评介Smith等的热河群^(40)Ar-^(39)Ar同位素测年结果

REVIEW OF THE ^(40)Ar-^(39)Ar DATES OF THE JEHOL GROUP BY SMITH ET AL.

对Ｓｍｉｔｈ等１９９５年热河群４０Ａｒ－３９Ａｒ同位素测年工作的背景以及各样品的采样地点、层位与测年目的等作了补充介绍．由于Ｓｍｉｔｈ等的论文中将３组后期火山岩的测年结果分别误解为义县组下部与上部地层的同位素年龄，因而整体上混淆了义县组不同层位的测年结果．Ｓｍｉｔｈ等１９９５年发表的６组样品的测年数据实际上至少代表了４个与热河群义县组相关层位的４０Ａｒ－３９Ａｒ同位素年龄：１２２．９±０．３Ｍａ（凌源大新房子沉积层下伏火山岩）、１２２．１±０．２～１２２．５±０．３Ｍａ（凌源二十里堡沉积层）、１２１．５±０．９～１２１．６±０．５Ｍａ（义县金刚山沉积层上覆火山角砾岩）、１２０．８±０．４～１２１．４±０．７Ｍａ（后期火山岩）。

The dates reported by Smith et al. in 1995 are the first 40Ar-39Ar ages for the Jehol Group in western Liaoning, Northeast China, which provide a considerably improved age calibration of the lowermost Yixian Formation of the Jehol Group, and indicate that the whole group was deposited entirely within the Early Cretaceous time. However, because Smith et al. 's report mistook the dates of three epigenetic igneous rock samples as the ages of the upper and lower parts of the Yixian Formation, respectively (samples 5, 10, 4), the isotopic dating results, as a whole, are perplexing and have been greatly misunderstood since they were published in 1995. In fact those dates for the Jehol Group by Smith et al. represent the 40Ar-39Ar ages of at least four horizons: 122.9 ± 0.3Ma (volcanics underlying the Daxinfangzi Bed of the Yixian Formation), 122.1 ± 0.2- 122.5 ± 0.3Ma (Ershilipu Bed of the Yixian Formation), 121.5 ± 0.9- 121.6 ± 0.5Ma (Volcanic breccias above the Jingangshan Bed of the Yixian Formation), 120.8 ± 0.4- 121.4 ± 0.7Ma (epigenetic igneous rocks overlying or intruding into the Yixian Formation).