An investigation of the origin of ultrabasic granofels, Fiordland, New Zealand

Ultrabasic granofels are a minor component of the Pembroke Granulite that is hosted within granulite facies gabbroic gneiss, in Fiordland, New Zealand. The ultrabasic granofels unit consists predominantly of pargasite ± clinozoisite. The unit has a rare earth element (REE) pattern that shows a typical amphibole cumulate middle-REE hump, which suggests an igneous cumulate origin. However, field relationships, including irregular contact boundaries and significant mineralogical changes, suggest the ultrabasic granofels has replaced part of the gabbroic gneiss as a result of metasomatic melt-rock interaction.Significant REE-enrichment in the ultrabasic granofels relative to the host gabbroic gneiss, low dihedral angles in plagioclase grains and geochemical evidence of mass increase all point to an influx of external melt. A model is presented for a reaction product that imitates an igneous cumulate; involving fluxing of granodioritic melt through a high to moderate strain zone. The system and melt are at an appropriate P-T where it is pargasite +clinozoisite-saturated. This results in the dissolution of plagioclase and quartz and crystallisation of pargasite + clinozoisite. Progressive melt-rock interaction leads to the formation of plagioclase-quartz-poor and pargasite-clinozoisite-rich rock domains. This model presents a new way of forming basic to ultrabasic rocks in the deep crust.