The Petrology and Geochemistry of Calc-Alkaline Andesites on Shodo-Shima Island, SW Japan

Abstract

Petrographical and geochemical characteristics of calc-alkaline andesites on Shodo-Shima Island, SW Japan, having bulk compositions largely identical to the continental crust, are presented. The following petrographic observations suggest a role for magma mixing in producing such andesite magmas: (1) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in high-Mg andesites and the other identical to those in basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+ relations, respectively; (2) the presence of orthopyroxene phenocrysts with mg-number >90 suggests the contribution of an orthopyroxene-bearing high-Mg andesite magma to production of calc-alkaline andesites; (3) reversely zoned pyroxene phenocrysts may not be in equilibrium with Mg-rich olivine, suggesting the involvement of a differentiated andesite magma as an endmember component; (4) the presence of very Fe-rich orthopyroxene phenocrysts indicates the association of an orthopyroxene-bearing rhyolitic magma. Contributions from the above at least five endmember magmas to the calc-alkaline andesite genesis can also provide a reasonable explanation of the Pb–Sr–Nd isotope compositions of such andesites.