Comparative Study And Spatial-Temporal Distribution Of Soil And Rock Geochemical Data From West Qinling-South China Transect

The current large-scale geochemical investigations mainly concentrate on soil samples but rarely involve rock samples.As a matter of fact,to establish the geochemical database of rock samples is important and essential,which would be greatly helpful for discuss the spatial and temporal distribution of various elements in the Earth surface as the following reasons:(1)by comparing the soil data and rock data,we can get a quantitative insight on the geogenic controls of soil geochemical patterns,and then unravel other anthropogenic and natrual infulences;(2)as rocks have relatively continuous time record and large time span,they are ideal objects for study the secular changes of the Earth surface.On account of this,this paper systematically studied both soil and rock samples form a 3800km transect crossing West Qingling to South China.In total 76 elements,81 indicators were precisely analyzed,the concentrations and distributions of multiple elements were discussed combined with some important geochemical indicators,the results were potrayed in the form of geochemical profiles to display the spatial variations in soils and rocks,as well the temporal distributions of multiple elements in fined-grained sedimentary rocks and granites were described.In the spatial distributions,Al2O3,CaO,Na2O,K2O and Sr have significante diffrences in soils and rocks,the concentrations of CaO,Na2O and Sr significantly decrease with the increase of precipitation,while K2O and Al2O3 slightly decrease and increase respectively.The results show that the concentrations and distributions of these elements in soils are closely related with precipitation.Chemical Index of Alteration(CIA)is a robust indicator to reflect the precipitation changes,the significant correlations of CIA and CaO,Na2O and Sr in soil show that the concentrations of these elements in soils can rough reflect the precipitation changes.Nevertheless,the data of CaO in soil are somewhat scatterd with CIA,while the variation trends of CaO in soil and CIA in the lithological units are decoupled.All that are caused by the mass carbonatites along the transect.The enrichment factors of soils relative rocks can largely address this phenomenon.The Ecao are less scatterd with CIA,while the variation trend of Ecao in the lithological units are coupled with that of CIA.All the results indicate that we must consider the influence of bedrocks on soil geochemistry when facing some speciall lithological units,and the enrichment factors may be a more reasonable indicators for this situation.With respect to the relatively immobile elements,La,Th,W and Sn,their geochemical patterns in soils are similar to that in rocks.Meanwhile,the rare earth elements(REE)patterns in soils and bedrocks,and the proportions of La:Th:W:Sn in soils and rocks are pretty similar.Thus it suggests soils can largely inherit and preserve the geochemical features of these elements from original bedrocks.The enrichment factor of W and Sn(Ew and Esn)are homogeneous in most areas of the transect with the exception of extremely high values in W-Sn mineralization districts.The results show that variables such as tectonic enviroment,degree of weathering,precipitation,lithology,and absolute concentration can only make Ew and Esn to fluctuate in a very limited range,only mineralization and mining activities can significantly elevate the Ew and ESn· Thus,the abnormal Ew and ESn can be used as robust indicators to recognize the soil pollution caused by mineralization and mining activities.The temporal distributions of multiple elements in fine-grained sedimentary rocks and granites were also described.Of particular importance is the fine-grained sedimentary rocks,they sufferred from long distance transport so that they can represent a large provenance;meanwhile,W and Sn are relatively stable so that their concentrations in fine-grained sedimentary rocks can reflect the secular changes.The results show that the Cathaysia Block have significantly high contents of W and Sn,and the peak values occur in Neoproterozoic to Ordovician.Thus it indicates the Cathaysia Block has possessed high geochemical background of W and Sn before the Mesozoic mineralization events.The preliminery enrichement of W and Sn in Cathaysia Block are related to the chemical heterogeneity of mantle,intense crustalmovement and highly evolved granitic magma events.