Transitional time of oceanic to continental subduction in the Dabie orogen: Constraints from U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric dating

We investigated the oceanic-type Xiongdian high-pressure eclogites in the western part of the Dabie orogen with combined U–Pb, Lu–Hf, Sm–Nd and Ar–Ar geochronology. Three groups of weighted-mean 206Pb/238U ages at 315 ± 5, 373 ± 4 and 422 ± 7 Ma are largely consistent with previous dates. In contrast, Lu–Hf and Sm–Nd isochron dates yield identical ages of 268.9 ± 6.9 and 271.3 ± 5.3 Ma. Phengite and amphibole Ar–Ar total fusion analyses give Neoproterozoic apparent ages, which are geologically meaningless due to the presence of excess 40Ar. Plagioclase inclusions in zircon cores suggest that the Silurian ages likely represent protolith ages, whereas the Carboniferous ages correspond to prograde metamorphism, based on the compositions of garnet inclusions. Despite weakly-preserved prograde major- and trace element zoning in garnet, a combined textural and compositional study reveals that the consistent Lu–Hf and Sm–Nd ages of ca. 270 Ma record a later event of garnet growth and thus mark the termination of high-pressure eclogite–facies metamorphism. The new U–Pb, Lu–Hf and Sm–Nd ages suggest a model of continuous processes from oceanic to continental subduction, pointing to the onset of prograde metamorphism prior to ca. 315 Ma for the subduction of oceanic crust, while the peak eclogite–facies metamorphic episode is constrained to between ca. 315 and 270 Ma. Thus, the initiation of continental subduction is not earlier than ca. 270 Ma.