Thermal Structure of the Paleo‐Continental Subduction Zone: Insights From Quantitatively Constrained Prograde P–T Paths of Exhumed LT/UHP Eclogites in the Dabie Orogen

Abstract

AbstractThe discrepancy of geothermal gradients predicted by numerical modeling and recorded by peak metamorphic conditions of exhumed high/ultrahigh pressure (HP/UHP) metamorphic rocks triggers many uncertainties on the thermal structure of subduction zones, which is especially significant for continental subduction zones. Well‐constrained prograde P‒temperature (T) paths of HP/UHP rocks reflect the change in P–T conditions during continental subduction and are insusceptible to later processes, making them robust recorders of the thermal structure of subduction zones. To make a reliable estimation of the thermal structure of a continental subduction zone, the prograde P–T paths of three low (L)T‒UHP eclogites in the Dabie orogen were robustly constrained using a multiple thermobarometry method. The results demonstrate that these eclogites underwent three stages of metamorphic evolution during continental subduction from epidote amphibolite‐facies at ∼450°C–470°C/11–13 kbar, through amphibole eclogite‐facies at 585°C–600°C/17–19 kbar, to peak UHP eclogite‐facies at 600°C–630°C/27–29 kbar, indicating a low and continuously changing geothermal gradient from ∼12°C/km at lower crust depth (40 km) to 10°C/km at depth of 60 km and to ∼6°C/km at sub‐arc depth (100 km). The consistency of this result and the average thermal gradient recovered by peak metamorphic conditions of exhumed HP/UHP metamorphic rocks of continental origin suggests that the continental subduction zones may have a common thermal structure characterized by strongly concave upward geothermal gradient. A comparison of the results of this study and analytical models shows that shear heating may play a crucial role in the thermal evolution of continental subduction zones.