长江流域沉积物黏土矿物组合特征及物源指示意义

Clay Mineral Assemblages in the Yangtze Drainage and Provenance Implications

通过X射线衍射方法系统分析了长江流域(干流和主要支流)表层沉积物(及部分表土)的黏土矿物,结果显示:长江流域黏土矿物主要由伊利石、绿泥石、高岭石和蒙脱石组成,伊利石含量最高,蒙脱石含量最低;长江干流上、中、下游黏土矿物含量基本一致,支流表现出明显区别,与流域不同的源岩类型和风化强度相对应。长江干流伊利石结晶度和化学指数自上游到下游均逐渐增大,说明长江流域从上游到下游,风化作用有增强的趋势。长江支流中,上游的伊利石化学指数均较低,中游湘江、赣江均为最高,代表上游以物理风化作用为主,中下游为化学风化作用。长江干流的黏土矿物面貌受到了上游支流的较大影响,不能只依靠干流信息,必须结合支流数据,判断长江风化作用强度。黏土矿物分布显示,长江流域上游雅砻江、大渡河、岷江、嘉陵江对中下游沉积物贡献较大,乌江贡献较小;中游支流中,汉江对下游干流有一定的贡献;湘江、赣江、洞庭湖对下游及三角洲地区贡献较少。

The X-rays diffraction analysis （Xt/D） is applied to measure the clay mineral compositions of the surface sediments in the Yangtze River drainage basin. The results show that the clay mineral compositions of the sediments display a similar pattern along the main stream, with highest content of illite and lowest content of smectite, but they are different from that in the tributary rivers, these can be responded to the heterogeneous source rocks and weathering intensity. The illite crystallity and the illite chemical weathering index （5/10 peak ratio） of the main stream indicate a gradually increased weathering from upstream to downstream. In the tributary rivers, the lower illite chemical index in the upper-reaches and the higher index in the middleand lower-reaches represent a transformation from relative dominance of physical weathering to chemical weathering. Since the upper-reaches tributaries have an important influence to the whole drainage, understanding the weathering intensity does not only rely on the information of main stream, but also on that of the tributaries. Based on the result derived from the clay mineral distributions and the illite indexes, the contribution of sediments from upper-reaches including the Yalong Jiang, the Dadu He, the Min Jiang and the Jialing Jiang is larger, with minor contribution from the Wu Jiang. As for the middle branches, the Han Jiang transports more sediments to the lower-reaches and the Delta, while the Xiang Jiang, Gan Jiang and the Dongting Lake contribute little.