琼北南渡江两岸火山岩分布区土壤地球化学分异及成因研究

Geochemical Differentiation and Genesis of Vol-canic Rock Weathered Soil from the Downstream of the Nandu River on Northern Hainan Is-land

海南岛北部的第四纪火山岩分布区是我国玄武岩风化成土最活跃的地区之一,对于理解热带成壤过程的元素地球化学行为具有重要意义。本文选取琼北南渡江下游两岸多期火山岩上的心土及风化剖面,系统探讨该区土壤的主量与稀土元素分异特征。研究发现,玄武岩风化土壤的主量元素组成均以SiO_(2)、Al_(2)O_(3)、TFe_(2)O_(3)为主,三者总含量在73.62%以上,MgO、CaO、Na_(2)O和K_(2)O强烈淋滤流失。除全新世石山组玄武岩风化的心土样品外,其它期次火山岩风化心土样品的风化程度均为极端风化,CIA值(化学蚀变指数)多处于91.0以上,且平均的CIA_(Qp^(2)d^(1))>CIA_(Qp^(2)d^(2))>CIA(Qp^(3)d^(2))>CIA(Qhs),表现为成土年龄越老,风化程度越高。本次研究还发现,无论是面上分布的心土样品还是剖面土壤样品,均为轻稀土富集的右倾模式,Eu异常不显著,剖面土壤样品ΣREE与CIA之间呈现负相关。不同期次玄武岩风化土壤均呈现显著的Ce正异常,可能与琼北地区处于热带湿热气候区,受强烈的风化淋滤作用有关。

The Quaternary volcanic region on northern Hainan Island is one of the most active areas of weathering in China,and is of great significance for understanding the element geochemical behavior during the basalt weathering.We analysis the major and trace element compositions of both subsoils from different periods of weathered basalts and soil samples from typical weathering profile in the downstream of the Nandu River on northern Hainan to investigate their differentiation mechanism.It is found that the major elements of volcanic weathering soils are SiO_(2),Al_(2)O_(3) and TFe_(2)O_(3) with the total content of more than 73.62%,and their MgO,CaO,Na_(2)O,K_(2)O were strongly leached.Except for the weathered core soil samples of the Holocene Shishan Formation basalts,the other subsoil samples derived from different periods of volcanic rocks were extremely weathered,and their CIA(chemical alteration index)values are all higher than 91.0 with the aver-age CIA value trend of CIA_(Qp^(2)d^(1))>CIA_(Qp^(2)d^(2))>CIA(Qp^(3)d^(2))>CIA(Qhs),which indicates that the older soils underwent more weathering.Besides,both the regional subsoil and soil samples from the profile of the weathering ba-salts share the similar REE characteristics,such as right-lean pattern of LREE enrichment and insignificant Eu anomaly,and negative correlation between ΣREE and CIA values on the profile.The significant positive Ce anomaly in both the subsoil and soil samples from the different basalt weathering profiles might be correlated with the strongly weathering and leaching under the tropical hot humid climate of northern Hainan Island.