Zones of progressive regional metamorphism in the Willyama complex, broken hill district, New South Wales

Abstract

Descriptions are given of basic, pelitic and granitic rocks belonging to the Willyama Complex which were recrystallized during its initial metamorphism, the Willyama Metamorphism. With progressive increase in metamorphic grade from north‐west to south‐east, rocks of basaltic composition change from amphibolites containing blue‐green hornblende (Zone A), to varieties with brown‐green hornblende (Zone B), then to hornblende‐two pyroxene gneisses (Zone C). Pelites pass from sillimanite‐bearing muscovite‐biotite schists into orthoclase‐bearing sillimanite‐garnet‐cordierite gneisses, while in granitic gneisses the predominant potassium feldspar changes from microcline to orthoclase. Fourteen new rock analyses and twelve rock compositions calculated from modes and mineral analyses are presented. The results of experimental studies of mineral breakdown reactions, including new data from two preliminary experiments, permit an estimation of physical conditions during metamorphism. Temperatures in the range 600–850°C and pressures of about 8 kilobars are indicated. Fluid pressures appear to have been considerably lower than total rock pressure. Details are given of 44 specimens for which mineral analyses are available.