Late Cenozoic magmatism in southwest Japan arc is characterized by voluminous eruptions of intermediate to felsic lavas and monogenetic mafic volcanisms. The intermediate to felsic lavas show geochemical characteristics of magmas commonly referred to as “adakite”. This peculiar magma is generally interpreted as partial melts of subducting oceanic crust formed at the depth of the garnet-stability field (> 50 km).
This research presents geochronological and geochemical analyses for the adakites in SW Japan arc. The eruption of adakites had occurred during the last 2 Myrs, concurrently with or following the eruption of mafic lavas in the adjacent (< 10 km) regions. Trace-element characteristics of adakitic lavas are consistent with partial melting of the basaltic layer of the subducting Shikoku Basin Plate. Mass balance modeling of trace-element concentrations and isotopic compositions suggests that the adakitic magmas are best explained by mixing of partial melts from oceanic crust (F = 15%) and sediment (F = 40%) at 90:10 to 60:40 ratios. Numerical modeling predicts that the slab temperature beneath the adakite volcanoes is too low to generate melts from the crustal layer previously extensively dehydrated in the fore-arc region. Instead, the sub-arc mantle temperature, estimated from the associated maﬁc lavas (1280–1420°C) appears to have been suﬃciently high to lead the slab melting. Spatial coincidence of adakite volcanoes and seismic gaps of the subducting slab demonstrates the causal link between slab melting and mantle upwelling at slab tears. This study suggest that these tears could have been formed by subduction of ridges on the plate. A warm mantle upwelled through tears, preventing the solidiﬁcation of the adakite melts in the mantle and facilitating the transportation of these melts to the surface.