Petrology of the alkali basalts in Mishima Island, Yamaguchi Prefecture, Southwest Japan with special reference to the primitive alkali basalt.

Cenozoic volcanism of Mishima Island in Japan Sea, 50 km off the coast of Yamaguchi Prefecture, is characterized by picritic basalts and associated andesites of alkaline suites and older sub-alkaline basalts. One picritic flow is of cummulative origin: it contains abundant (22-25%) olivine which shows disequilibrium with the host rock. Another flow (15% olivine, Fo=89%), however, shows equilibrium between olivine and host rock, and its pressure was estimated to be near 10 kb from the CMAS diagram. Associated basalts and andesites are not the fractionation products of this primary picritic basalt, because their K2O contents are not as high as those expected by crystal fractionation model.