Late Cretaceous redbeds from the Gan-Hang Belt in Southeast China: petrography and geochemistry implications for provenance, source weathering, and tectonic setting

Abstract

ABSTRACTThe distinct basin and range tectonics in Southeast China were generated by crustal extension associated with subduction of the Palaeo-Pacific plate during the late Mesozoic. Compared with 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 sandstones from the Late Cretaceous Guifeng Group in the Yongchong Basin, Southeast China. Detrital grains are subangular to subrounded, poorly sorted, and rich in lithic fragments. Variable Chemical Index of Alternation values (59.55–79.82, avg. 66.79) and high Index of Compositional Variability (ICV) values (0.67–3.08, avg. 1.40) indicate an overall low degree of chemical weathering and rapid physical erosion of source rocks. Such features are consistent with an active extension tectonic setting. Other chemical indices (e.g. Al2O3/TiO2, Th/U, Cr/Th, Th/Sc, Zr/Sc) also suggest significant first-cycle sediment input to the basin and a dominant felsic source nature. Thus, the Guifeng Group possibly underwent moderate to low degrees of weathering upwards. Sandstone framework models and geochemical characteristics suggest the provenance was likely a combination of passive margin (PM) and active continental margin (ACM) with minor continental island arc (CIA) tectonic settings. Sediment derivation from Neoproterozoic metamorphic rocks and Cambrian to Triassic granitoids indicates PM provenance, whereas sediments derived from Jurassic to Cretaceous granitoids suggest ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben basin under the back-arc extension regime when Southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.