Origin of atoll garnets in ultra-high-pressure eclogites and implications for infiltration of external fluids

Abstract

Atoll-shaped and normal garnets in a low-temperature (LT) and ultra-high-pressure (UHP) metamorphic eclogite of the Dabie orogen were studied by detailed petrographic observation and pseudosection modelling. The normal and atoll garnets exhibit pronounced chemical variations, characterized by the overall increase of pyrope and decrease of grossular content from core to rim. A compositional discontinuity between the inclusion-rich core and the inclusion-poor mantle was defined by a sharp decrease in grossular content and an increase in pyrope. Pseudosection modelling using different effective bulk compositions (EBCs) obtained using XRF and the point-counting method shows that the eclogite experienced two stages of eclogite-facies metamorphism from a prograde amphibole-eclogite-facies (∼500–700 °C, <2.2 GPa) to the peak UHP lawsonite-eclogite-facies (∼650 °C, ∼3.3 GPa), followed by an isothermal decompression (ITD) exhumation. Garnet essentially did not grow owing to the parallels between the prograde P–T path and the garnet mode isopleths after the formation of the garnet core. The stable field and the compositions of the peak mineral assemblage at LT-UHP conditions insignificantly depend on the scale of equilibrium. Partial lawsonite and omphacite should be subtracted from the EBCs to obtain reasonable P–T estimates/pseudosections that are consistent with the genesis of atoll-garnet-bearing eclogite. Pseudosection modelling indicates that the infiltration of external fluid into the “dry” eclogite weakened the rigid garnet and produced micro-cracks. These cracks served as permeable pathways, leading ultimately to the consumption of previously formed garnet core and the formation of atoll texture. Previous P–T estimates using conventional garnet-omphacite-phengite thermobarometry are less reliable because omphacite was highly altered owing to net transfer reactions involving omphacite. The atoll texture was developed from prograde to peak UHP metamorphism, and the atoll-garnet forming event in the Dabie eclogite predates the final stage of the UHP metamorphism. The previous Lu–Hf date of c. 221 Ma is thus a maximum estimate of the termination of UHP metamorphism.