K－Ar测年法在确定沉积岩成岩时代中的应用——以鄂尔多斯盆地为例

根据伊利石Ｋ－Ａｒ测年法研究，确定鄂尔多斯盆地存在两期与构造运动有关的成岩作用事件：即与早期燕山运动有关的侏罗纪（１７０—１６０Ｍａ）和与晚期加里东运动有关的志留－泥盆纪（４２０－３７０Ｍａ）成岩作用时代。为了揭示碎屑物质对Ｋ－Ａｒ年龄的影响和伊利石成岩作用的机制，本文提出了一个通过观察Ｋ－Ａｒ年龄随深度变化的趋势来进行判断的模式。在Ｋ－Ａｒ年龄小于地层时代的条件下，Ｋ－Ａｒ年龄与深度呈正相关，或者很相近的Ｋ－Ａｒ年龄与深度变化无关均标志着没有或很少有碎屑物质的影响；同时，前者指示逐渐埋藏条件下的成岩作用，而后者则反映短暂的热事件引起的成岩作用事件。

The Application of the K Ar Dating to the Determination of Diagenetic Ages of Sedimentary Rocks-A Case Study from the Ordos Basin

The method of K Ar dating and its application to the determination of diagenetic ages of sedimentary rocks is introduced and discussed using the K Ar data from the Ordos Basin. In the basin two distinct diagenetic stages were identified by means of K Ar dating, which can be correlated with two thermal events resulting from major tectonic movements. A Jurassic diagenetic event(170 160 Ma) documented in the Permian-Triassic rocks is related to the early Yanshanian movement, whereas a Silurian-Devonian diagenetic stage (420 370 Ma) recorded in the Cambrian-Ordovician sequences corresponds to the late Caledonian movement. These suggest that the diagenetic processes of illites were mainly controlled by tectono-thermal events in the basin. The key problem of evaluating the reliability of K Ar ages from sedimentary rocks is not diffusional loss of radiogenic argon, but detrital contamination. A model of estimating detrital contamination by observation of the variations of K Ar ages with depth is proposed and discussed using K Ar data from the Ordos Basin and many other basins of the world. A positive correlation between K Ar ages and depth or no correlation between similar K Ar ages and depth are indications of little or no influence of detrital minerals and predominance of authigenic illites. Thus, such K Ar data reflect diagenetic ages. A negative correlation between K Ar ages and depth suggests detrital contamination. Therefore, these K Ar data give mixed ages of less geological meanings. Furthermore, the correlation between K Ar ages and depth can be used to decipher the mechanisms of the diagenetic process of illites. A positive correlation between K Ar ages and depth reflects burial diagenesis, while no correlation between similar K Ar ages and depth indicates a diagenetic event caused by a short thermal event, as shown in this study.