岷江上游晚更新世黄土粒度与元素组成特征及其物源指示意义

Grain Size & Elements Composition Characteristics and Their Implications for Provenance of the Late Pleistocene Loess in the Upper Reaches of the Minjiang River,China

岷江上游地区广泛分布黄土地层,揭示其成因和物源对深入理解该区环境演变具有重要意义。以位于四川省茂县叠溪镇的晚更新世黄土剖面为研究对象,分析了黄土粒度分布、粒径频率曲线和粒度参数以及元素地球化学特征,并与北方黄土和邻近的成都粘土对比。结果表明叠溪黄土剖面粉砂(2~50μm)平均含量达81. 59%,且变异较小,而粘粒(<2μm)和砂粒(> 50μm)平均含量分别为13. 77%和4. 95%,相对北方黄土含较多1μm左右的细颗粒。粒径频率曲线和粒度参数特征表明叠溪黄土具有典型风成沉积特征,沉积物主要由<1000 m高度的近地表风力搬运沉积为主。叠溪黄土常量元素含量与北方黄土基本相同,表现为明显的Na淋失而K相对稳定,风化程度较低。Ti O2/Al2O3、K2O/Al2O3、Eu/Eu^*、(La/Yb)N、(Gd/Yb)N和(La/Sm)N等特征值以及Zr、Th、Sc、La、Th、Ba、Sr、Rb、Pb和U含量分布反映出叠溪黄土与北方黄土物源具有相似性,均明显区别于成都粘土。末次冰期亚洲冬季风和邻近山原地带产生的重力风可能均对岷江上游黄土沉积发挥了重要作用。

Investigation on the origin and provenance of the loess deposits, dotted extensively in the upper reaches of the Minjiang river, is necessary for further understanding regional environmental evolution. The Diexi loess, located in Maoxian county in Sichuan Province and deposited in the late pleistocene, were investigated and compared with the windblown sediments in northern China and clay in Chengdu plain, in terms of grain size distribution, frequency curve, particle size parameters and element geochemical features. The results showed that the silt fraction (2~50 μm) was dominant in the Diexi loess with an average concentration of 81.59% and a low coefficient of variation, while the clay (<2 μm) and sand (>50 μm) average concentrations are 13.77% and 4.95%, respectively. More fine particles of ~1 μm presented in the samples compared to northern loess. From the grain size frequency curves and particle size parameters, the Diexi loess were windblown dusts and were transported by near-surface wind that less than 1000 m high. The major elements contents of the Diexi loess were similar to those of the loess of northern China, as both showed leaching Na and stable K. The provenance of the Diexi loess was similar to that of the loess in northern China while significantly different from that of the Chengdu clays, demonstrated by the characteristic values of TiO 2/Al 2O 3、K 2O/Al 2O 3、Eu/Eu^*、(La/Yb) N、(Gd/Yb) N and (La/Sm) N , and the concentration distributions of Zr、Th、Sc、La、Th、Ba、Sr、Rb、Pb and U. Our study suggested that the remote windblown dusts influenced by Asian winter monsoon and neighboring particles transported by katabatic winds may be both material sources of the loess in the upper reaches of the Minjiang river.