Petrography and geochemistry of the Late Cretaceous redbeds in the Gan-Hang Belt, southeast China: implications for provenance, source weathering, and tectonic setting

ABSTRACT

The distinct basin and range tectonics in southeast China were generated in a crustal extension setting during the late Mesozoic. Compared with the adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstone samples from the Late Cretaceous Guifeng Group of the Yongchong Basin in the Gan-Hang Belt, southeast China. Detrital grains are commonly subangular to subrounded, poorly sorted, and are rich in lithic fragments. The variable pre-metasomatic Chemical Index of Alternation (CIA* = 62–85), Chemical Index of Weathering (CIW = 70.90–98.76, avg. 85.62), Plagioclase Index of Alteration (PIA = 60.23–98.35, avg. 79.91), and high Index of Compositional Variability (ICV = 0.67–3.08, avg. 1.40) values collectively suggest an overall intermediate degree of chemical weathering and intense physical erosion of the source rocks, but a relatively decreased degree of chemical weathering during the late stage (Lianhe Formation) of the Guifeng Group is observed. Several chemical ratios (e.g. Al₂O₃/TiO₂, La/Th, Cr/Th, Th/Sc, Zr/Sc) also suggest a dominant felsic source nature, significant first-cycle sediment supply, and low sedimentary recycling. Such features are consistent with active extension tectonic setting. Sandstone framework models and geochemical characteristics suggest the provenance is related to passive margin (PM), active continental margin (ACM), and continental island arc (CIA) tectonic settings. Sediment derivation from the Neoproterozoic metamorphic rocks and Silurian–Devonian granites indicates a PM provenance, whereas sediments derived from the Early Cretaceous volcanic-intrusive complexes suggest an ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben under the back-arc extension regime when southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.

KEYWORDS:

• Late Cretaceous redbeds
• Guifeng Group
• provenance
• source weathering
• Gan-Hang Belt

Acknowledgements

The authors thank the Editor-in-Chief, Dr Robert J. Stern, and the reviewer, Dr Francesco Perri from the University of Calabria, and two other anonymous reviewers for their constructive comments which greatly improved the quality of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

Financial support was provided by China Geological Survey projects [grant numbers 1212011120836 and 1212011220248], China Scholarship Council [grant number 201308360142], Gan-Po Excellent Talents 555 Project of Jiangxi Province [grant number GCZ 2012-1], Research Foundation of Jiangxi Education Department [grant number GJJ13438], Open Fund of Fundamental Science on Radioactive Geology and Exploration Technology Laboratory [grant number RGET1304].