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

Abstract

Petrographical and geochemical characteristics of calc-alkaline McLennan, 1995) that are produced at convergent plate boundaries and are characterized by lower FeO∗/MgO andesites on Shodo-Shima Island, SW Japan, having bulk comratios than tholeiitic andesites (Miyashiro, 1974). An positions largely identical to the continental crust, are presented. understanding of the origin of such orogenic, calc-alkaline The following petrographic observations suggest a role for magma andesites is thus essential for comprehending not only mixing in producing such andesite magmas: (1) two types of olivine the magma flux in the modern Earth but also the role phenocrysts and spinel inclusions, one with compositions identical of subduction-related magmatism in the evolution of the to those in high-Mg andesites and the other identical to those in solid Earth. Since the pioneering work of Eichelberger basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe (1975, 1978), Anderson (1976), Sakuyama (1979, 1981, relations, respectively; (2) the presence of orthopyroxene phenocrysts 1984), and Luhr & Carmichael (1980), the majority of with mg-number >90 suggests the contribution of an orthopyroxenepetrologists have considered mixing of mafic and felsic bearing high-Mg andesite magma to production of calc-alkaline magmas to be one of the major mechanisms of generation andesites; (3) reversely zoned pyroxene phenocrysts may not be in of calc-alkaline andesites. Continental crust formation, equilibrium with Mg-rich olivine, suggesting the involvement of a however, may not be elucidated solely by such magma differentiated andesite magma as an endmember component; (4) the mixing processes. The reasons for this are twofold. First, presence of very Fe-rich orthopyroxene phenocrysts indicates the calc-alkaline andesite magmatism typifies continental arcs association of an orthopyroxene-bearing rhyolitic magma. Conat least on the modern Earth, suggesting that the contributions from the above at least five endmember magmas to the calctinental crust itself plays an important role in such andesite alkaline andesite genesis can also provide a reasonable explanation of formation, possibly as the source of the felsic endmember the Pb–Sr–Nd isotope compositions of such andesites. component. Second, there is a slight but significant difference in MgO contents between the bulk continental crust and the typical calc-alkaline andesite (e.g. Kelemen,