Geochemical diversity of chromian spinel bearing alkaline Matsue Basalt in the Shimane Prefecture, southwest Japan

Matsue city located in Japan Sea coast side of southwest Japan arc. There are many alkaline basic- intermediat lavas that we called "gMatsue Basalts"h in Matsue city area. The activity of the Matsue Basalt is about 10 Ma ago. Matsue basalt basically consist of 6 basic lavas and some it"fs pyrocrastic deposite (sandstone and tuff). The pourpose of this study is clarifing geochemical characteristics of Matsue basalt. We here report the Bulk chemical composition, Mineral assemblege and mineral chemistry of Matsue Basalt. This is first report of systematic chemical study about Matsue Basalt. And we found chromian spinels in the many rock samples. Acording to our result of bulk rock chemical compositions, Matsue Basalt are divided into 6 groups, which are 3 basalt groups (Chausuyama, Hanamagari and Agenogi groupes), 2 basaltic andesite groups (Tsuda and Rakuzan groups) and 1 andesite group (Toukodai groupe). However it was clarified that it was a chemical same feature with Tsuda and Rakuzan groups among Matsue Basalt. That is Matsue Basalt is chemically consists of 5 groups. Chausuyama and Agenogi basalts show particularly with respect to their low FeO*/MgO ratios and high Cr content. And these two basalts have chromian spinels in and around olivine grains. Chromian spinels from Chausuyama and Agenogi basalts have Cr# (Cr/(Cr+Al) atomic ratio) of 0.16- 0.40, and olivines from these basalts have Fo of 88-76. Arai (1987, 1994) proposed that Fo-Cr# relations depend on the tectonic setting, due to differing melting conditions (pressure, temprature and water vaper conditions). That is Chausuyama and Agenogi basalts of Matsue Basalt derived from the relativery depleated lerzolitic mantle.