Daniel Coulthard and his colleagues tell us about how the magma source might evolve following SZI:
The origin of crust created along the leading edge of the Philippine Sea Plate after the Pacific Plate began subducting beneath it about 52 million years ago was investigated by analyzing volcanic glasses recovered by drilling the Bonin forearc during an International Ocean Discovery Program (IODP) Expedition. The glasses have not been affected by alteration and thus preserve pristine compositions of the erupted lavas. Two deep water drill sites recovered basalts and rare andesites produced by crystal removal and crust assimilation. The compositions of the basalt glasses suggest that they were produced by depressurization of hot mantle during near-trench seafloor spreading in the presence of minor amounts of water-rich fluid from the newly subducting plate. Younger, more silica-rich lavas termed "Boninites" were recovered from two sites drilled in shallower water. Boninite glass compositions demonstrate that these Boninites were generated when shallow hot mantle was invaded by water-rich melts derived first from the subducting basaltic crust and, then, this crust plus sediment.
Coulthard et al. (2020, G-cubed)