Neoproterozoic geochronologic framework of magmatism in Fanjingshan area and its tectonic implications

The Jiangnan orogen is located between the Cathaysian and Yangtze blocks, where numerous Neoproterozoic igneous rocks have been identified, which provide a chance to study the mantle-crust interaction and plate tectonics of the earth's earlier history. The tectonic evolution of the eastern segment of the Jiangnan orogen seems to be clear, but the tectonic evolution of its southwestern segment remains controversial mainly due to absence of field relationship-consistent geochronologic framework. The Fanjingshan area is located in the northern portion of the southwestern segment of the Jiangnan orogen. 1:50000 geological mapping of that area revealed detail field relationship between different geological units, thus it is an ideal area to construct a reasonable geochronologic framework. This paper reports four in- situ zircon U/Pb dating results of mafic to ultramafic plutons which intruded the Neoproterozoic Fanjingshan Group. (deposition time 850- 830 Ma). The new data obtained by the authors demonstrate that the three gabbroic plutons crystallized at 813 Ma, 804 Ma, and 748 Ma, respectively. Combined with previously published geochronologic data, the authors hold that the magmatism that occurred in the southwestern segment of the Jiangnan orogen consisted of two stages: the Jinning-I stage (850 to 825 Ma) and the Jinning-II stage (820 to 750 Ma), which correspond to the sedimentation time of Fanjingshan and Xiajiang Groups, respectively. Between them is an angular unconformity. Such a magmatic geochronological framework is consistent with that of the sedimentary rocks. The tectonic setting of the Neoproterozoic tectonostratigraphy is not very clear due to the fact that regional tectonic setting remains unclear.