First application of titanium-in-zircon thermometry to ultrahigh-temperature metamorphism

Titanium-in-zircon thermometry has been applied to evaluate the temperatures of metamorphism for two ultrahigh-temperature (UHT) granulite localities, the Neoproterozoic Anápolis-Itauçu Complex in central Brazil, and lower crustal xenoliths from the Neoarchean Kaapvaal craton in South Africa. UHT metamorphism in the Anápolis-Itauçu Complex occurred ca. 640 Ma. Ti-in-zircon thermometry on metamorphic zircon yields temperatures of 965-811 °C; a population of late zircon that formed from ilmenite and rutile breakdown records temperatures of ∼780 °C. Maximum temperatures are less than those inferred from quantitative phase equilibria modeling. Based on textural setting (inclusions in peak phases and as matrix grains), these zircons likely record growth during the prograde and/or postpeak evolution. In contrast, the Kaapvaal xenoliths record two phases of zircon growth: during UHT metamorphism at 2720-2715 Ma, followed by growth during isobaric cooling in kyanite-bearing leucosomes ca. 2690 Ma. Temperatures recorded by the two phases of growth are 1024-878 °C and 936-839 °C, respectively, suggesting that zircon growth occurred during UHT conditions and continued throughout the postpeak evolution over a 200 °C interval. These examples demonstrate that the Ti-in-zircon thermometer is a powerful new tool with the potential to determine crystallization temperatures of zircon during granulite facies metamorphism, throughout the prograde to postpeak evolution of such terranes.