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Recent studies on clay minerals in the boundary strata from the Cambrian-Ordovician Boundary at Jilin showed that the clay minerals may fit in with the Rb-Sr determination of the boundary age. Therefore, the authors have collected fresh rock samples from the boundary strata of the Xiaoyangqiao section and applied the new clay mineral method to the determination of Rb-Sr ages. In order to make a comparison among different methods, the total-rock samples have also been determined. This paper is to report and discuss the results from RbSr dating by both methods on the samples collected from the Cambrian-Ordovician Boundary strata at Xiaoyangqiao section.
Our studies showed that the clay of HDA 9B1 is rich in 1Md illite, which is neither of detrital nor of diagenetic origin. This means that the homogenization time of the Rb-Sr system can be assumed to be at or close to the sedimentary event, and the clay of HDA9B1 can be reasonably considered as representing the age of the Cambrian-Ordovician Boundary strata at Xiaoyangqiao. The intercept of the HDA 9B1 shale clay mineral isochron and the lowest Sr isotopic ratio of limestones are all identical with the Sr isotopic ratio of sea water in Cambrian-Ordovician time; this is another important support to the conclusion that the Xiaoyangqiao section belongs to the Cambrian-Ordovician Boundary strata and the clay mineral isochron age(487±5.6Ma) of HDA9B1 is reliable. This study also demonstrates that reliable ages can often be obtained by using the clay mineral method for Rb-Sr dating of sedimentary rocks, but the related mineralogy and geochemistry must be studied beforehand in order to select the optimum clay and carbonate samples and to interpret age results. Total-rock samples only yield an approximately estimated the age(490-500Ma).
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•Wang Yinxi :Corresponding author
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