热带地区碳酸盐岩上覆红色风化壳的成因机理及元素演化

开展热带岩溶地区红色风化壳元素地球化学演化规律研究,有助于丰富碳酸盐岩风化成土理论的认识。在越南北部选取典型的碳酸盐岩风化剖面,分析热带气候条件下碳酸盐岩上覆红色风化壳中主量元素的物质来源和演化特征。结果表明:无论Ti/Zr的元素比值分析,Hf-Zr、Nb-Ta及Sm-Nd的元素对协变分析,还是上地壳元素平均值(UCC)标准化分析,两个剖面的上覆风化壳均显示原地残积的特征,即两个剖面是碳酸盐岩的原位风化产物。碳酸盐岩风化成土过程中,CaO、MgO淋失明显,Al_2O_3和Fe_2O_3发生富集,显示两个剖面均经历较强的风化过程,但是白云岩和灰岩剖面有一定的分异特征。越南北部白云岩风化剖面从基岩到上部土层显示出稳定的Ti-Fe元素共富集特征,而灰岩剖面中Fe的增长速率明显超过Ti。白云岩上覆风化壳经历了强烈的风化作用,其脱硅作用弱于灰岩风化剖面,而富集铝的作用强于灰岩。迁移系数的演化规律说明两个剖面中长石成分(钾长石、钠长石等)或次生矿物(伊利石等)在风化成土过程中逐渐分解,同时Al、Fe、Si等稳定元素的在风化剖面中不断富集。

Genetic mechanism and elemental evolution of weathering laterite crust overlying carbonate rocks in tropical areas

The study on the geochemical evolution of elements in the red weathering crust in tropical karst area is helpful to the understanding of the theory of weathering and soil-forming of carbonate rocks. Two typical weathering profiles of carbonate rocks in northern Vietnam were selected to analyze the material source and evolution characteristics of major elements in the red weathering crust overburden of carbonate rocks under tropical climate conditions. The results show that regardless of the element ratio analysis of Ti/Zr, the element pair covariant analysis of Hf-Zr, Nb-Ta and Sm-Nd, or the normalized analysis of the average value of upper crustal elements (UCC), the weathering crust of the two profiles all show the character of in-situ residual, that is, the two profiles are weathering products of carbonate rocks in situ. In the weathering process of carbonate rocks, the leaching of CaO and MgO is obvious, and the enrichment of Al₂O₃ and Fe₂O₃ occurs, indicating that both sections have undergone a strong weathering process, but the dolomite and limestone sections have certain differentiation. The weathering profile of dolomite stone in northern Vietnam exhibits a stable Ti-Fe co-enrichment from the bedrock to the topsoil, while the growth rate of Fe in the limestone profile is significantly higher than that of Ti. The crust over dolomite stone underwent intense weathering process. Though its desilication was weaker than that of limestone weathering profile, its aluminum enrichment was much stronger than that of limestone profile. The evolution law of migration coefficient indicates that the components of feldspar (potassium feldspar, sodium feldspar, etc.) or secondary minerals (illite, etc.) in the two profiles were gradually decomposed in the process of weathering, while the stable elements such as Al, Fe and Si were continuously enriched in the weathering profiles.