TRACE ELEMENT ACCESSORY-PHASE THERMOMETRY IN ULTRAHIGH-TEMPERATURE GRANULITES: AN EVALUATION AND ASSESSMENT OF POTENTIAL FOR ROBUST TEMPERATURE-TIME POINTS ALONG THE P-T PATH

Trace element thermometry (Ti-in-zircon (Watson & Harrison, 2005) and Zr-in-rutile (Watson, pers. comm.)) is uniquely suitable for potentially recording crystallization temperatures of dateable accessory phases. If temperatures can be linked to U-Pb dates, then temperature-time points along a P-T path may be established. The closure temperature for the Ti-in-zircon thermometer is likely to be exceptionally high since other 4+ cations (Hf, Th, U) are effectively immobile at all plausible geologic conditions. In contrast, Zr in rutile is more mobile, particularly at granulite facies conditions, unless rutile is encapsulated and armored in porphyroblasts. The applicability of these thermometers is assessed through a study of two ultrahigh-temperature (UHT) granulite complexes: the Anápolis-Itauçu Complex (AIC) of central Brazil and crustal xenoliths from the central Kaapvaal craton of South Africa. In each case, the rocks have been metamorphosed at Ts of around 1000°C, based on the presence of UHT indicator assemblages such as spinel-quartz, sapphirine-quartz and orthopyroxene-sillimanite-quartz.

In the AIC granulites, metamorphic zircon interpreted to have formed during prograde or peak conditions records Ti-in-zircon Ts of 910-860°C. Individual U-Pb dates in the range 645-635 Ma are inferred to record the interval of UHT conditions during the Brasiliano orogeny, but they have not yet been linked to individual zircon crystallization temperatures. A second population of zircons formed from the breakdown of ilmenite records lower Ts of ~780°C, interpreted to record crystallization as melt-producing reactions are re-crossed during decompression and cooling. Zr-in-rutile thermometry of inclusions in garnet porphyroblasts yields a wide range of Ts from 960-650°C.

In a contrasting example, zircons from the Lace kimberlite in the Kaapvaal craton record higher Ti-in-zircon Ts of 990-960°C that are linked to the deep crustal manifestation of Neoarchean Ventersdorp extension at c. 2720-2715 Ma. A second population of c. 2690 Ma zircon in kyanite-bearing leucosomes records Ts of ~800°C, indicating initial cooling rates of 6-8°C/Ma. Zr-in-rutile yields Ts of 710-550°C, which are interpreted to record the closure T of Zr diffusion during cooling, and rutile exhibits exsolution lamellae of zircon, providing empirical evidence for the mobility of Zr at high-temperature crustal conditions.