Volcaniclastic sandstones record the influence of subducted Pacific MORB on magmatism at the early Izu-Bonin arc

The remnant rear-arc segment of the early Izu-Bonin arc, known as Kyushu-Palau Ridge (KPR), is a key location where magmatic outputs can be constrained during lifetime an island arc. We present new geochemical data for coarse-grained basaltic to andesitic volcaniclastic sandstones derived from KPR and deposited in Amami Sankaku Basin (IODP Site U1438, Unit III rocks) time period 40–30 Ma. Bulk disaggregated cleaned at U1438 retain primary signatures used infer evolution sources juvenile arc through time. A sharp increase slab-derived components source magmatism developed about 35 Ma, indicated by increasing Th/La decreasing Sm/La, Yb/Hf Nb/Nd. Systematic variations trace element ratios abundances younger samples Ma window are decoupled Hf-Nd isotope ratios, which measurably more depleted (?Hf = 16.5–15, ?Nd 9.6–8.2) than boninites produced preceding phase sampled modern forearc. Hafnium isotopic compositions remain little-changed similar subducting Pacific Plate after 40 do not revert highly radiogenic Indian-type MORB mantle wedge reflected highly-depleted basalts forearc (commonly ?Hf ? 18.0). overall pattern recorded indicates that Pacific-type slab-melt component, was boninite magmatism, persisted while subducted sediment component lost or significantly reduced. Variations (at constant near-constant ?Hf) high field strength Izu Bonin controlled addition melt supercritical fluid wedge. (slab melt) thus throughout life currently active arc-backarc system.