SHRIMP U-Pb zircon dating of a metagabbro and eclogites from western Dabieshan (Hong'an Block), China, and its tectonic implications

In order to better understand the collision tectonics between the Sino-Korean and Yangtze cratons, we performed ion microprobe (SHRIMP) dating on zircons of a metagabbro and four eclogites from the high-pressure/ultrahigh-pressure (HP/ UHP) metamorphic terrane of western Dabieshan (also known as the Hong'an Block), China. New zircon U-Pb data of eclogites from the Huwan and Hong'an HP units suggest that their protoliths were formed during the Neoproterozoic, from 733 +/- 10 to 684 +/- 15 Ma, whereas the metamorphic ages are 229 +/- 12 Ma for an eclogite of the Huwan IIP unit, and 213 +/- 5 Ma for that of the Xinxian UHP unit. The age data indicate that western Dabieshan is an integral part of the Dabie-Sulu Triassic HP/ UHP metamorphic belt. A few eclogites with Carboniferous or older zircon ages reported in the literature are limited in the northwestern corner of western Dabieshan. They should not be considered as representative of the whole Huwan unit, and the geological meaning of their occurrence still retrains obscure. The identical ages of eclogites from the Huwan and Hong'an units, together with their comparable P-T conditions and similar country rocks, suggest that the two units belong to the same HP slice overlying the UHP slice (=main part of the Xinxian unit). Zircons from a metagabbro in the Balifan tectonic melange unit located to the north of the HP/UHP complexes yield a Neoproterozoic emplacement age at 532 +/- 11 Ma. Taking into account the Yangtze signature for the adjacent Nanwan flysch unit, we infer that the Balifan and Nanwan units represent the basement and cover accretionary complexes, respectively. They were sliced from the Yangtze craton during it northerly subduction Accordingly, the northern boundary fault of the Nanwan unit should be the Triassic suture between the Sino-Korean and Yangtze cratons. The cooling ages obtained from different units suggest diachronous exhumation of the subducted slab during Triassic time, from ca. 240 Ma for the hanging wall accretionary complex to ca- 210 Ma for the footwall UHP slice. This scenario could be explained by a successive subduction of underlying slices with a nearly concomitant exhumation of overlying slices. (C) 2004 Elsevier B.V. All rights reserved.