Provenance of the Nb-rich bauxite and Li-rich claystone at the base of the Heshan Formation in Pingguo, Guangxi, SW China: Constrained by U–Pb ages and trace element contents of detrital zircon

Abstract

The base of the Upper Permian Heshan Formation in Pingguo County, Guangxi Province, SW China, can be divided into two layers: a lower Nb-rich bauxite layer and an upper Li-rich claystone layer. The sources of these layers may have played a critical role in their formation. Detrital zircon provenance analysis is an effective method for identifying sediment provenance; however, it has not been applied previously to the Nb-rich bauxite and Li-rich claystone, thus limiting our knowledge of the sources of these two layers. Detrital zircons from the bauxite and the claystone layers yield a single age peak at ∼ 260 Ma, which is consistent with the age of the Emeishan Large Igneous Province (LIP; 263–257 Ma) and the Permian Palaeo-Tethyan magmatic arcs (300–260 Ma). In contrast, detrital zircons from the two layers yield different trace element signatures. Yttrium and Hf contents and U/Yb ratios of detrital zircon grains from the bauxite layer overlap with those of continental and oceanic crust and have similar Hf/Th, Th/Nb, Th/U, and Nb/Hf ratios to those of zircons of within-plate and magmatic arc origins. These results indicate that the bauxite was sourced from volcanic rocks or ash related to the Emeishan LIP and the Permian Palaeo-Tethyan magmatic arcs. However, detrital zircons from the claystone have similar Hf/Th, Th/Nb, Th/U, and Nb/Hf ratios to only those of zircon from magmatic arcs, suggesting a single Permian Palaeo-Tethyan magmatic arc source. Thus, our results reveal an abrupt change in the provenance of zircons at the base of the Heshan Formation. The lower Nb-rich bauxite layer was derived from the weathering of volcanic rocks or ash related to the Emeishan LIP and the Permian Palaeo-Tethyan magmatic arcs, whereas the upper Li-rich claystone layer was derived principally from the weathering of volcanic ash related to the Permian Palaeo-Tethyan magmatic arcs. Since Nb is an immobile element, volcanic rocks/ash related to the Emeishan LIP and the Permian magmatic arc could represent a source for bauxite and Nb. In contrast, Li is chemically highly mobile in the Earth’s surface environment and its origin cannot be constrained easily, further work is needed to identify the sources of Li.