Geochronology of Neoproterozoic mafic rocks and sandstones from northeastern Guizhou, South China

The mafic-ultramafic rocks in the Fanjingshan Group from northeastern Guizhou have been classified as the calc-alkaline series. These mafic rocks have prominent negative Nb, Ta and Ti but positive Pb anomalies and show typical geochemical signatures of arc basalts and decoupling Nd-Hf isotopic characters. SHRIMP and LA-ICP-MS U-Pb zircon datings for four samples of the mafic rocks yield U-Pb zircon ages of 814 ± 15 Ma, 814 ± 6 Ma, 831 ± 6 Ma and 827 ± 24 Ma with an average age of 822 ± 15 Ma. The U-Pb ages of the youngest detrital zircon grains in four samples of the sandstones from the Fanjingshan Group are 873 ± 4 Ma, 874 ± 10 Ma, 870 ± 10 Ma and 869 ± 9 Ma with an average age of 872 ± 3 Ma that is considered as the maximum depositional age of these sandstones. It suggests that the Fanjingshan Group, the equivalent of the Sibao and Lengjiaxi Groups occurring in the western Jiangnan orogen, is Neoproterozoic rather than Mesoproterozoic as previously considered. Combined with published age data, the U-Pb zircon dating of both mafic rocks and sandstones in the Fanjingshan Group provides new constraints on the timing of various geological events during the development of the Jiangnan orogen. It is evident that there existed coeval magmatism (878-822 Ma) with arc signatures and sedimentation (872-835 Ma) in the foreland basin along the southeastern margin of the Yangtze Block during the Neoproterozoic. Compared with the Grenvillian-age orogenic event, the collision between the Yangtze and Cathaysia Blocks was postponed for 355-160 Ma. Moreover, the basement strata in the Jiangnan orogen underwent thorough greenschist-facies metamorphism. The prevalent granulite-facies metamorphism in the Grenvillian orogenic belts has not been found in the metamorphosed sedimentary-igneous rocks of the basement strata exposed in the Jiangnan orogen. All these suggest that the Jiangnan orogen should not be considered as a Grenvillian-age orogenic belt. It is proposed that the Jiangnan orogen might be the product of collision between two neighboring blocks during the transition of Rodinia to Gondwana.