HIGH-MG ANDESITES IN THE SETOUCHI VOLCANIC BELT, SOUTHWESTERN JAPAN: Analogy to Archean Magmatism and Continental Crust Formation?

The occurrence of unusual high-Mg andesite (HMA) characterizes the Setouchi volcanic belt in SW Japan, which was activated at 13.7±1.0 Ma by subduction of the young and hot Shikoku Basin lithosphere into the high-temperature upper mantle. This tectonic setting may be analogous to the thermal regime during Archean times, which suggests more ubiquitous production of HMA. A plausible process that can comprehensively account for the petrological and geochemical characteristics of Setouchi HMAs involves partial melting of subducting lithosphere, subsequent melt-mantle interactions, and final equilibration with the upper-most mantle. HMAs and more differentiated andesites, which are coined sanukitoids, are distinct in that they are phenocryst-poor (<10%), compact, and nearly anhydrous, despite HMA magmas originally containing ∼7 wt% H2O, and commonly form composite lava flows. One mechanism for explaining these features is formation of a mostly solidified HMA pluton, remelting of the HMA pluton by intrusion of a high-temperature basaltic magma, consequent production of a nearly dry HMA magma, and mixing of this HMA magma with overlying residual felsic melts during ascent to form a zoned magma reservoir.