Magmatic Processes in Southwest Japan Volcanism
Summary
Subduction of the Philippine Sea Plate beneath the Eurasian margin in southwest Japan drives a suite of magmatic processes, from dehydration of the slab and mantle‐wedge melting to back-arc extension and crustal assimilation. Magma compositions span alkaline basalts in forearc settings to silicic granodiorites and high-Fe/Mg granitoids in the outer arc. Middle Miocene forearc alkaline magmatism records low-degree partial melting of a deep mantle source with ocean-island basalt signatures, linked to initial subduction of a young oceanic basin. Continued opening of the Japan Sea facilitated upwelling of high-temperature mantle that induced partial melting of continental crust, producing diverse plutonic suites within the accretionary Shimanto Belt. Intrusive bodies such as the Osumi Granodiorite document variable contamination by sedimentary country rock, magma differentiation under evolving stress regimes and distinct Rb–Sr isotopic trends. These processes underpin the formation of rare-metal deposits, inform volcanic hazard assessments and enhance understanding of geothermal potential. Integration of petrological, geochemical and geophysical evidence reveals temporal and spatial interconnections between tectonic rotations, arc-arc collision and magmatic evolution in a globally significant convergent margin.
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Technical terms
Forearc alkaline magmatism: Magma generation in the region between the ocean trench and volcanic arc, enriched in sodium and potassium.
Partial melting: Process in which only a portion of a rock melts, yielding magma with a composition distinct from the source.
Assimilation: Incorporation of surrounding country rock into a magma body, altering its chemistry.
Granodiorite: Coarse-grained intrusive igneous rock similar to granite but with a higher proportion of plagioclase feldspar.
Rb–Sr isochron: Radiometric dating method that uses decay of rubidium-87 to strontium-87 to determine magma crystallisation ages.
References
- Middle Miocene forearc alkaline magmatism in Amami-Oshima Island, central Ryukyu Arc: implications for paleoreconstruction of Shikoku Basin. Earth, Planets and Space (2023).
- Petrogenesis of Granitic Rocks in the Hisakajima Island, Goto Archipelago, Southwestern Japan: A Geochemical Study. Minerals (2021).
- Geochemical Study of the Osumi Granodiorite, Southwestern Japan. Minerals (2024).
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