| 研究概要 |
Three projects have been undertaken.The first involved sampling and analysis of ll Ma old basalts of the Matsue Formation,which outcrops within a 5 km radius of Matsue city.Representative samples were analysed for major and trace element oxides,rare earth elements,and ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd.three samples were dated by K/Ar whole rock.Preliminary results show that two basalt types are present,one representing melting of an isotopically depleted mantle source,and the other,melting of an isotopically and trace element enriched source.Most Matsue Formation basalts are mixtures of these two end members.The range in isotope chemistry spans that known for all of southwest Japan.The limited age and extent of the Matsue Formation basalts is interpreted as representing interaction of a small mantle plume with overlying relatively depleted mantle.The second project has involved whole-rock and isotope analysis,with K/Ar age dating of Miocene volcanic rocks from Oki Dozen Island.This approach has revised the volcanic history,showing that the central cone trachyte,and syenite on Oki Dozen evolved together as an early igneous phase,before the eruption of basaltic through to rhyolitic rocks that make up the main volcanic shield.Earlier studies had argued that the central cone trachyte occurred part the way through the shield building phase,and the position of the syenite was unknown.The third project has been to examine the whole-rock and isotope chemistry of Quaternary volca
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