Abstract The Dabie–Sulu orogenic belt in east-central China contains one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world. The UHP eclogites are associated with gneiss, peridotite and marble. But all these rocks underwent in situ UHP metamorphism during the continental collision in the Triassic. Although fluid action is not significant during cold subduction, it becomes prominent during hot exhumation of UHP slices. Whereas the composition of UHP metamorphic rocks is primarily inherited from their protoliths, they were locally modified by partial melting to varying extents. The partial melting of UHP rocks is highly heterogeneous along the collisional orogen, and achieves a maximum during rifting orogeny at the post-collisional stage. This paper outlines the petrographical features and geochemical compositions of UHP metamorphic rocks, and presents an overview of collisional modification and post-collisional reworking of the subducted continental crust. Further discussions are devoted to continental subduction tectonics, as well as to subduction style, subduction polarity and exhumation mechanism. The results also provide insights into the initiation of continental rifting in a Wilson cycle. Consequently, studies of the Dabie–Sulu orogenic belt have contributed greatly to our understanding of tectonic processes, fluid regime and chemical geodynamics in continental subduction zones.

Abstract In the Sm-Nd and Rb-Sr isotopic geochronology of metamorphic rocks, an important question is whether radiometric systems of mineral isochrons have achieved isotopic equilibrium during a given metamorphic event and preserved the equilibrium afterwards. An analogue to mineral chronometry is O isotope geothermometry. Because the rates of Sm-Nd, Sr and O diffusion in metamorphic minerals are comparable in many cases, the state of O isotope equilibrium between metamorphic minerals can provide a test for the validity of mineral Sm-Nd and Rb-Sr chronometers. In order to illustrate this applicability, O isotope geothermometry was carried out for Sm-Nd and Rb-Sr isochron minerals from ultrahigh-pressure (UHP) eclogites and gneisses at Shuanghe in the Dabie terrane of east-central China. Although the Sm-Nd isochrons give consistent Triassic ages of 213 to 238 Ma for UHP metamorphism, the Rb-Sr isochrons give Jurassic ages of 171 to 174 Ma for the same samples. O isotope geothermometry of the gneiss, eclogite and amphibolite minerals yields two sets of temperatures of 600 to 720 °C and 420 to 550 °C, respectively, corresponding to cessation of isotopic exchange by diffusion at about 225 ± 5 Ma during high pressure eclogite-facies recrystallization and at about 175 ± 5 Ma during amphibolite-facies retrogression. The preservation of Triassic Sm-Nd isochron ages, but the occurrence of Jurassic Rb-Sr isochron ages and the regular O isotope temperatures for the same samples, suggest that rates of Sr and O diffusion in such hydroxyl-bearing minerals as biotite and hornblende are faster than rates of Nd diffusion in garnet and Sr diffusion in phengite on the scale of a hand-specimen during the amphibolite-facies retrogression. While the mineral with slow diffusivity has exerted the primary control on the homogenization rate of initial isotope ratios among isochron minerals during retrograde metamorphism, the mineral with high parent/daughter ratio has exerted the principal control on the initiation of the mineral isochron clock in response to retrogression. Valid mineral isochrons can be expected to date the timing of metamorphic resetting only if the mineral with high parent/daughter ratio has a fast rate of radiogenic isotope diffusion during the metamorphic resetting.