Zircon U-Pb age recording couple subduction of oceanic and continental eclogites in the Huwan shear zone, western Dabie Mountains, and its geodynamic implications.

The Huwan high-pressure ( HP) metamoiphic zone is one of the key regions to investigate the evolution of the Qinling-Dabie -Sulu orogenic belt. Two eclogite samples from Xiongdian and Xuehe in this zone were selected for LA-ICPMS zircon U-Pb dating, trace element and Hf isotope analysis to constrain the eclogite-facies metamorphism and protolith ages and their origin. U-Pb and trace element analyses of the magmatic zircon cores in the Xiongdian eclogite constrain their protolith formation at 406±14Ma. The zircons have high e_(Hf)(t) values (e_(Hf)(t) =11.3±1.3) and young t_(DM) ages (t_(DM)= 578±52Ma). It suggests that the protolith of the Xiongdian eclogite was derived from the depleted mantle, and thus confirmed its derivation from the Paleotethyan oceanic crust. In contrast, U-Pb and trace element analyses of the magmatic zircon cores in the Xuehe eclogite yield Neoproterozoic protolith age of 703 ±8Ma, suggesting that its protolith was continental crust derivation from the Yangtze Craton. The zircons in Xuehe eclogite have relatively low e_(Hf)(t) values (e_(Hf)(t) =4.11±0.94) and old t_(DM) ages (t_(DM) = 1105±37Ma). This demonstrates that the protolith of the Xuehe eclogite was probably derived from remelting of Neoproterozoic juvenile continental crust materials by rift magmatism in the northern margin of the Yangtze Craton. Both magmatic and metamorphic zircons in the Xiongdian eclogite have similar trace element characteristic. It suggests that the metamorphic zircons formed due to complete recrystallization, and the weighted average age of 316± 1Ma is probably a maximum estimate of the peak eclogite-facies. The metamorphic zircons in the Xuehe eclogite are characterized by low Th/U ratios, insignificant negative Eu anomalies, relatively flat HREE patterns, and low Nb/Ta ratios. These signatures reflect that these zircons formed in the presence of garnet and rutile, and absence of feldspar, and thus under eclogite-facies conditions. The temperatures of the metamorphic zircons in the Xuehe eclogite were calculated of 704～741℃ from their Ti contents, consistent with their formation under eclogite-facies conditions. Their weighted mean ~(206)Pb/~(238) U age of 312±3Ma is thus taken as the best estimate age of the eclogite-facies metamorphism in the Huwan shear zone. These results provide unambiguous evidence for the Carboniferous eclogite-facies metamorphism of both oceanic and continental rocks in this zone. The coexistence of the Paleozoic oceanic crust and Neoproterozoic continental crust with similar metamoiphic ages in the Huwan shear zone implies that the HP continental rocks may have played a key role in exhumation and preservation of the oceanic rocks through buoyancy-driven uplift.