Metamorphic evolution of the central Southern Cross Province, Yilgarn Craton, Western Australia

Two contrasting styles of metamorphism are preserved in the central Souther n Cross Province. An early, low-grade and low-strain event prevailed in the central parts of the Marda greenstone belt and was broadly synchronous wit h the first major folding event (D-1) in the region. Mineral assemblages si milar to those encountered in sea-floor alteration are indicative of mostly prehnite-pumpellyite facies conditions, but locally actinolite-bearing ass emblages suggest conditions up to mid-greenschist facies. Geothermobarometr y indicates that peak metamorphic conditions were of the order of 250-300 d egrees C at pressures below 180 MPa in the prehnite-pumpellyite facies, but may have been as high as 400 degrees C at 220 MPa in the greenschist facie s. A later, higher grade, high-strain metamorphic event was largely confine d to the margins of the greenstone belts. Mineral assemblages and geothermo barometry suggest conditions from upper greenschist facies at P-T condition s of about 500 degrees C and 220 MPa to upper amphibolite facies at 670 deg rees C and 400 MPa. Critical mineral reactions in metapelitic rocks suggest clockwise P-T paths. Metamorphism was diachronous across the metamorphic d omains. Peak metamorphic conditions were reached relatively early in the lo w-grade terrains, but outlasted most of the deformation in the higher grade terrains. Early metamorphism is interpreted to be a low-strain, ocean-floo r-style alteration event in a basin with high heat flow. In contrast, diffe rential uplift of the granitoids and greenstones, with conductive heat inpu t from the granitoids into the greenstones, is the preferred explanation fo r the distribution and timing of the high-strain metamorphism in this regio n.