Feedback of Slab Distortion on Volcanic Arc Evolution: Geochemical Perspective From Late Cenozoic Volcanism in SW Japan

Southwest Japan is an island arc formed by subduction of the Philippine Sea (PHS) plate. The Quaternary magmatism in this region is characterized by eruptions of high‐Sr andesites and dacites, considered to have been derived by melting of the PHS plate. The loci of these volcanoes spatially coincide with seismic discontinuities of the subducted PHS plate. Thus, the magmatism is interpreted as the result of slab melting at the plate tears. However, the processes that promote slab tearing remain unclear. In this study, we applied geochronological and geochemical analyses to late Cenozoic volcanic rocks in southwest Japan as tracers of slab morphology. Two different magma types, ocean‐island basalt (OIB) and island‐arc basalt (IAB), have occurred over 12 million years (Myr). These two magmas are attributed to different integrations of melts extracted from an originally fertile mantle; the OIBs from high temperature melt (1,300–1,400°C) were extracted at a depth of 40–80 km, whereas the IABs were extracted from a shallower, lower temperature region (30–60 km, 1,200–1,350°C). Secular change in Sr enrichment of IAB likely arose due to a transition of slab‐derived fluids, incorporated into magmas as they formed, from water‐ to melt‐dominant one. Progressive shallowing of the subducted PHS plate is responsible for secular change in the properties of slab‐derived fluids as well as rollback of OIB volcanoes. Production of chemically variable magmas in the Chugoku district is the surface expression of distorting slab morphology by interaction between mantle and the subducting plate. Key Points Late Cenozoic magmatism in the Chugoku district in SW Japan began at 12 Ma and has continued to the present, having produced various types of magmas with mafic and intermediate‐felsic compositions Basaltic magmas in this region were formed by progressive extraction of melts from adiabatically upwelling mantle with various extents of hydration by slab‐derived fluids. The mantle sourced by basaltic magmas younger than 5 Ma was metasomatized by siliceous slab melt similar in composition to high‐Sr andesites and dacites, erupted at 2 Ma to the present, in this region The secular evolution in the composition of slab‐derived fluids is attributed to a change in the subduction angle of the slab, which became flatter over time and was eventually torn apart. Distortion of the subducting slab is due to the collision of topographic prominences within oceanic lithosphere with the trench and