Garnet porphyroblast-bearing leucosomes in metapelites: mechanisms, phase diagrams, and an example from Broken Hill, Australia

Abstract

Mafic porphyroblasts are commonplace in leucosomes in metamorphic rocks at uppermost amphibolite facies and in the granulite facies. Large, 5 cm or more in diameter, garnet porphyroblasts are common in leucosomes in low-to medium-pressure granulite facies metapelites such as those in the Broken Hill area, Australia. Although there is significant consensus regarding the origin of those porphyroblasts — that they form as a consequence of incon-gruent melting reactions in which hydrous minerals are consumed — there are various interesting questions which need to be answered. For example: if the porphyroblasts are melting products, why then are they preserved during cooling when the melt crystallizes and the hydrous mineral becomes stable again? Intuitively, one would expect complete, or at least major retrogression of the porphyroblasts. This is not usually observed, and is generally absent in the garnet porphyroblasts at Broken Hill.