KAr systematics of illite/smectite in argillaceous rocks from the Ordos basin, China

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Conventional KAr dating, X-ray diffraction analyses, illite crystallinity determinations and quantitative infrared spectroscopy mineral analyses were carried out on clay materials of the Mesozoic and Paleozoic argillaceous rocks from the Ordos basin (China), in order to date the illite/smectite diagenesis, and to evaluate the influence of detrital contamination on the KAr ages. A clay mineral assemblage of illite + I/S + chlorite ± kaolinite was determined. Two different stages of illite diagenesis were distinctly identifiable, which can be correlated to two thermal and/or fluid flow events accompanying well-known major tectonic movements in China. A Jurassic diagenetic event (170-160 Ma) is related to the Early Yanshanian movement, whereas a Silurian-Devonian diagenetic age (420-370 Ma) corresponds to the Late Caledonian movement. No influence of the Jurassic event is evident on the Silurian-Devonian diagenetic illite, attesting to the good retentivity of diagenetic illite for argon. Principles for estimating any influence on the KAr data of a size fraction by detrital contamination are discussed. Decreasing illite crystallinity and increasing or constant KAr data with increasing depth indicate that authigenic illite predominates and that the KAr data are diagenetic with little detrital influence. Furthermore, the correlation between KAr ages and depth can reveal the temperature-controlled mechanisms of the diagenetic illitization process. Increasing KAr ages with depth reflects burial diagenesis, while constant KAr ages with depth indicates a diagenetic event caused by a short-lived thermal event, as shown in our study.