Genesis of Island Arc Magmas by Mantle-Derived Bimodal Magmatism: Evidence from the Shirahama Group, Japan

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Abstract
Volcanic rocks in the Mio-Pliocene Shirahama Group, Japan, resulted primarily from parallel fractional crystallization of basalt and an inferred boninite. Those formed via fractionation from basalt through andesite to dacite are referred to as rocks belonging to the tholeiitic series; those formed via fractionation of assumed boninite are referred to as rocks belonging to the calc-alkaline series. Mineral chemistry of plagioclase and titanomagnetite, plus magmatic temperatures inferred from the two-pyroxene thermometer, show unambiguous differences between the tholeiitic and calc-alkaline series and support a genetic classification based on parallel fractionation models. Many transitional rocks exhibit profound reversed zoning of orthopyroxene phenocrysts and are interpreted as mixed magmas. A model for genesis of are magmas is presented, which is constrained by geological and experimental evidence, as well as comparisons with boninite-tholeiite associations elsewhere. A hydrated, wet peridotite diapir generated above the cool subducting slab ascends and is heated as it penetrates the overlying hot mantle wedge. Temperature and H2O gradients established in the diapir result in simultaneous generation of basalt in hotter and drier regions and boninite in cooler and wetter regions. These two parental melts evolve into a series of tholeiitic and calc-alkaline magmas in higher-level chambers, with or without mixing.

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