Microgeochemistry of rutile and zircon in eclogites from the CCSD main hole: Implications for the fluid activity and thermo-history of the UHP metamorphism

Rutile and zircon in eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD-MH) were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron microprobe (EMP). Both single spot and profile analyses of large matrix rutiles (> 60 μm) generally demonstrate core–rim variability of Pb, Sr, Zr and U. Pb and Sr show enrichment at the rims, while Zr and U decrease in concentration from the cores to the rims. Correspondingly, temperatures estimated by Zr-in-rutile geothermometers are higher within the cores (mainly in the range of 680–740 °C) than at the rims by 10–30 °C. We note that estimated temperatures within the cores of matrix rutiles correlate positively with grain diameter. This suggests that the Zr content in rutiles is not only dependent on temperature and pressure, but is also affected by fluid activity and Zr diffusion. Estimated temperatures within the cores of matrix rutiles merely represent lower limits of the peak ultra-high pressure (UHP) metamorphic temperatures.Combined with Th/U and Ce/Lu ratios, rare earth element (REE) patterns and 206Pb/238U ages, Ti-in-zircon geothermometers can be used to distinguish the thermal evolution of eclogites during the UHP metamorphism. Zircons of 745 ± 8.0 Ma (2σ, n = 3) show evidence of the temperatures at which they crystallized from the original magma. Zircons of 240 ± 7.0 Ma (2σ, n = 11) with flat HREE patterns suggest a definite temperature range (689 ± 18 °C (2σ, n = 11)) for the prograde metamorphism. Although 226 Ma is generally considered the time of the peak UHP metamorphism for the Dabie–Sulu orogen, except for a few grains with a maximum temperature of 744 °C, most of the 226 ± 2.0 Ma (2σ, n = 12) zircons indicate lower temperatures (average = 698 ± 12 °C (2σ, n = 12)). These data imply that the 226 Ma zircons were probably not products of the peak UHP metamorphism, but might instead be associated with the pressure decrease in the post-peak metamorphism. The youngest zircons (< 220 Ma) provide evidence for temperatures (682 ± 14 °C (2σ, n = 11)) associated with the retrograde metamorphism. Geothermometers that incorporate both rutile and zircon indicate that there may have been experienced a locally short-lived heating event for eclogites from the CCSD-MH.