Zircon U-Pb age and Hf isotope compositions of Lengjiaxi and Baxi Groups in middle Hunan Province： implications for the Neoproterozoic tectonic evolution in South China

Abstract：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 is 860-820Ma. On the other hand, the lower limit of sedimentation of the Banxi Group which overlies the Lengjiaxi Group in angular unconformity is ca.820Ma, suggesting the quick transformation from the assemblage between Cathysia and Yangtze to the extension of the Jiangnan orogenic belt. Zircon Hf isotope compositions exhibit a rapid increase of the initial Hf values during 850-820 Ma, indicating significant addition of the depleted mantle materials during this period. The quick increase of εHf(t) could be ascribed to the following factors: (1) most clastic materials were derived from arc igneous rocks (e.g., I-type granites) and (2) some materials were derived from the mixed substances (such as tuff) between crust-derived magma and mantle-derived magma during the quick collapse after the orogeny). In combination with previous studies, the authors hold that the plate-rift model is a most plausible geodynamic model for the Neoproterozoic tectonic evolution of South China.