Construction of P‐T‐t paths for eclogite in the Tongbai orogen by combining phase equilibria modelling with zircon inclusion composition

The interpretation of metamorphic ages for zircon domains in eclogite remains challenging because of the uncertainties in interpreting their rare earth element (REE) patterns and mineral inclusions. This has seriously hindered the construction of robust pressure (P)-temperature (T)-time (t) paths for eclogites from collisional orogens. In this study, a composite approach is used to obtain a reliable P-T path for eclogite from the Tongbai orogen and quantitatively attribute zircon U-Pb ages to specific P-T intervals. An integrated study using isochemical phase diagram section (pseudosection) modelling, multi-equilibrium thermobarometry, and the Zr-in-rutile thermometry with petrographic observations indicates that the target eclogite experienced multistage metamorphic evolution. The evolution is characterized by a clockwise P-T path with compressional heating for peak eclogite facies metamorphism at 588 +/- 20 degrees C and 26.7 +/- 2.8 kbar and two stages of isothermal decompression from an early stage of amphibole eclogite facies metamorphism at 15-10 kbar to a late stage of amphibolite facies metamorphism at pressures of <10 kbar. Metamorphic zircon domains in eclogite and quartz vein show relatively flat patterns of heavy REE without notable Eu anomalies but different cathodoluminescence responses and middle REE (MREE) contents. This indicates their growth in different stages of eclogite facies metamorphism, which are categorized into two groups. The first group is composed of bright rims or homogenous grains that are characterized by low MREE contents, suggesting the existence of abundant amphibole during the metamorphism. The compositional isopleths of amphibole inclusions indicate that this group of zircon domains would form by hydration during crustal exhumation to amphibole eclogite facies at 240 +/- 2 Ma. The second group consists of dark sector mantles and shows high MREE contents, indicating the prograde to peak metamorphism with significant decomposition of amphibole or lawsonite during crustal subduction to eclogite facies at 245 +/- 2 Ma. Therefore, the combination of the isopleth thermobarometry of mineral inclusions with the Zr-in-rutile thermometry and the zircon REE partitioning is an effective means to link the metamorphic P-T conditions to the ages of metamorphic zircons in the different stages of collisional orogeny.