『Abstract
　In this study, an assessment of the lithogenic concentrations of trace metals in soils and saprolite over basement rock units in Ibadan, SE-Nigeria is presented in respect of bedrock types and geochemical controls on the weathering-associated release of trace metals. Consequently, soil, weathered and fresh rock samples from the Precambrian Basement of SW Nigeria were collected from three different bedrock units within Ibadan metropolis and subjected to mineralogical and geochemical analyses. The analytical results revealed major proportions of oxides in the range of 18-20％ Al2O3, 2-6％ Na2O and 1-6％ K2O for weathered profiles over granite-gneiss and pegmatite units, compared to 2-3％ Al2O3, ＜0.5％ Na2O and ＜1.0％ K2O over schist-quartzite. For the trace elements, weathered profiles on granite-gneiss and schist-quartzite settings exhibit similar enrichment trends (enrichment factor,EF≧1) for most of the trace elements, unlike the pegmatite bedrock. However, enrichments are relatively greater in the top unit compared to the intermediate saprolite unit, especially for Pb, Ni, Zn, Cr, Co, Rb, Sr and Ba, a situation attributed to leaching and redistribution within the weathered profiles through pedogenetic process and percolating groundwater.
　Furthermore, the estimated weathering indices using Ruxton Ratio (RR = {SiO2/Al2O3}) and Chemical Index of Alteration (CIA = 100 {Al2O3/[Al2O3 + CaO + Na2O + K2O]}) revealed RR of 2.9-3.7 and CIA of 54-73％ for granite-gneiss and pegmatite units, implying medium levels of weathering, compared to RR of 30.8-35.5 and CIA of ＞60％ for schist-quartzite units, which suggest weak chemical weathering. Also the estimated high percentage loss, especially for Pb, Rb, Sr, Ba relative to the bedrocks, shows that the trace elements can be mobilized within the weathering profiles even at a low degree of chemical weathering. Such weathering-induced release of trace metals is of environmental significance as natural lithogenic input sources and as background reference for future monitoring of possible human/anthropogenic impacts.

Keywords: Trace metals; soils; saprolite; Lithogenic concentration; Basement complex rocks; Weathering indices; Trace metal mobilization; Nigeria』

1. Introduction
2. Study area
3. Geological setting and soil description
4. Methodology
　4.1. Profile description and sampling
5. Laboratory analyses and data evaluation
6. Results and discussion
　6.1. Major elements
7. Trace metals
8. Chemical ternary plots and weathering indices
9. Trace metal mobility (enrichment and loss)
　9.1. Environmental implications
10. Summary and conclusion
Acknowledgements
References

Fig. 2. Diagramatic sketches of the representative weathered profiles over the different bedrock types in the study area. Fig. 6. A conceptual representations of geo-pedological processes controlling the release of trace metals within a typical weathering profiles under tropical humid climatic conditions (modified after Taylor and Eggleton, 2001). 〔Tijani,M.N., Okunlola,O.A. and Abimbola,A.F.(2006): Lithogenic concentrations of trace metals in soils and saprolites over crystalline basement rocks: A case study from SW Nigeria. Journal of African Earth Sciences, 46, 427-438.から〕

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