Neoproterozoic post-collisional extension of the central Jiangnan Orogen: Geochemical, geochronological, and Lu-Hf isotopic constraints from the ca. 820-800Â Ma magmatic rocks

Neoproterozoic magmatic rocks in South China contain important information on evaluating crustal reworking during the Yangtze-Cathaysia collision and post-collisional extension. We present here geochemical, geochronological, and Lu-Hf isotopic data of a suite of granitoids and diorites from the Jiuling and Meixian plutons of the central Jiangnan Orogen. The granitoids display intermediate geochemical features between S-type and I-type granites. They exhibit arc-like patterns of REE and trace element partition, reflecting inheritance of juvenile arc-derived crust. Igneous zircons from the Jiuling and Meixian granitoids yield U-Pb ages of 822-819 Ma and 816-815 Ma, respectively. Inherited zircons from the granitoids commonly yield a major age population of 980-860 Ma, consistent with the time of the Shuangxiwu volcanic arc. The igneous and inherited zircons have highly variable εHf(t) values ranging from −9 to +12.2. We interpret the granitoids to have been generated by remelting and binary mixing of (1) juvenile arc-derived crustal material and (2) supracrustal material of the Shuangqiaoshan meta-sedimentary rocks. The previous crustal thickening achieved by the Yangtze-Cathaysia collision is a prerequisite for their generation. The two granitoids, together with regional 825-815 Ma S-type granites, indicate incipient post-collisional collapse of thickened crust, heralding tectonic switch from compression to extension. The Meixian diorites display arc-like geochemical characteristics, and yield zircon U-Pb ages of 805-804 Ma and neutral to positive zircon εHf(t) values (−0.1-+7.5). The diorites were generated by decompression melting of arc-derived lower crust, associated with asthenospheric mantle upwelling during continental rifting. Such rifting may be part of global Neoproterozoic rifting events that finally led to breakup of the Rodinia supercontinent.