Petrogenesis and tectonic evolution of Lianyunshan complex, South China: Insights on Neoproterozoic and late Mesozoic tectonic evolution of the central Jiangnan Orogen

Abstract

The Yangtze and Cathaysia blocks in South China are separated by the ca. 1500km long Jiangnan Orogen. The Lianyunshan complex, located in the central segment of Jiangnan Orogen, includes wide exposures of late Mesozoic granites. Here we report geological, geochronological and geochemical data from the Lianyunshan late Mesozoic granites and their Neoproterozoic host rocks belonging to the Lengjiaxi Group. Detrital zircon U–Pb ages reveal a single prominent peak at ca. 840Ma in the metamorphosed and deformed Lengjiaxi Group samples. Ten of the youngest zircon grains in these rocks have a weighted mean age of 828.8±7.1Ma, which is tentatively interpreted as the maximum depositional age of the Lengjiaxi Group. Two stages of late Mesozoic S-type granites are dated at ca. 150Ma and ca. 140Ma from the Lianyunshan domain. From early to late, these rocks show decreasing MgO, CaO, Fe2O3T, TiO2 contents and increasing SiO2, K2O, Eu/Eu*, Rb/Sr ratios and differentiation index, suggesting continuous magmatic evolution dominated by fractional crystallization. The εHf(t) values of zircons with late Mesozoic ages are all negative, with their two-stage model ages mainly ranging from 1.9 to 1.4Ga, which is significantly different from the model age of the host rocks belonging to the Lengjiaxi Group. These results indicate that the late Mesozoic granites were mainly derived from the partial melting of Paleo-Mesoproterozoic basement rocks that are older than Lengjiaxi Group. Major and trace element features suggest that the protoliths of these early stage granites in Lianyunshan are probably mixed greywacke and shale. The arc-like trace elements signature of all the late Mesozoic granites were possibly inherited through crustal contamination during the magma genesis caused by slab rollback associated with Paleo-Pacific subduction during 150- to 140Ma beneath the central Jiangnan belt.