海南岛第四纪玄武岩的红土化作用

我国海南岛北部广泛分布着第四纪玄武岩,其中喷出规模最大、分布面积最广的是早更新世玄武岩(Q₁),其上覆盖着一层较厚的红土风化壳,有的成为可供开采的铝土矿和褐铁矿。晚更新世玄武岩(Q₃)出露范围较小,地表亦多风化成红土,但发育较差。笔者研究的红土风化剖面便发育在早更新世橄榄玄武岩上(图1)。

LATERITIZATION OF QUARTERNARY BASALTS IN HAINAN ISLAND

1.Lateritic weathering crust is widely developed in Early Pleistocene basalts in the northern part of Hainan Island.In the northeastern part of Hainan Island,the alumino-ferruginous laterite was formed in good topographic condition owing to greater annual precipitation and higher mean annual temperature.In the northwestern part,however,ferruginous laterite was formed on the smooth littoral platform of basalts in relatively dry climate.2.Alumino-ferruginous lateritic profile can be subdivided into 5 zones from bottom to top : fresh basalts,crystalline halloysite-allophnae zone,halloysite-allophane zone,halloysite-gibbsite zone and ocher-b-axis disordered kaolinte-gibbsite zone.Ferruginous laterite can be subdivided into 4 zones from bottom to top: fresh basalts,halloysite-allophane-chlorite zone,ocher-b-axis disordered kaolinite-nontronite zone and ocher-b-axis disordered kalinite-gibbsite zone.3.In the formation of laterite,3 processes were principally involved: clay formation,ferritization and bauxitization.Clay formation took place in the silica- and base-leaching stage,and ferritization and bauxitization appeared in the end-leaching stage.Finally,hydrolytic zone,relatively rich in Fe and Al hydroxides was formed.Because of the migration of Fe in weathing profile,the rate of ferritization was more rapid than that of bauxitization.Ferruginous materials were often transported to the top of weathering profile,which promotes the formation of concretionary iron ore bed or iron pan bed.4.During weathering course,the trace-elements in aluminoferruginous lateritic profile have mostly been leached away,while the ones in ferruginous lateritic profile have been more stable.5.Atmospheric water is major factor controlling the leaching process in weathering profile.In the case of that atmospheric rainfall is the same or similar,microtopog-raphy plays a leading role because it may dominate drainage condition.In the northeastern part of Hainan Island the climatic and topographic condition favourable for forming laterite with bauxite is present,but for forming high-grade lateritic bauxite ore deposits its latitude is too high and the atmospheric temperature is too low and weathering time duration is too short.