南方红土形成过程及其稀土元素地球化学

LATERITE FORMATION PROCESS IN SOUTHERN CHINA AND ITS RARE EARTH ELEMENT (REE) GEOCHEMISTRY

南方不同红土的化学蚀变指数（CIA），硅铝系数（Sic／川。0）、铝铁系数（则产。／Fe；q）、硅铁铝系数（R2O3／SiO2）反映红土的形成经历了脱钙、脱碱基和初级脱硅富铝富铁3个主要的风化阶段；稀土在前两个阶段中主要发生淋失，而在后一阶段中富集，其配分曲线具Eu亏损的轻稀土显著富集型特征。红土中稀土元素的赋存状态以晶格相为主，主要与基者密切有关，而其它赋存状态则与红土化过程紧密相连。红土中可溶态稀土含量的高低是风化作用强度的有效指标。

Chemical index of alteration (CIA) for different laterites released that laterites derived from basalt, Quatemary red clay and gneiss hold the highest CIA values,followed by granite laterites, then limestone and sandstone; other parameters SiO2/Al2O3,Al2O3/Fe2O3 and R2O3 /SiO2 ratio exposed the results which agreed well with that of CIA; Normalized transitions for major element components were catried out to remove the false phenomena of enrichment or loss of inert components from which three stages for laterizahon process can be inferred, that is, leaching stage of CaO followed by leaching of K2O, Na2O, MgO and primary desiliciflcation with Al-enricIiment, then the stage of desilicification and Fe and Al-enrichment.Total REE contents in laterites were much higher than that in their basal rocks, and gneiss, limestone and gdrite laterites possessed the highest contents of total REE but lowest in sediments rich in qtal, t tests show no obviously REE content variance in different laterite horizons; Soluble REE richened apparently in the lower horizons in laterite profiles, varied a lot with their derived basal rocks and extremely obviously exponential relation can be observed between total REE and soluble REE also extremely obviously positive relations can exist for soluble REE with CIA values and clay contents while extremely obviously negative relations with organic matter, pH values and Al2O3 / Fe2O3 ratios, which means soluble REE contents can be an effective indicator for laterization sthength. Total REE contents showed extremely apparent negative relations with SiO2/ Al2O3, Al2O3/ Fe2O3 ratios and positive with R2O3 /SiO2 ratio which reflected the reliance on parent rocks. After being normalized tusitiory laterite REE can be seen leached in the eary laterization stage while enriched in the later stage. The main chemical forms of REE in laterites were mainly crysed phases which inherited from parent rocks, while other forms mainly related closely with laterization process.