Petrogenesis of some palaeozoic rhyolites in Victoria

Thick rhyolite sheets occur within the middle Palaeozoic cauldron subsidence complexes in Victoria. Well‐preserved eutaxitic and vitroclastic textures in the basal regions of the Rubicon, Jemba, and Ryans Creek rhyolites provide evidence of an ash‐flow origin. Subsequent recrystallization during cooling has masked these primary textures towards the central and upper parts of the rhyolitic sheets. All three rhyolites show dense welding throughout and the major zonation observed is in the degree of granophyric recrystallization.

Despite the turbulent ash‐flow emplacement and the possibility of post‐eruptive chemical modification, the mineralogical and chemical trends within vertical sequences in the Rubicon and Jemba Rhyolites are essentially continuous and represent the inverted variation within the original magma body. The bulk compositions of the rhyolites suggest that conditions of water saturation were closely approached or attained within the magma chamber. The chemical variation with increasing height and phenocryst content in the rhyolites can be summarized as: increasing K, Ca, Ba and Sr, and decreasing Na and Rb. Whereas this variation is in accord with frac‐tionation theories, it is more likely to have resulted from the partial fusion process believed to have been responsible for the generation of the acid magmas.