U-Pb and Hf Isotopic Analyses for Detrital Zircon of the Danzhou Group in the Western Jiangnan Orogenic Belt and Tectonic Implications

In order to better constrain the specific depositional age and provenance of the Danzhou Group and understand the geological evolution of the Jiangnan Orogenic Belt, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from the Gongdong and Hetong formations of the Danzhou Group in the Longsheng area of the Western Jiangnan Orogenic Belt. Detrital zircons from the Gongdong Formation yield three age populations of 2658-2517 Ma, 2427-1678 Ma and 891-781 Ma, and the youngest ages suggest that the sedimentation began after ca. 783 Ma. U-Pb ages of detrital zircons from the Hetong Formation yield major populations at 2769-2502 Ma, 2492-2100 Ma, and 991-731 Ma, and the youngest ages redefine the maximum depositional age of this unit is 760 Ma, much younger than previously considered. Thus, the upper part of the Hetong Formation in the Longsheng area is newly subdivided into the Sanmenjie Formation, which is characterized by a large amount of 765-761 Ma volcanic rocks. The dominant 991-731 Ma detrital zircons for all samples were likely sourced from the Neoproterozoic igneous rocks of the southeast margin of the Yangtze Block. The subordinate 2494-1678 Ma detrital zircons were probably sourced from the Cathaysia Block. Minor amounts of 2769-2502 Ma detrital zircons may have been sourced from the Yangtze Block. Detrital zircons from the Gongdong Formation have mainly negative epsilon Hf (t) values (-1.1 to 21.8, 90%), suggesting that the detritus of the Gongdong Formation is dominated by the recycling of old crustal materials. The epsilon Hf (t) values of detrital zircons from the Hetong Formation have a large spread of -22.2 to +9.7, indicating that the source material of the Hetong Formation includes both the juvenile crustal materials and the recycled ancient crustal materials. The above age populations and Hf isotopic characteristics are consistent with the magmatic rocks in the Jiangnan Orogenic Belt and the Southeast Yangtze Block. Taking into account the lithostratigraphic features, provenances, and depositional ages, the Danzhou Group in the Western Jiangnan Orogenic Belt was deposited in a back-arc basin.