Porphyritic Magnesian Andesites in the Setouchi Volcanic Belt, SW Japan

Abstract

Porphyritic magnesian andesites (PMAs; SiO 2 ∼55 wt. %; MgO ∼7 wt.%) are found in the Miocene Setouchi volcanic belt, SW Japan. The PMAs are characterized by the presence of plagioclase phenocrysts, whereas the rather aphyric, mantle-derived high-Mg sanukitoid andesites (HMAs) found in the region do not contain such phenocrysts. The following petrographic observations suggest a role of mixing of magmas in producing the PMA magma: (1) reversely zoned pyroxene phenocrysts, both clino- and ortho-pyroxenes, are observed in PMAs; (2) normally zoned clinopyroxene may be in equilibrium with olivine but not with normally zoned orthopyroxene in terms of Fe-Mg partitioning; (3) plagioclase displays a wide compositional range (An 80-45 ) with a bimodal distribution; (4) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in HMA sanukitoids and the other identical to those in basalts, are recognized in terms of Ni-Mg and Cr-Al-Fe 3+ relations, respectively. The above petrographic characteristics may be best explained by the presence of three end-member magmas, namely, Mg-rich basalt and HMA magmas, both having olivine and clinopyroxene phenocrysts and a low-Mg, plagioclase-phyric andesite magma. Major, trace, and isotope compositions of these magmas may also support the magma-mixing origin for PMAs.