Modelling the partial melting of metasediments in a low-pressure regional contact aureole: the effect of water and whole-rock composition

Abstract

AbstractLow-pressure regional aureoles with steep metamorphic field gradients are critical to understanding progressive metamorphism in high-temperature metasedimentary rocks. Delicately layered pelitic and psammitic metasedimentary rocks at Mt Stafford, central Australia, record a greenschist- to granulite-facies Palaeoproterozoic regional aureole, associated with S-type granite plutons, reflecting metamorphism in the range 500–800 °C and at ∼3 kbar. The rocks experienced minimal deformation during metamorphism and partial melting. Partial melting textures evolve progressively along the steep metamorphic field gradient from the incipient stages of melting marked by cuspate grains with low dihedral angles, to melt proportions sufficient to form diatexite with schollen. Phase equilibria modelling in the NCKFMASHTO system for pelitic, semi-pelitic and high- and low-ferromagnesian psammitic samples quantitatively illustrates the dependence of partial melting on rock composition and water volume. Pelitic compositions are more fertile than psammitic compositions when the water content in the rocks is low, especially during the early stages of melting. The whole-rock ferromagnesian component additionally influences melt fertility, with ferromagnesian-rich psammite being more fertile than psammite with a lower ferromagnesian component. Subtle variations in free water content can result in obvious changes in melt volume but limited variation in melt composition. Distinct melting histories of pelitic and psammitic rocks inferred from field relationships may be partially attributed to potential differences in water volume retained to super-solidus conditions. Melt composition is more dependent on the rock composition than the variation in water content.