Chronological and geochemical studies of the Middle Neoproterozoic mafic rock and turbidite in the Dahongshan orogenic belt on the northern margin of the Yangtze Block: Implications for the evolution of the back-arc basin

The northern margin of the Yangtze Block may have witnessed the tectonic shift from a compressional setting to an extensional setting at similar to 820Ma, which is the key to reveal Neoproterozoic tectonic evolution process of the Yangtze Block. This study identified a set of mafic rock and turbidite assemblage in Yuantan area of Dahongshan orogenic belt in the northern margin of the Yangtze Block. The whole-rock Sm-Nd isochronal age of mafic rock is 845 +/- 81Ma (MSWD = 1.5), and the turbidite detrital zircon age spectrum includes four major peak ages (similar to 835Ma, similar to 1597Ma, similar to 2002Ma, similar to 2482Ma) and two minor peak ages (similar to 2680Ma, similar to 2934Ma). Together with previous research results, the formation age of turbidite is constrained at 835 similar to 820Ma. This is consistent with the formation age of the Huashan Group, and the turbidite should represent the northward extension of the Huashan Group. Zircons of turbidite are mainly sourced from the Tongbai-Dabie area in the northern margin of the Yangtze Block, different from those of the Huashan Group in previous study, which indicates that the Huashan Group may have multiple provenances. Detrital zircon age spectrum and stratigraphic sequence characteristics are similar with those in the back-arc basin. Mafic rocks in this study have low SiO2 (45.41% similar to 50.29%), K2O (0.03% similar to 1.49%), high TiO2 (1.42% similar to 3.03%), and low U/Th (0.15 similar to 0.29), Th/Zr (0.003 similar to 0.011), Nb/Y (0.059 similar to 0.147) and positive epsilon(Nd) (t) (+1.18 similar to +1.57) are similar with N-MORB. These mafic rocks should be derived from the asthenosphere, experienced < 3% partial melting and suffered low degree assimilation during the ascent. In conclusion, mafic rock and associated turbidite were suggested to represent a rock assemblage of back-arc setting.