A rhythmic source change of the Neoproterozoic basement meta-sedimentary sequences in the Jiangnan Orogen: Implications for tectonic evolution on the southeastern margin of the Yangtze Block

Abstract

The amalgamation of South China Block and its role in the assembly and breakup of Rodinia supercontinent have been hotly debated in recent decades. However, the intensity of accreted arcs around South China and the following exhumation were poorly constrained. Here we present combined U–Pb dating and Hf isotopic analyses of detrital zircons for a stratigraphic section of the upper Shuangqiaoshan Group in the eastern part of the Jiangnan Orogen (JO). U–Pb dating results suggest a general younging trend of the maximum depositional ages from the Anlelin to the Xiushui Formations of the upper Shuangqiaoshan Group, indicating a co-magmatic sedimentation in association with continental-arc magmatism along the SE margin of the Yangtze Block. The Anlelin and Xiushui Formations were constrained to form in the periods of ca. 845–836Ma and ca. 835–815Ma, respectively. Provenance analysis suggests two sources for these Neoproterozoic basement sequences. Neoproterozoic zircons with positive-εHf(t) values represent detritus derived from the arc-related magmatic rocks to the southeast, whereas >1000Ma zircons and the other Neoproterozoic detrital zircons with negative εHf(t) values are proposed to be sourced from the interior of the Yangtze Block to the northwest. Simultaneously, the Hf isotope results of detrital zircons indicate that there was a rhythmic source change for the sediments in the upper Shuangqiaoshan Group, with the amounts of two separate sources from the Yangtze Block to the west and from the arc terranes to the east increasing alternatively. This provides solid evidence for a bidirectional water flow and supports a back-arc basin setting for the deposition of the sediments. Combined with the published U–Pb and Hf isotopic data for detrital zircons from the Neoproterozoic folded sedimentary sequences in the JO, we calculated the relative contribution of arc-derived crustal materials in the basement sequences of the JO. The calculations suggest that the proportion increases progressively from west to east and the back-arc basins represented by the folded sedimentary sequences could be divided into three regions, plus an arc terrane region to the east. This comparison further suggests that the orientation of Neoproterozoic accretion on the Southeastern margin of the Yangtze Block was in a W–E direction, and the JO in Neoproterozoic may not have extended 1500km as what we see today.