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

The 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 similar to 450 degrees C-470 degrees C/11-13 kbar, through amphibole eclogite-facies at 585 degrees C-600 degrees C/17-19 kbar, to peak UHP eclogite-facies at 600 degrees C-630 degrees C/27-29 kbar, indicating a low and continuously changing geothermal gradient from similar to 12 degrees C/km at lower crust depth (40 km) to 10 degrees C/km at depth of 60 km and to similar to 6 degrees 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. Plain Language Summary Subduction zones are important sites for material cycles, volcanic and seismic activity, and metallogenesis. The thermal structure (change of temperature with depth) is considered to place a first-order control on these processes. By measurement of surface heat flow on the active subduction zones combined with formal modeling the thermal structure, thermal structure of oceanic subduction zones has been well constrained. In contrast, the thermal structure of continental subduction zones is far from clear as its geophysical information is usually modified by later continental collision. Metamorphic evolution paths of exposed rocks preserve complete information on the change of temperature and pressure with time, making it a reliable recorder of the thermal structure of a paleo-continental subduction zone. In this contribution, we carried out a detailed petrological study to trace the metamorphic paths of ultrahigh pressure eclogites in the Dabie orogen, China and then recover the thermal structure of the continental subduction zone. The results demonstrate that the target rocks record a continuously decreasing thermal gradient with depth. Combining the results of this and previous studies, we suggested that the continental subduction zones have a common thermal structure characterized by varied thermal gradients from 10 to 12 degrees C/km at crustal depth to 6-7 degrees C/km at sub-arc depth.