Neoproterozoic Tectonic Setting of Southeast China: New Constraints from SHRIMP U-Pb Zircon Ages and Petrographic Studies on the Mamianshan Group

Precambrian tectonic history of Zhejiang, Fujian, and Jiangxi provinces of south China is important for understanding the tectonic evolution of South China but its magmatic activity, petrogenesis, stratigraphic sequence of the Mamianshan Group is still strongly controversial. Here we present new sensitive high resolution ion micro-probe (SHRIMP) U-Pb zircon geochronological data for the Mamianshan Group and petrographical data to constrain the tectonic framework of the regions. Our results showed that the SHRIMP U-Pb zircon age of green schists of the Dongyan Formation is 796.544.3 Ma, the Daling Formation is 756.2 +/- 7.2 Ma, and mica-quartz schist of the Longbeixi Formation is 825.5 +/- 9.8 Ma. These data indicate that the Mamianshan Group was formed not in the Mesoproterozoic, but in the Neoproterozoic and its stratigraphic sequences should be composed of Longbeixi, Dongyan, and Daling Formations from the bottom to the top. Rocks from this Group, from Zhejiang, Fujian and Jiangxi provinces, constituted the upper basement of the Cathaysia Block that overlay the lower basement of the Mayuan Group. Detailed petrographic studies demonstrate that the amphibole schists of the Dongyan Formation in the Mamianshan Group were formed within an intra-arc rift setting rather than a continental rift as previously suggested. Rather, this island-arc type formation was developed by collision and/or subduction between various blocks resulting from the breakup of the supercontinent Rodinia at c.850-750 Ma. The Zhuzhou conglomerate, distributed near Dikou Town, Jian'ou City, Fujian Province and previously considered as evidence of the Mesoproterozoic Dikou movement, is shown here not to be the basal conglomerate above the angular unconformity between the upper and lower basements. Our conclusions have important implications for understanding the Precambrian tectonics of South China.