Ultramafic and Mafic Xenoliths in Alkali Basalts from Iki Island

Abstract

Mafic-ultramafic xenoliths of alkali basalt from Iki Island were examined to investigate the petrological constitution of the lower crust of southwestern Japan. Mafic and ultramafic rocks as xenoliths from Iki Island can be divided into two groups, I and II, in terms of the texture. Xenoliths of Group I consist of relatively fine-grained granular two-pyroxene gabbro with metamorphic texture. Orthopyroxene is hypersthene with Mg/(Mg+Fe) atomic ratio, 0.60 to 0.70, and clinopyroxene is augite or salite. Plagioclase (An₅₅-An₇₄) is homogeneous and simply twinned. Those of Group II, on the other hand, are the mixtures of various amounts of olivine, clinopyroxene and plagioclase, with subordinate amounts of spinel, magnetite, ilmenite, kaersutite and biotite. Group II xenoliths are relatively coarse-grained and usually still retain igneous textures. Fo content of olivine varies from 67 to 78. Clinopyroxene (augite or salite) is more Mg-rich than that of Group I xenoliths. Plagioclase (An₆₄-An₇₉) is usually zoned and has polysynthetic twins. Plagioclase and olivine often reacted to make spinel-pyroxene symplectite. Two-pyroxene geothermometer denotes the equilibrium temperature of about 1, 000℃ in both groups of xenoliths. Xenoliths of Group I are metamorphic rocks of pyroxene granulite facies, into which basaltic magmas had intruded to have piled mafic and ultramafic rocks of Group II. This petrological constitution of the lower crust may be common to southwestern Japan.