The influence of bulk rock MnO on garnet development in metamorphic rocks of andalusite-staurolite grade; Kanmantoo, South Australia

Abstract

An outcrop of meta-pelitic schist from near the township of Kanmantoo, South Australia, was chosen to demonstrate the effect that increased bulk MnO content has on metamorphic assemblage development. From bulk composition and petrological analyses it is observed that the presence of small amounts of MnO in high XFe pelitic rocks correspond with the development of garnet. The incorporation of Mn into the KFMASH model system describes the existence of garnet with andalusite and staurolite that KFMASH does not predict. Furthermore it is shown that modal abundances of garnet are proportional to bulk MnO rock contents, enhancing the prediction that MnO causes an expansion of all garnet-bearing assemblages. This allows for the description of the numerous appearances of the garnet mineral in rocks that have calculated temperatures and pressures, for which it is not predicted to exist at.