Volcanic textures in quartzo-feldspathic gneiss of the Willyama Supergroup, Olary Domain, South Australia

Several occurrences of quartzo-feldspathic gneiss forming part of the Palae oproterozoic Willyama Supergroup in the Olary Domain, South Australia, reta in textural indications for a felsic volcanic origin. These examples preser ve relict quartz phenocrysts ('quartz eyes') and, at a few sites, relict fe ldspar phenocrysts and fragmental textures are retained. The rocks contain an assemblage of albite and quartz, with minor muscovite, biotite, K-feldsp ar, magnetite, hematite, pyrite, zircon and xeno-time. They share chemical characteristics with other felsic gneisses in the Willyama Supergroup with U-Pb zircon ages of ca 1700 Ma which have been inferred to have formed from A-type rhyolite precursors, it is interpreted that the rocks represent rhy olitic pyroclastic or epiclastic deposits (with associated minor iron-forma tion and cherty exhalites), which subsequently underwent pre- and/or syn-me tamorphic Na (-Fe) metasomatism and amphibolite-grade metamorphism.