Whole-rock Ar-Ar dating for low-grade metavolcanics within the Dabie orogen and its geological significance

Abstract

The genetic relationship between lowgrade and ultrahigh-pressure (UHP) metamorphic units in the interior of the Dabie orogen has been controversial with respect to preservation of volcanic texture during continental subduction to mantle depths. In order to resolve this issue, whole-rock Ar-Ar dating was carried out for greenschist-facies metatuff that is in contact with UHP eclogite in Yuexi County, Anhui Province. One sample gave a plateau age of 784.4±2.0 Ma and an isochron age of 785.0±4.7 Ma, and the other sample a plateau age of 770.9±2.0 Ma and an isochron age of 769.5±3.1 Ma. It appears that the Ar isotopic system was not disturbed since the volcanic eruption at the middle Neoproterozoic. Because of the low closure temperature of Ar diffusion in volcanic rocks, retention of the Neoproterozoic ages indicates that the low-grade metavolcanics did not experience high-grade metamorphism up to eclogite-facies conditions during the Triassic subduction of continental crust. Furthermore, the Neoproterozoic Ar-Ar ages are also in agreement with protolith ages of UHP metaigneous rocks in the Dabie orogen. Therefore, the metatuff is interpreted as a tectonic relict that was scraped off during the Triassic subduction and thus was not subducted to mantle depths like the adjacent eclogite. This lends support to the model for crustal detachment between cover and basement during continental collision. A tectonic melange model is proposed to explain the occurrence of contrasting grades of metamorphic rock within the UHP metamorphic zone.