“巫山黄土”的稀土元素特征与成因

REE Compositions of the“Wushan Loess”and Its Origin

为了探讨"巫山黄土"的成因,对剖面样品进行了稀土元素含量、稀土分布模式和稀土特征值计算分析.结果表明:"巫山黄土"稀土元素特征与现代河流沉积物差别明显,而与北方黄土及长江中下游的下蜀土却基本一致.稀土分布模式与典型风尘沉积物相似,物源判别函数得到的物源指数(PI)均与风积黄土的最为接近,显示其风积成因的特点."巫山黄土"稀土元素平均含量216.39×10-6高于地壳平均值178.0×10-6和两块中国黄土标样平均值178.22×10-6,但与长江中下游镇江下蜀土样品的平均值207.7×10-6比较接近.样品的轻稀土较富集,重稀土略有淋失,轻重稀土比值为8.62,Eu呈较明显的负异常,Ce异常不明显.表明"巫山黄土"风尘沉积后经历了较北方黄土更为强烈的风化作用,其形成环境与下蜀土比较接近.

The rare earth element （REE） compositions of the ＂Wushan Loess＂ are studied in order to encipher information on the genesis of the ＂Wushan Loess＂. The REE patterns, concentrations and indices are used for the discussion. The results show that REE compositions of the ＂Wushan Loess＂ are quite different from those of the modern river sediments and similar to those of the Loess Plateau and the ＂Xiashu Loess＂ in the middle and lower Yangtze reaches. REE distribution patterns are similar to those of the typical eolian deposits. The index （PI） calculated by source discriminant function is similar to those of the eolian deposits, indicating the eolian origin. The average REE content （216.39 × 10^-6） is significantly higher to the average rate of the earth crust （178. 0 × 10^-6）, two samples from the Loess Plateau （178. 22 × 10^-6）, and fairly close to the average rate of the samples from the Xiashu loess in the middle and lower Yangtze reaches （207. 7 × 10^-6 ）. The light rare earth element （LREE） are relatively enriched, while the heavy rear earth element （HREE） are slightly missing. The ratios of LREE and HREE is 8. 62. The element Eu negative anomaly is obvious; while the element Ce anomaly is not obvious. It indicates that ＂Wushan＂ loess has suffered more intense weathering than the Loess Plateau. The ＂Wushan Loess＂ formed under the same weathering conditions as the ＂Xiashu Loess＂.