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Radiogenic isotope dating of illitic clays has been widely used to reconstruct thermal and fluid flow events in siliciclastic sedimentary basins, the information of which is critical to investigate mechanisms of hydrocarbon maturation. This study carried out Rb−Sr and 40Ar−39Ar dating of authigenic illitic clay samples separated from the Palaeogene sandstone in the northern South China Sea. Our Rb−Sr data further confirm the previously reported three periods of fluid flow events (at 34.5 ± 0.9, 31.2 ± 0.6, and 23.6 ± 0.8 Ma, respectively) in the northern South China Sea, which are related to regional episodic tectonism. However, 40Ar−39Ar ages of illite obtained in this study are significantly younger than the corresponding Rb−Sr ages. The significantly younger 40Ar−39Ar ages were probably due to 40Ar loss caused by later dry heating events on the Hainan Island that have not affected the Rb−Sr isotopic systematics. The inconsistency between Rb−Sr and 40Ar−39Ar data should be attributed to different isotopic behaviors of K−Ar and Rb−Sr isotopic systematics in illite. Our results indicate that Rb−Sr isotopic dating method may be a preferential approach for clay dating in geological settings where exist younger dry heating events. [ABSTRACT FROM AUTHOR]