A window into the lower crust: Trace element systematics and the occurrence of inclusions/intergrowths in granulite-facies rutile

Abstract

Rutile occurs as an accessory mineral in many high-temperature metamorphic assemblages and has the potential to identify and investigate UHT metamorphic terranes. Whilst the use of Zr-in-rutile geothermometry to identify ultrahigh-temperature terranes is appealing, its application to granulites can be difficult owing to diffusional resetting of Zr concentrations during cooling and decompression. In order to provide constraints on P-T conditions and trace element systematics during ultrahigh-temperature metamorphism, metamorphic rutile in granulites from the Archean Napier Complex and the Palaeozoic Rauer Group, Antarctica have been investigated for trace element composition and mineral inclusions/intergrowths. Textural observations and Zr-in-rutile temperatures show that unlike the large proportion of rutiles grains in the matrix, rutile grains that are shielded by phases with low Zr-diffusivities (e.g. orthopyroxene) and/or are in chemical isolation from zircon, have the potential to retain Zr concentrations that correspond to ultrahigh-temperatures. Principal Component Analysis reveals that V, Cr, Nb, Mo, Ta and W are significantly enriched in rutile from Archean and Proterozoic ultrahigh-temperature terranes. High concentrations of elements such as Cr and V indicate a spinel-rich source, demonstrating the effect of protolith bulk chemical composition on rutile trace element signatures. Rutile is shown to contain inclusions and intergrowths of aluminium silicate, quartz, corundum and feldspar, including the first reported occurrence of prograde kyanite, which provides direct evidence that the Napier Complex experienced a typical clockwise P-T evolution.