干旱区生态环境知识资源中心

The Huwan high-pressure ( HP) metamorphic 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 celogite-facies metamorphisin 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 epsilon(Hf)(t) values (epsilon(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 Xuelle eclogite have relatively low epsilon(Hf)(t) values (epsilon(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 +/- 1 Ma is probably a maximum estimate of the peak eclogite-facies. The metamorpbic zircons in the Xuehe eclogite are characterized by low Th/U ratios, insignificant negative Eu anomalies, relatively flat HREE patterns, arid 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 similar to 741 degrees C from their Ti contents, consistent with their formation under eclogite-facies conditions. Their weighted mean Pb-206/(238) U age of 312 +/- 3 Ma 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 metamorphic 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.