A Neoproterozoic low-δ18O magmatic ring around South China: Implications for configuration and breakup of Rodinia supercontinent

We report Neoproterozoic (ca. 785-780 Ma) granites from the western margin of the Yangtze Block that are characterised by magmatic zircons with δ18O values as low as 2.98‰. The lack of low-δ18O magmatic zircons in the ca. 820-805 Ma rhyolite samples from the Neoproterozoic Suxiong Formation indicates that there is no recycling of pre-existing hydrothermally altered crust in the study area prior to the emplacement of the ca. 785-780 Ma granites. Thus the ca. 785-780 Ma granites with low-δ18O values from the western margin of the Yangtze Block can be linked to assimilation of syn-magmatically altered rocks (rather than the assimilation of a pre-existing hydrothermally altered crust). The granites have a source constrained by their depleted Hf isotopes and low-δ18O values resulting from high temperature hydrothermal alteration. The δ18O values of the zircons from core to rim exhibit a decrease indicative of the remelting of material during the interaction between magma and water at high temperatures. In combination with the widespread low-δ18O signatures that occur in the northern and southern margins of the Yangtze Block and Cathaysia Block, the locations of low-δ18O magmatic zircons exhibit a Neoproterozoic low-δ18O magmatic ring around South China. This continent-scale Neoproterozoic extensional and magmatic event cannot be attributed to subduction processes in South China resulting in the emplacement of a magmatic ring of felsic igneous rocks with low-δ18O values. We therefore propose a model involving a Neoproterozoic super-mantle plume with a diameter of approximately 1500 km controlling the development of the rift systems around South China.