Neoproterozoic active margin of the SW South China Block: Constraints from U[sbnd]Pb ages, Sr[sbnd]Nd isotopes and geochemical data for the gabbro and granodiorite along the Ailaoshan tectonic belt

Abstract

The Yangtze Block plays a crucial role in the revelation of the Precambrian geology of the South China Block (SCB). A series of Neoproterozoic igneous rocks have been reported along the north, west and east Yangtze Block over the past two decades. In this study, gabbros and granodiorites in the south Yangtze Block are investigated with the aim of constraining the timing and tectonic background of the SCB during the Neoproterozoic. Representative samples for the gabbro and granodiorite yielded similar zircon UPb ages of ca. 780 Ma. The gabbros have low Nb/U ratios, Nb, Ta, and light rare earth element (LREE) content, and have εNd(t) values of +2.10 to +6.15. The granodiorites exhibit enrichments in Th, U, Zr, Hf, and LREE; depletions in high field–strength elements (HFSEs, e.g., Nb, Ta and Eu); and εNd(t) values of −3.01 to −11.85. Petrogenetic analysis of the gabbros suggests that two different mantle sources (i.e., E–MORB and N–MORB) existed in the region, and the granodiorites originated from partial melting of lower continental crust with the input of mantle materials. Our data, combined with the available geological observations, show that the gabbros and granodiorites in the Ailaoshan tectonic belt were generated in an active continental margin. The SCB was an isolated continent and continuous oceanic crust subduction occurred around it during the Neoproterozoic (ca. 876–706 Ma).