Geochronology and Hf isotopes of zircon from volcanic rocks of the Shuangqiaoshan Group, South China: implications for the Neoproterozoic tectonic evolution of the eastern Jiangnan orogen

The Shuangqiaoshan Group is an important stratigraphic unit in the Jiangnan orogen, however, prior to this study its age was unclear. Zircon from the intruding gabbros, interleaved quartz-keratophyres and tuffs yielded ages of 801 ± 4 Ma, 878 ± 5 Ma and 879 ± 6 Ma, respectively. The ages of the quartz-keratophyres and tuffs suggest that the depositional age for the Shuangqiaoshan Group is ca. 880 Ma, rather than Mesoproterozoic as previously suggested. This Shuangqiaoshan Group, and temporally equivalent folded sedimentary sequences along the Jiangnan orogen, may have possibly formed within a back-arc foreland basin setting. The ca. 800 Ma gabbros have arc-like geochemical features, high positive ε_Hf(t) values (mean value of 5.2 ± 0.7), and Mesoproterozoic Hf model ages of 1163 ± 26 Ma, suggesting that they may generate from the partial melting of juvenile crustal materials resulting from the early subduction. Zircon from the interleaved quartz-keratophyres and tuffs have high positive ε_Hf(t) values, ranging from 3.3 to 18.8. They, as well as their equivalent volcanic rocks, may have acted as a main source for 880-850 Ma detrital zircon in folded sequences along the Jiangnan orogen, and their mantle source would represent the depleted mantle end-member for Neoproterozoic crustal-mantle interactions in the Jiangnan orogen.