Zr-in-rutile resetting in aluminosilicate bearing ultra-high temperature granulites: Refining the record of cooling and hydration in the Napier Complex, Antarctica

Abstract The relative validity and closure temperature of the Zr-in-rutile thermometer for recording UHT metamorphism are process dependent and hotly debated. We present an integrated petrological approach to Zr-in-rutile thermometry including phase equilibrium (pseudosection) modelling in complex chemical systems with updated mineral a-X models and systematic in-situ microanalysis of rutile. This study is centred on high-pressure rutile bearing UHT granulites from Mt. Charles, Napier Complex, Antarctica. P–T phase equilibrium modelling of two garnet bearing granulites (samples 49677, 49701) constrains an overall post-peak near isobaric cooling (IBC) evolution for the Napier Complex at Mt. Charles; from ~ 14 kbar, 1100 °C with moderate decompression to ~ 11 kbar, 800–900 °C. Local hydration on cooling over this temperature range is recorded in a kyanite bearing granulite (sample 49688) with an inferred injection of aqueous fluid equivalent to up to 9 mol% H 2 O from T–MH 2 O modelling. Further late stage cooling to 2 O liberated by melt crystallisation, interacting with rutile on cooling of sample 49688 to reset Zr-in-rutile temperatures (606–780 °C) at the Mt. Charles locality. The wide range of geochemical and petrological characteristics of Napier Complex rutile highlights that Zr-in-rutile reintegration and a broad petrological approach are required for successful interpretation of Zr-in-rutile geothermometry for long-lived regional UHT metamorphism.