REE Compositions of the "Wushan Loess" and Its Origin
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摘要: 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.Abstract: 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".
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Key words:
- "Wushan loess" /
- rare earth element (REE) /
- genesis /
- geochemistry
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图 2 “巫山黄土”稀土元素分布模式(a，b) 及与黄土高原黄土的比较(c)
Fig. 2. Comparison between Wushan Loess in Wushan (a, b) and the loess in the Loess Plateau in REE distribution pattern (c)
表 1 “巫山黄土”与不同沉积类型地质体稀土元素丰度比较(10-6)
Table 1. The contents of rare earth elements for different sediments(10-6)
地质体 网纹红土 地壳均值 黄土标样 镇江下蜀土 武都黄土 洛川黄土 巫山黄土 La 37.60 39.00 36.00 43.71 39.70 35.43 43.32 Ce 84.90 43.00 75.20 84.04 78.30 62.37 82.18 Pr 7.93 5.70 8.65 10.41 8.18 8.10 10.26 Nd 28.90 26.00 32.00 38.34 34.50 30.20 48.81 Sm 5.00 6.70 6.30 7.43 5.97 5.80 7.86 Eu 0.91 1.20 1.16 1.47 1.10 1.15 1.44 Gd 3.96 6.70 5.40 6.37 4.77 5.37 6.89 Tb 0.65 1.20 0.84 0.90 0.82 0.82 1.11 Dy 3.37 4.10 5.19 6.31 4.93 4.53 5.98 Ho 0.74 1.40 0.98 1.31 1.06 0.96 1.20 Er 2.14 2.70 2.85 3.42 2.47 2.65 3.25 Tm 0.42 0.25 0.42 0.49 0.38 0.44 0.54 Yb 2.57 2.70 2.79 3.08 2.29 2.52 3.08 Lu 0.41 0.82 0.40 0.46 0.30 0.39 0.48 ∑REE 179.57 178.00 178.22 207.70 184.77 160.74 216.39 ∑LREE 193.20 121.60 159.31 185.40 167.75 143.05 193.37 ∑HREE 18.40 19.87 18.87 22.35 17.02 17.68 22.54 数据来源：网纹红土、地壳均值、黄土标样(黎彤和倪守斌，1990；叶玮等，2008); 武都黄土(张虎才，1996); 洛川黄土(陈骏等，1996); 镇江下蜀土(李徐生等，2006); “巫山黄土”为本文测试数据(25个样品平均值). 
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表 2 “巫山黄土”与不同沉积类型地质体稀土元素特征值
Table 2. Rare earth element ratios of Wushan Loess and the other sediments
不同环境沉积物 δCe δEu (La/Yb)N (Gd/Yb)N (Eu/Sm)N (Nd/Yb)N (Nd/Lu)N (La/Lu)N ∑LREE/∑HREE 巫山黄土 0.96 0.60 10.13 1.85 0.48 5.77 5.53 9.67 8.62 洛川黄土 0.90 0.63 10.07 1.76 0.52 4.36 4.25 9.82 8.09 镇江下蜀土 0.97 0.65 10.18 1.71 0.52 4.53 4.53 10.18 8.31 黄土高原马兰黄土 1.1 0.66 8.64 1.47 0.52 3.75 3.57 8.22 8.05 华南网纹红土 1.21 0.63 10.49 1.27 0.48 4.09 3.83 9.83 11.59 洞庭湖网纹红土 1.25 0.69 10.69 1.28 0.57 3.69 3.42 9.90 10.50 黄土标样 1.04 0.58 9.26 1.75 0.49 4.18 4.35 9.65 8.44 长江河漫滩 0.83 0.76 14.77 0.24 5.02 5.02 14.76 10.79 数据来源：华南网纹红土、黄土标样、长江河漫滩(黎彤和倪守斌，1990；叶玮等，2008); 洛川黄土(陈骏等，1996); 镇江下蜀土(李徐生等，2006); 洞庭湖网纹红土(毛龙江等，2008); 黄土高原马兰黄土(吴明清等，1991).
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表 3 “巫山黄土”成因判识结果
Table 3. Distinguish Result on Origin of Wushan Loess
1组 2组 3组 4组 5组 6组 D1 D2 D1 D2 D1 D2 D1 D2 D1 D2 D1 D2 洛川黄土 长江河漫滩 镇江下蜀土 长江河漫滩 马兰黄土 长江河漫滩 华南网纹红土 长江河漫滩 洞庭网纹红土 长江河漫滩 黄土标样 长江河漫滩 PI 0.24 0.76 0.22 0.78 0.40 0.60 0.44 0.56 0.50 0.50 0.21 0.79 注：表中D1和D2分别代表端元1和端元2；PI为物源指数值.
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