Ages Of The Lengjiaxi And Banxi Groups In The Middle Jiangnan Orogenic Belt:Implications On The Neoproterozoic Tectonic Evolution Of The Jiangnan Orogenic Belt

Zircon LA-ICPMS U-Pb ages and Hf isotope compositions of the tuff and clastic rocks from Lengjiaxi Group and Banxi Group in the middle section of the Jiangnan Orogenic Belt were determined. The results reveal that the sedimentary time of the Lengjiaxi Group was860-820Ma. On the other hand, the lower limit of sedimentation of the Banxi Group, which was angular unconformably overlies on the Lengjiaxi Group, was at ca.820Ma. This scenario argued a quick transformation from assemblage between Cathysia and Yangtze to extension of the Jiangnan orogenic Belt. Zircon Hf isotope compositions exhibit a quick increasing of the initial Hf values during850-820Ma, indicating significant addition of the depleted mantle materials during this period. The quick increasing of εHf(t) could be ascribed to (1) most clastic materials deriving form arc igneous rocks (e.g., I-type granites) and (2) mixing between crustal-derived magma and mantle derived magma during quick collapse after the orogeny (e.g., the tuff sample). Combing our results and previous studies, we suggest that plate-rift model is a most plausible geodynamic modelfor the Neoproterozoic tectonic evolution of the South China.