Triassic high-pressure metamorphism in the Huwan shear zone: Tracking the initial subduction of continental crust in the whole Dabie orogen

Abstract

LA-(MC)-ICPMS trace element, U–Pb and Hf isotope analyses were performed on zircons from continental-derived retrograde high-pressure metamorphic rocks, which were attested by the occurrence of HP mineral inclusions in metamorphic zircons, from the Huwan shear zone in the western Dabie orogen. Zircons in a schist sample are mostly detrital and gave U–Pb ages from 1212 to 2312Ma, suggesting that the detritus were derived from multiple sources or suffered partial Pb-loss during subsequent metamorphism. Magmatic zircons in an amphibolite sample yielded an upper intercept U–Pb age of 1806±63Ma, which was interpreted as its protolith formation age. The zircons in the amphibolite show a weighted εHf (t) value of −16.3, and a weighted Hf model age of 3455Ma, suggesting that the protolith was derived from reworking of ancient continental materials. Metamorphic zircons in a quartz-rich leucosome sample and the amphibolite sample gave weighted mean 206Pb/238U ages of 241±1 and 243±4Ma, respectively. These metamorphic zircons contain mineral inclusions of garnet and omphacite and are characterized by relatively flat HREE patterns with slight negative Eu anomalies. This indicates that they formed in the presence of garnet but the absence of plagioclase, and thus under eclogite facies conditions. The U–Pb age of ca. 243Ma is consistent with eclogite-facies metamorphic ages of the juvenile oceanic crust in the western Dabie orogen, suggesting that the initial continental subduction for high-pressure metamorphism of the whole Dabie orogen can be traced back to ca. 243Ma. In addition, the western Dabie HP-UHP orogenic belt might have suffered multi-slice subduction and exhumation during the Triassic.