Geochemical Charcateristics And Their Provenance Implication Of The Silt Fraction From Aggradation Red Earth

In recent years,the research on the genesis of accretion type laterites shows that this type of laterites of aeolian deposits.Provenance consistency is one of the basic tasks in the study of accretionary laterites.In-depth exploration of this issue can not only provide data for the depositional environment of accretionary laterites,but also is of great si gnificance for further understandin g the evolution of the paleoenvironment in southern China durin g the Quaternary.Previous research work on a g gravated laterite provenance mainly focused on whole-rock samples or<20μm grain size,and did not remove the interference of weatherin g si gnals on its provenance identification.In order to weaken the influence of weatherin g on the identification of provenance,and the sand（>63μm）content in the a g glomerated laterite samples is extremely low,the research group carried out geochemical provenance tracin g work on the 4～63μm components of the a g glomerated laterite.Support the conclusion that near-and far-source dust mixed sources.However,the provenance study of the U-Pb a ge spectrum of heavy minerals and zircon in the same profile does not support the contribution of distant dust in the accretionary laterite,which may be related to the particle size of heavy minerals and zircon>20μm.So will the signal of distant dust be related to the 4～20μm component?This is a question worthy of further discussion in the research on the provenance of a g glomerative laterite.In this paper,the Haihuizhen section L-HH of Jiujian g city is selected,and based on the element geochemical provenance tracin gmethod,the provenance information of the components of the accretionary laterite silt sand is discussed in detail,and the provenance information of the 4-20μm components is further clarified,which provides effective evidence for the interpretation of the provenance of the accretionary laterite and the re-understandin g of the Quaternary dust accumulation environment in southern China and its possible drivin gmechanism.The followin gconclusions are initially drawn:（1）The L-HH profile has a good aeolian similarity with the northern loess.The grain size composition of agglomerated laterite is dominated by silt 4～63μm components;the proportion of clay-grade weathering products is relatively large,and the effect of weatherin g to form soil is significant;the content of sand（>63μm）is extremely low,ranging from0.65%to 4.58%The content of silt 4～63μm gradually increased from bottom to top in the profile,from 70.16%in the reticulated laterite layer to 72.99%in the yellow-brown soil layer,and the content of clay（<4μm）gradually decreased,and the reticulated laterite layer The avera ge particle size 6.94φis finer than the yellow-brown deposits 6.71φ.（2）The geochemical characteristics of the silt components in the L-HH section are similar to those of the whole rock sample,and the major elements are mainly Si O₂,Al₂O₃and Fe₂O₃.However,the content of Si O₂ is hi gher than that of the whole rock sample,and the content of iron and aluminum oxides is si gnificantly lower than that of the whole rock.The remainin g contents are all less than 0.5%;the content of trace elements such as Cu,Zn,Rb,Sr in the silt component is significantly reduced.The CIA value of chemical weathering index decreased from 80.93 for the whole rock to 66.28 for the silt component,and the weathering degree weakened.The silt component samples still inherit the weathering trend of the whole section on the basis of weakening the weathering signal of the aggravated laterite.（3）The provenance tracing results using the northern loess as end-member 1 and the Yan gtze River sediments as end-member 2 show that the provenance index PI_FS value of the silt component in the L-HH section and the provenance index PI_QY value of the whole rock sample.There is a big difference,and the PI_JR value after weakening the weathering effect is similar to the PI_FS value.The provenance information indicating the whole rock sample may not reflect the real provenance signal,and the strong weathering of the aggravated laterite will interfere with it.It is reasonable to use the silt components to exclude the influence of the weathering signal to identify the source of the material.（4）The element geochemistry and provenance discrimination index of the silt components in the L-HH section show that the aggravated laterite provenance is a mixture of near and far materials.The contribution of the northern provenance is mainly concentrated in the 4～20μm fine silt component,of which the PI_XFS value of the yellow-brown soil is 0.37,and the PI_XFS value of the weakly reticulated yellow-brown soil is 0.40,both less than 0.5,which are close to the end-member 1 Northern loess.The 20～63μm coarse silt components are mainly contributed by the near-source,the PI_CFS value of the yellow-brown soil is 0.52,and the PI_CFSvalue of the weakly reticulated yellow-brown soil is 0.54,both greater than 0.5,which are close to the end-member 2 Yangtze River sediments.（5）From the bottom to the top of the L-HH profile,the clay content decreases,the silt content increases,the iron and aluminum oxides decrease,and the weathering pedo genesis weakens.Moreover,from the provenance tracin g results,the contribution of lon g-ran ge materials gradually increases from the profile upward,and this trend may be a response to the gradual stren gthenin g process of the East Asian winter monsoon.