Application of Ti-in-zircon and Zr-in-rutile thermometers to constrain high-temperature metamorphism in eclogites from the Dabie orogen, central China

Ultrahigh-pressure (UHP) eclogites from the northern Dabie orogen, central China underwent a complex metamorphic evolution during Triassic continental deep subduction and subsequent exhumation. The eclogites were strongly affected by multiple decompression and re-crystallization processes during exhumation, thus making the determination of peak metamorphic conditions particularly challenging. The recently calibrated Ti-in-zircon and Zr-in-rutile thermometers provide new tools to estimate the peak and post-peak temperatures. Rutiles within UHP mineral-bearing domains of zircon have Zr contents of 1030−4310 ppm and record a temperature range of 880−1080 °C, with a peak at 970 °C. Rutiles within high-pressure (HP) mineral-bearing domains contain Zr contents of 800−5800 ppm and define a temperature range of 780−1030 °C with an average value of 873 °C. Rutiles in granulite-facies mineral-bearing rims of zircon yield a temperature of 845 °C. In contrast, rutile inclusions within garnet have lower Zr contents of 100−800 ppm, yielding lower temperatures of 600−850 °C. The UHP and HP mineral-bearing mantle domains of zircon have Ti contents of 2.15–97.8 ppm, yielding Ti-in-zircon temperatures of 620−1020 °C. Ti contents in the rim domains of zircon are low (b7 ppm), indicating a temperature of 650−700 °C, except for two granulite-facies overgrowth rims which yield a temperature of 793−840 °C. Thus,most of the temperatures for different domains ofmetamorphic zircons are b800 °C and generally lie around 650 ± 50 °C. Few measures of temperature are the N900 °C for the UHP and HP domains of zircon. Therefore, this study combined with the previously published data suggests that the eclogites in the region experienced a protracted high-temperature (HT) metamorphic evolution from UHP eclogite-facies to HP granulite-facies metamorphic stages. This long-lived HT evolution coupled with slow cooling and partial melting may explain the rare preservation of UHP traces in the area. HP/UHP mineral inclusions in garnet and zircon experienced variable degrees of decompression breakdown or retrogression. Therefore, also Ti concentrations in zircons and Zr concentrations in rutiles within garnet and zircon may have been strongly modified by re-crystallization or partial melting. As a result, only few zircons record their actual crystallization temperatures. In comparison, the rutile inclusions in zircon generally define higher temperatures. We therefore suggest that rutile inclusions in zircon are the most suitable candidates for HT estimates, especially in strongly retrogressed eclogites.