A re-evaluation of prograde pressure-temperature paths of eclogites from the Tongbai orogen by quartz-in-garnet and Ti-in-calcic amphibole thermobarometry

The prograde pressure (P)-temperature (T) path of eclogites reflects important information on the geodynamic evolution and the thermal structure of subduction zones. The robustness of garnet allows its compositions to be commonly utilized in conjunction with thermodynamic equilibrium modelling to trace the prograde P-T paths of eclogites. Nevertheless, some recent studies have demonstrated that garnet may undergo disequilibrium nucleation and growth, which can lead to an inaccurate reconstruction of the prograde evolution by using phase equilibrium modelling. This suggests that the prograde P-T conditions determined by phase equilibrium modelling should be verified using other independent thermobarometric methods. In this study, the quartz-in-garnet (QuiG) barometry was combined with the Ti-in-calcic amphibole (TiCA) thermometry to re-evaluate the prograde P-T evolution of eclogites from the Tongbai orogen in central China, which has previously been constrained by phase equilibrium modelling. The results show that the Tongbai eclogites experienced an early blueschist facies prograde stage under 380–520 °C and 1.5 GPa and a late epidote eclogite facies prograde stage under 520–600 °C and 1.8 GPa. These pressure conditions are significantly lower than those constrained by phase equilibrium modelling (2.3–2.6 GPa), but are within the stability fields of mineral inclusions in garnet porphyroblasts. This suggests that the results of this study provide a reliable constraint on the prograde evolution experienced by the Tongbai eclogites. T-composition modelling indicates that the discrepancy of CaO between the effective equilibrium compositions for garnet growth and the whole-rock compositions of eclogite can lead to an overestimation of prograde pressures when using phase equilibrium modelling based on whole-rock compositions of eclogites. The combination of the prograde P-T conditions determined in this study with the well-established peak P-T conditions from previous research (~ 590 °C and 2.7 GPa) suggests that the Tongbai eclogites underwent a prograde evolution characterized by an initial heating followed by a later compression, with continuously decreasing geothermal gradient from ~ 8–11 °C/km at depth of ~ 50–60 km to ~ 6–7 °C/km at depth of ~ 80–90 km. This is comparable with the reported thermal structure of other continental subduction zones, indicating that the continental subduction zones may have a common thermal structure characterized by strongly concave upward geothermal gradients.