南方更新世红土氧化物地球化学特征

Geochemical behavior of major elements of Pleistocene red earth in South China

江西、浙江和湖南等地更新世红土化学组成分析结果表明,各地各类红土的化学组成具有较好的一致性。以SiO2、Fe2O3和Al2O3为主,K2O、Na2O、CaO和MgO大量淋失,相对于上陆壳(UCC)平均组成富Ti、Fe和Al,贫K、Na、Ca和Mg。符合亚热带地区脱硅富铝或脱硅富铝铁的基本成土过程,总体形成于湿热环境。其烧失量(LOI)对红土脱硅富铝程度具有一定的指示意义,LOI越大,脱硅富铝程度越强。同一剖面从网纹红土至均质红土或从网纹红土至棕色黄色风化程度渐弱。然而,全岩样品的SiO2/Al2O3、SiO2/R2O3分子比在直接指示红土风化强度方面不够有效。白网纹与红色基质的物源一致,化学组成却存在一定分异。以UCC为背景的元素盈亏状态和以均质红土为背景的元素绝对迁移量特征一致表明,物质迁移,尤其是Fe迁移与网纹红土网纹化过程之间存在必然的联系。Fe迁出区形成白网纹,Fe加入区继承红土化特征。

The analytical results of Pleistocene red earth from Jiangxi, Zhejiang and Hunan provinces show that SiO2, Fe2O3 and Al2O3 are dominant chemical compositions for all samples and Ca, Na, Mg and K are leached out in large quantities, CIA value ranges from 77.49% to 90. 54% and ba value is usually lower than 0. 3. The upper continental crust-normalized value for major elements displays Ti, Fe and Al enrichment and K, Na, Ca and Mg eluviation. Therefore, the Pleistocene red earth underwent wet-hot weathering and desilicification-allitization process, its loss on ignition （LOI） may be regarded as an indicator for degree of desilicification-allitization： the higher LOI values are, the lower SiO2/AL2O3 or SiO2/R2O3 molecular ratios are. At the same section, the weathering is weakening from vermiculated red earth unit to typical red earth unit or to brown-yellow soil unit. Furthermore, chemical compositions of ＂white vermiculated reticulations＂ differ from those of their red background materials. Their material sources are the same, but Fe content are different. Both UCC-normalized values and the migration coefficient （δj, w） based on mass balance calculation for major elements indicate correlation between element migration and reticular process, especially Fe element migration played an important role in reticular process of red earth. Fe is loss from white vermiculated reticulations and rich in red background materials.