Decoding Mesoproterozoic and Cambrian metamorphic events in Willyama Complex metapelites through the application of Sm–Nd garnet geochronology and P–T pseudosection analysis

Abstract

The pressure–temperature ( P–T) path for the Palaeoproterozoic Willyama Supergroup rocks in the Olary Domain, South Australia has been reconstructed through detailed petrographic observations, in conjunction with calculation of compositionally specific P–T pseudosections of metapelitic rock units and Sm–Nd garnet geochronology. The P–T path for the Willyama Complex has historically been interpreted to follow a single anticlockwise path, however the results of this study demonstrate that this path can be better described by two metamorphic events (M 1 and M 2) separated by 1100 Ma. The M 1 event occurred at c. 1600 Ma and was associated with high temperature–low pressure metamorphism. Sm–Nd garnet geochronology constrains the timing of garnet growth at c. 1585 Ma. The growth of large andalusite porphroblasts during M 1 exerted a first order control on the bulk composition during the subsequent M 2 event. Mineral chemistries coupled with quantitative phase diagrams constrain peak conditions to be in the order of c. 550°C and 5.5 kbar during M 2. The identification of two metamorphic events calls into question interpretations of metamorphic core complex formation and the single anticlockwise P–T paths being associated with the early stages of the tectonic evolution of the terrain.