Determination of Volcanic Rocks of Niutoushan Formation in Luliang Area,western Margin of Yangtze Block and Its Indication to the Evolution Law of Rodinia Supercontinent

Located at the western margin of the Yangtze Block, the central Yunnan region has Neoproterozoic volcanic rocks, which is an important window to study the evolution of the Rodinia supercontinent. The Niutoushan Formation is an important Neoproterozoic stratum in the central Yunnan region. Due to the lack of accurate geochronology, the division of the regional stratigraphic framework and the tectonic evolution of the Rodinia supercontinent had been seriously restricted. In this paper, the petrography, as well as zircon U-Pb geochronology, in-situ Hf isotope, and trace elements of the rhyolite tuff from the Niutoushan Formation have been studied. The results show that the tuff is composed of volcanic materials, and the zircons without grinding have obvious magmatic oscillating zones. The zircon U-Pb ages of tuff are 817.9 +/- 7.7Ma, 818.3 +/- 8.3Ma, and 822 +/- 13Ma, and epsilon(Hf)(t) values are both positive and negative. The magma source is associated with the mixing of mantle material, and mainly related to the melting of Paleoproterozoic continental crust material (1523.9 similar to 2107Ma). Zircon studies show that tuff was formed in a magmatic arc and belongs to the continental crust. The petrogeochemistry shows that the tuff belongs to siliceous volcanic rocks and has arc volcanic properties. It is concluded that the Niutoushan Formation is a sedimental-magmatic response to the subduction process of the Cathaysia block to the Yangtze block. It was formed in the back-arc basin environment of the western margin of the Yangtze block, and this back-arc basin might be the early stage of the Kangdian rift in Neoproterozoic South China. The end of the Niutoushan Formation sedimentation indicated that the Yangtze and Cathaysia blocks had converged, and the Rodinia supercontinent might have formed a unified plate. At the same time, the Rodinia supercontinent began to break up and the transition time was about 820Ma.