Temperature effect over garnet effect on uptake of trace elements in zircon of TTG-like rocks

To assess the garnet effect on uptake of rare earth elements in TTG petrogenesis, zircon trace elements were analyzed for TTG-like rocks in a Neoproterozoic composite batholith at Huangling in the Yangtze Gorge, South China. A TTG-like suite has higher whole-rock La/Yb and Sr/Y ratios, lower whole-rock Nb/La ratios and εNd(t) values, and lower zircon εHf(t) values than the other three non-TTG suites. Zircons from this suite have higher Hf, Nb and Ta contents than those from the other three non-TTG suites. However, REE profiles and ratios cannot be unambiguously distinguished between residued cores and co-magmatic domains despite significant differences in their U-Pb ages and Lu-Hf isotopes. All zircons from the four suites are enriched in HREE but depleted in LREE, with positive Ce anomalies. They display no to very weak Eu anomalies, contrasting to the typical zircon of magmatic origin. This suggests that plagioclase was neither a stable phase during partial melting nor a removed phase during fractional crystallization. Most of the zircons from the TTG-like rocks have Ti contents lower than 30 ppm, yielding a range of Ti-in-zircon temperatures from 600 to 880 °C. While a part of zircons from all granitoids show various degrees of LREE enrichment, the absence of flat HREE patterns is evident for all zircons, including both residued cores and co-magmatic domains in the TTG-like rocks. This suggests that there is no preferential uptake of HREE in garnet relative to zircon even if garnet could exist as a stable phase during partial melting to form the TTG and their source rocks. Because partition coefficients for garnet/melt, zircon/melt and zircon/garnet decrease with increasing temperature during partial melting, comparison of calculated apparent DHREE values with experimental DHREE values between zircon and melt allows estimate of temperatures for zircon crystallization. This yields a range of 750 to 800 °C for the TTG-like rocks, which is consistent with whole-rock zircon saturation temperatures of 756 to 793 °C. However, they are significantly lower than the crossover temperatures of 900 to 950 °C for preferential partition of HREE in the zircon-garnet-melt system. Thus, the temperature is suggested as a key cause for the lack of garnet effect on the uptake of HREE in zircons from the TTG-like rocks. Therefore, the temperature effect can play a critical role in dictating the partition of HREE in the zircon-garnet-melt system of adakitic and TTG-type rocks.