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

摘要

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 degrees 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.