Plate tectonics get weird in the western pacific. They get weird in the eastern pacific too. Pretty much everywhere, actually. But the western Pacific has its own type of weird. Double subduction for starters. Nakamura and Iwamori (2013) use the term double subduction to describe where one plate is subducting beneath another subducting plate. In this case beneath the south of Japan, the Pacific plate is subducting from the east, while the Philippine plate is subducting from the southeast and overlaps the Pacific plate (see map below). To add further confusion, the Pacific plate is also subducting beneath the Philippine plate further to the east. As you might guess, this weird configuration results in some weird volcanoes. Nakamura and Iwamori (Nakamura and Iwamori, 2013) investigate a subset of these lavas that contain the controversial adakite geochemical signature. Adakites once described magmas that were derived from melting of the subducting slab, but now they describe any magma that has the composition originally attributed to melts of the subducting slab. In this case, Nakamura and Iwamori (2013) eschew plate melting and argue for more-or-less normal subduction-induced melting of the asthenosphere, but with a heightened fluid flux from the two subducting slabs and deep residual garnet.
Image from figure 1 of Nakamura and Iwamori (2013). Note teeth denote subduction plate boundary with the arrows pointing in the direction of the downgoing slab.
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