Geochemical mapping in Aichi prefecture, Japan: Its significance as a useful dataset for geological mapping

Since 1994 a geochemical mapping project has been undertaken in Aichi prefecture, central Japan, with the aim of establishing a database to contribute to environmental assessment in the region. To date more than 1200 stream sediments have been collected and analyzed. The samples were analyzed for 10 major elements by X-ray fluorescence spectrometry and loss-on-ignition. The analyses show that stream sediments from areas with sedimentary bedrock are enriched in SiO2 and mafic elements such as TiO2 and Fe2O3 compared to areas with granitic bedrock. The SiO2 enrichment is mainly due to the amount of quartz and the enrichment in mafic elements is due to biotite, both of which are derived from the sedimentary bedrock. In contrast, sediments derived from granitic bedrock are characterized by high abundances of elements contained in feldspars such as Na2O and CaO. This contrast in elemental distribution (e.g., Na2O/TiO2 ratio) can be useful for geological mapping in the areas covered by vegetation and with poor bedrock exposure. The relative abundances of Ca, Na and K in the stream sediments suggest that the Inagawa granite, one of the main granite bodies in the study area, can be classified into four distinct types. This classification is distinct from previous classifications based on geological and petrochemical studies. It is confirmed that analysis of geochemical compositions of stream sediments has considerable potential as a tool in geological studies including lithological mapping.