Multistage high-temperature metamorphism and partial melting of the North Dabie complex zone, central China, and their geodynamic processes and tectonic implications

The integrated investigations on eclogites from the Luotian dome in the North Dabie complex zone(NDZ), central China suggest that the NDZ wholly underwent high-T/UHP metamorphism and multistage exhumation history as a coherent slice, resulting in the common formation of peculiar multiple decompression textures and the extremely rare preservation of UHP metamorphic traces. The eclogites were strongly affected by multiple decompression and re-crystallization processes, and modified by multistage partial melting during exhumation, thus making the determination of peak metamorphic conditions particularly challenging. The recently calibrated Ti-in-zircon and Zr-in-rutile thermometers provide new tools to estimate the peak and post-peak temperatures. Multistage high-T(>900°C) conditions of the eclogites are indicated by garnet-omphacite thermometer and hypersthene+quartz lamellae in clinopyroxene. It is also supported by high-T indicators including high Zr and Ti contents in metamorphically grown rutile and zircon, respectively. The temperatures estimated using zircons and rutile inclusions within zircon from the NDZ eclogites reveal a protracted HT metamorphic history with at least three metamorphic stages from UHP eclogite-facies to HP granulite-facies, occurred at significantly different pressures and ages, during Triassic continental subduction and subsequent exhumation. In addition, this study presents field, petrographical and geochemical evidences on multiple partial melting of the NDZ such as decompression melting at 207±4 Ma and heating melting at ~130 Ma. Therefore, the long-lived HT evolution coupled with slow cooling and partial melting may explain the rare preservation of UHP traces such as coesite and related minerals in the NDZ. These observations provide further insights into the petrogenesis and evolution of the NDZ, and processes of multi-slice exhumation, whereby the detachment and decoupling occurred at variable depths within the subducted continental crust and the multi-slices in the Dabie orogen were successively exhumed during the continental collision between the South China and North China Blocks.