Zircon U-Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China

The crustal structure of the Dabie orogen was reconstructed by a combined study of U-Pb ages, Hf and 0 isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith Origin with respect to continental subduction and exhumation. Zircon U-Pb dating provides consistent ages of 751 +/- 7 Ma for protolith crystallization, and two group ages of 213 +/- 4 to 245 +/- 17 Ma and 126 +/- 4 to 131 +/- 36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative epsilon(Hf)(t) values of -5.1 to -2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive epsilon(Hf)(t) Values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile Crust. Zircon 0 isotope analyses yield delta O-18 values of -3.26 to 2.79%., indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low delta O-18 rocks. North Dabie shares the same age of Neoproterozoic low delta O-18 protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement. The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower. (C) 2008 Elsevier B.V. All rights reserved.