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.