昆明石林碳酸盐岩红色风化壳元素地球化学特征

摘要

以昆明石林两个典型红土剖面为研究对象,探讨了主量元素及稀土元素在化学风化作用下的迁移特征及分布规律,为进一步认识碳酸盐岩风化剖面的演化过程以及红土剖面的物质来源提供地球化学依据.两个红土剖面的研究结果显示:(1) SiO2、A12O3和Fe2O3T是两个剖面红土样品的主要成分,三者占到总量的80％以上,其中又以SiO2的含量最高;(2)化学蚀变指数CIA(90以上)较高,硅铝系数和硅铁铝系数较低,显示出两个红土剖面经历了较强的风化淋溶作用,体现出研究区亚热带高原气候,温暖湿润的化学风化环境;(3)两个剖面稀土元素含量较高,范围分别在211.51×10-6～416.40×10-6和290.67×10-6～626.89 × 10-6之间,其稀土配分曲线均具有Eu亏损的轻稀土显著富集型特征,两个剖面都表现出轻稀土元素间分异较大而重稀土元素间分异较小的特征,这主要与剖面风化成土过程中的三价氧化物的强烈固定作用有关,且KP剖面的轻稀土元素分异更明显;(4)两个剖面经历了相似的风化迁移过程,结合元素对Al2O3-TiO2 、Al2O3-Fe2O31、Y-Yb相关性分析以及SiO2/Al2O3-Na2O/Al2O3相关图解可以得出,KP和CH剖面是由于下伏基岩风化而来.%In order to investigate the alteration process of carbonate weathering and the origin of the weathering profile in Karst area,two laterite profiles (KP and CH) from Shilin County of Kuming,Yunnan Province are reported to analyze concentrations of major elements and rare earth elements (REEs).The results of element behavior during chemical weathering and the geochemical characteristics of carbonate-derived laterite are summarized as follows:(1) The major element compositions of the two profiles are dominated by SiO2,At2O3,Fe2O3T(more than 80％),with abundant SiO2.(2) The range of chemical index of alteration (CIA) is above 90 with low ratios of SiO2/Al2O3 and SiO2/(Fe2 O3 + Al2 O3),indicating that there is an intensive leaching during chemical weathering in the subtropical warm and humid climate.(3) The KP and CH profile showed relatively high contents of REE,ranging from 211.51 × 10-6 to 416.40 × 10-6 and from 290.67 × 10-6 to 626.89 × 10-6,respectively.The REE patterns are consistent with enriched LREE,negative Eu anomalies,and significant fractionation in LREE and less in HREE.These characteristics can be attributed to the fixation of trivalent oxide during alterations.The KP profile is distinctive with more intensive fractionation in LREE.(4) Similar weathering processes in the two profiles are suggested.Further evidence from Al2O3-TiO2,Al2 O3-Fe2 O3T,Y-Yb correlation analysis and SiO2/Al2 O3-Na2 O/Al2 O3 diagrams support the hypothesis that the two profiles are derived from the underlying bedrock.