Element Geochemical Differentiation And Genesis Of Volcanic Rock Weathered Soil From Downstream Of Nandu River In Northern Hainan Island

The weathering in the north area of Hainan Island cropping different period of volcanic rocks is most active and important for understanding the element geochemical action in the basalt weathering process.In this study,we analysis the major and trace elements composition of subsoil samples from different periods of weathered basalt and soil samples from typical weathering profile in the downstream of Nandu River in Northern Hainan and investigate their differentiation mechanism.It is showed that the contents of major elements in the weathering soil samples are mainly SiO₂,Al₂O₃and TFe₂O₃with a mass fraction of more than 73.62%.With the age of the parent rock from old to new,the elemental mobility roughly shows a trend from strong to weak,especially SiO₂,TiO₂,Al₂O₃,CaO,Na₂O,and K₂O are the most obvious.In Qhs weathered soils,rapid loss of MgO,CaO and Na₂O,with leaching loss rates ranging from 29.5%to 56.5%,while in Qp²d¹,Qp₂d₂and Qp³d²,there is almost no more variation,and the leachate is almost exhausted.This indicates that the rapid leaching loss of MgO,CaO and Na₂O occurred in the early stage.From Qhs to Qp²d¹,SiO₂,TiO₂,Al₂O₃,TFe₂O₃show a gradual change,with SiO₂gradually lost and TiO₂and Al₂O₃gradually enriched.From Qhs to Qp²d²to Qp²d¹,TFe₂O₃showed first enrichment and then deficit.Compared to the background values of fresh basaltic elements,there is a large loss in trace elements from profile soils.With the extreme weathering of the basalt,alkali and alkaline earth metals in the soil are lost and transported out of the profile,Rb and Sr were almost completely lost.The behavior of redox-sensitive elements(Co,Cr and U)is restricted by Fe-oxides?Mn-oxides?secondary phosphates.Elemental behaviors of the immobile Zr,Hf,Nb and Ta still have strong activity in Basalt weathering.Except for the subsoil samples weathered by the volcanic rock from Shishan Formation,all of other subsoil samples derived from different period of volcanic rocks were show extremely weathering characteristics and the CIA values are all higher than 91.0.CIA(Qp²d¹)>CIA(Qp²d²)>CIA(Qp³d²)>CIA(Qhs),the average CIA values show the following trend.The average CIA values are higher as the age of soil formation getting old.Our result also shows that all the subsoil samples and soil sample in the basalt weathering profile share the similar characteristics,such as consistently LREE enriched patterns and insignificant Eu anomaly,except for the negative correlation between?REE and CIA values only in the soil samples from the weathering profile.From top to bottom in each section,there is no regular change of REE contents,but there are some differences between each section,which reflects the influence of parent rock on soil and shows that the REE composition has inheritance to the parent rock.The Eu anomaly is not significant,the Ce is positive anomaly.With the increase of the soil forming age,Ce positive anomaly tends to increase.The significant Ce positive anomaly ether in the subsoil samples or in the soil samples in the basalt weathering profile maybe correlated with the strongly weathering and leaching action under the tropical hot humid climate in north of Hainan Island.