U-Pb and Lu-Hf isotopes of detrital zircon grains from Neoproterozoic sedimentary rocks in the central Jiangnan Orogen, South China: Implications for Precambrian crustal evolution

The Neoproterozoic sedimentary strata in the central Jiangnan Orogen of South China are separated into the Lengjiaxi and Banxi groups by a mid-Neoproterozoic angular unconformity. This unconformity, previously interpreted as the evidence of continental collision between the Yangtze and Cathaysia blocks, is crucial for understanding the Neoproterozoic tectonic regime in the South China Craton. Here we report U-Pb ages and in situ Lu-Hf isotopes of detrital zircon grains from the sedimentary rocks near this unconformity. Zircon grains in meta-sandstone samples collected from the top part of Lengjiaxi Group yield maximum depositional ages of ∼827 Ma, ∼828 Ma and 836 Ma, and combined with other available age data, we suggest that the upper limit of the sedimentation for Lengjiaxi Group is at ca. 825 Ma. Zircon U-Pb analysis for two sandstone samples collected from the bottom of Banxi Group and Nanhuan strata yields two youngest ages of 797 Ma and 719 Ma, marking the initial depositional ages for these strata. All the samples contain abundant Neoproterozoic euhedral zircon grains, suggesting their near-source provenance. The Hf isotopic model ages of detrital zircons with positive εHf(t) values reveal two periods of juvenile crustal growth at 1.5-0.9 Ga and 2.8-1.6 Ga in the Yangtze Block. Several grains display core-rim structure with ages of rim part ranging from 864 to 851 Ma. We correlate these metamorphic ages with the amalgamation of Yangtze and Cathaysia blocks in the central segments. Our data in combination with those from other studies suggest that a back-arc basin existed in the central Jiangnan belt during ca. 860 Ma to ca. 825 Ma. We propose a four-phase crustal evolution model for the central Jiangnan belt.