中国中亚热带网纹红土的地球化学特征与沉积环境

SEDIMENTARY ENVIRONMENT OF VERMICULAR RED EARTH IN MID-SUBTROPICAL CHINA

130个样品的粒度、黏土矿物组成、地球化学特征以及Sr、Nd同位素分析表明，广泛分布在中亚热带地区并且具有一定厚度的第四纪网纹红土为加积型沉积。该类沉积粒度组成与北方风尘沉积类似，粉砂含量占优势，〉50μm含量小于5％，5～50tan组分41．2％-61．5％，〈5μm黏粒26．9％-55．8％；黏土矿物组成中含有较多的石英、云母和高岭石；全土体的平均硅铝率为7．96，硅铁铝率6．11；稀土元素丰度179．6mg kg^-1，与地壳平均值（178mgkg^-1）、北京郊区1998年12月大气粉尘稀土总量（178．0mgkg^-1）以及两块中国黄土标样（178．2mgkg^-1）非常接近，并且表现为轻稀土强烈富集，重稀土淋失，Ce元素正异常；网纹红土全岩样品的^87Sr／^86Sr值界于0．725657～0．727714之间，Nd同位素0．511989～0．512098，与佳县上新世红黏土和黄土的酸不溶相性物质接近。理化特征指示，第四纪加积型网纹红土与北方黄土中的古土壤以及上新世红黏土的形成环境类似，并且具有稳定的物质来源。

Accumulating conditions of vermicular red earth in South China have been explored for decades, but its origin and sedimentary environment are still unknown. Based on the analysis of physico-chemical properties of 130 samples which were sampled from Jiujiang Xinzi, and Shahe in Jiangxi Province, Jinhua and Anji in Zhejiang Province, and Zhuzhou in Hunan Province, the authors try to have a better understanding of how vermicnlar red earth （VRE） was accumulated. Particle size of the soil samples was measured using the Malvern 2000, major elements using XRF, REE using ICP-MS, and isotope ratios of Sr and Nd using TIMS. Results suggest that the widely distributed Pleistocene vermicular red earth is an aggrading kind of sediment, which is characterized by thickness, homogenous texture, and no bedding. Its particle-size distribution is similar to that of eolian deposits. The soil is dominant with silt. Particles 〉 50μm are less than 5 %, 5 - 50μm in the range of 41.2 % - 61.5 %, and clay 〈 5 μm in the range of 26.9 % - 55.8 %. Clay minerals are mainly composed of quartz, mica and kaolinite. The average silica-alumina ratio is 7.96, and silica sesquioxide ratio is 6.11, higher than the respective value of the laterite. The average REE content of vermicular red earth is 179.6 mg kg^-1 , similar to that of the earth crust（178 mg kg^-1） , dust in the suburbs of Beijing in December 1998（ 178.0 mg kg^-1）, and two of Chinese standard loess samples （178.2 mg kg^-1）. The REE pattern is identical with that of typical loess, suggesting that light REE are extremely abundant, while heavy REE are low. Ce element oc- curs in positive anomaly. The ratio of ^87Sr/^86Sr in vermicular red earth ranges from 0.722 001 to 0.727 741, and Nd istope ranges from 0.511 989 to 0.512 098, which are similar to those of Pliocene red clay in Jiaxian and acid insoluble substance of the loess in North China. Physico-chemical properties indicate that accumulating environment of vermicular red earth in the sub- tropic area is identical with those of the paleosol and Pliocene red clay, and is a stable dust source.