australia and western New South Wales, Australia, is an excellent example of geological region that has been significantly altered by metasomatic mass-transfer processes associated with regional metamorphism. Examples of metasomatically altered rocks in the Olary Domain are ubiquitous and include garnet–epidote-rich alteration zones, clinopyroxene- and actinolite-matrix breccias, replacement ironstones and albite-rich alteration zones in quartzofeldspathic metasediments and intrusive rocks. Metasomatism is typically associated with formation of calcic, sodic and/or iron-rich alteration zones and development of oxidised mineral assemblages containing one or more of the following: quartz, albite, actinolite–hornblende, andradite-rich garnet, epidote, magnetite, hematite and aegerine-bearing clinopyroxene. Detailed study of one widespread style of metasomatic alteration, garnet–epidote-rich alteration zones in calc-silicate host rocks, provides detailed information on the timing of metasomatism, the conditions under which alteration occurred, and the nature and origin of the metasomatic fluids. Garnet–epidote-bearing zones exhibit features such as breccias, veins, fracture-controlled alteration, open space fillings and massive replacement of pre-existing calc-silicate rock consistent with formation at locally high fluid pressures and fluid/rock ratios. Metasomatism of the host calc-silicate rocks occurred at temperatures between ∼400°C and 650°C, and involved loss of Na, Mg, Rb and Fe 2+, gain of Ca, Mn, Cu and Fe 3+ and mild enrichment of Pb, Zn and U. The hydrothermal fluids responsible for the formation of garnet–epidote-rich assemblages, as well as those involved in the formation of other examples of metasomatic alteration in the Olary Domain, were hypersaline, oxidised, and chemically complex, containing Na, Ca, Fe 3+, Cl, and SO 4 2−. Sm–Nd geochronology indicates that the majority of garnet–epidote alteration occurred at 1575±26 Ma, consistent with field and petrographic observations that suggest that metasomatism occurred during the retrograde stages of a major amphibolite-grade regional metamorphic event, and prior to the latter stages of regional-scale intrusion of S-type granites at 1600±20 Ma. The fluids responsible for metasomatism within the Olary Domain are inferred to have been derived from devolatilisation of a rift-related volcano-sedimentary sequence, perhaps containing oxidised and evaporitic source rocks at deeper structural levels, during regional metamorphism, deformation and intrusion of granites. At the present structural level, there is no unequivocal evidence for the fluids to have been directly sourced from granites.