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 ~14kbar, 1100°C with moderate decompression to ~11kbar, 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 9mol% H2O from T–MH2O modelling. Further late stage cooling to <740°C is recorded by voluminous retrograde mica growth and partial preservation of a ky-pl-kfs-bt-liq bearing equilibrium assemblage. Overall, Zr-in-rutile temperatures at 11kbar (Tomkins et al., 2007) are reset to between 606°C and 780°C across all samples, with flat core–rim Zr concentration profiles in all rutiles. However, zircon precipitates as inclusions, needle exsolutions, or rods along rutile grain boundaries are recrystallised from rutiles in qz/fsp domains. Reintegrating the Zr-in-rutile concentration ‘lost’ via the recrystallisation of these zircon precipitates (e.g. Pape et al., 2016) can recover maximum concentrations of up to ~2.2wt% and thus maximum peak temperatures of ~1149°C at 11kbar. Rutile Nb–Ta signatures and rounded rutile grains without zircon precipitates in hydrated mica domains in sample 49688 provide evidence for fluid-mediated mobility of Zr and Nb during retrograde cooling in hydrated lithologies. Aqueous fluid supplemented excess H2O 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.