Regional retrograde metamorphism of a high grade terrain: the Willyama Complex,Broken Hill,Australia

Retrograde metamorphism has been a major influence in the development of the presently observed lithologies of the Willyama Complex, Broken Hill. Two broad types of retrogression are distinguished: pseudomorphous and kinematic retrogression. The former type of retrogression involves replacement of prograde phases without complete loss of the high grade fabric; hence the prograde assemblage can frequently be inferred. Kinematic retrogression involves the development of a new schistose fabric and, like pseudomorphous retrogression, is commonly related to F₃ deformation. Retrogression was initiated during the waning stage of prograde metamorphism and was accompanied by an influx of aqueous fluid at similar pressure, but lower temperature conditions than prograde metamorphism. The source of the water is believed to be the crystallization of cooling partial melts. The regional nature of this ‘Willyama-style’ of retrogression is attributed to the metasediment-rich nature of the sequence, the high geothermal gradient and the considerable amount of in situ partial melting.