日本海形成に伴う西南日本の火山岩類の岩石化学-化学組成と同位体組成からの検討

第一个项目是对松江市周围5公里范围内露头的松江组11 Ma老玄武岩进行采样和分析。对具有代表性的样品进行了常量和微量元素氧化物、稀土元素、锶和锶的分析。三个样品用K/Ar全岩测年。初步结果表明，存在两种玄武岩类型，一种代表同位素亏损的地幔来源的熔融，另一种代表同位素和微量元素富集源的熔融。松江组玄武岩大多是这两个端元的混合物。同位素化学的范围与日本西南部已知的范围相同。松江组玄武岩的有限年龄和范围被解释为代表小的地幔热柱与上覆的相对亏损的地幔相互作用。第二个项目涉及全岩和同位素分析。用K/Ar法测定了奥基岛中新世火山岩的K/Ar年龄。这一方法修正了火山史，表明奥基岛的中央锥体粗面岩和正长岩是作为早期火成岩共同演化的，直到玄武岩喷发，直到构成主火山屏蔽体的流纹岩。早期的研究认为，中央锥体粗面岩在屏蔽建造阶段的部分过程中出现，正长岩的位置尚不清楚。第三项工作是研究第四纪火山岩的全岩和同位素化学

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