Application of Zr-in-rutile thermometry: a case study from ultrahigh-temperature granulites of the Khondalite belt, North China Craton

TLDR: Zr-in-rutile thermometry was applied to ultra-high-temperature granulites from three localities, Dongpo, Tuguishan, and Dajing/Tuguiwula of the Khondalite belt, North China Craton as mentioned in this paper.

Abstract: Zr-in-rutile thermometry was applied to ultrahigh-temperature (UHT) granulites from three localities, Dongpo, Tuguishan, and Dajing/Tuguiwula of the Khondalite belt, North China Craton. Zr concentrations of analyzed rutiles were detected by LA-ICP-MS and EMP, which display a mutative composition zoning, a large inter-grain variation, a bimodal distribution at around 1,500 and 6,000 ppm, and no relationship with the textural setting (matrix vs. inclusion). These characteristics were likely caused by post-peak diffusional resetting associated with slow cooling rates and the presence of a CO2-rich fluid. The grains with lower Zr concentrations (~500 to ~3,000 ppm) and temperature estimates (~650 to ~850°C) occur close to or in contact with zircon, which was easily affected by post-peak processes (for example: diffusion, dissolution/reprecipitation). The lowest temperatures (~650 to ~700°C) we obtained represent the closure temperature of Zr-in-rutile. Rutiles with higher Zr concentrations (~3,000 to ~8,000 ppm) and calculated temperatures (~850 to ~1,000°C) were least affected by late resetting, giving near-peak metamorphic temperatures. These temperature results higher than 900°C, even in excess of 1,000°C from the three localities, reconfirm the presence of UHT metamorphism. Our results also suggest that Zr-in-rutile thermometry is valid for ultrahigh-temperature estimates. In addition, there are positive correlations between concentrations of Zr and Hf, Nb and Ta of the investigated rutiles, but the correlations weaken as the concentrations increase, especially for Nb and Ta, implying fractionation of Nb and Ta.