Lithosphere Erosion and Crustal Growth in Subduction Zones: Insights from Initiation of the Nascent East Philippine Arc

The Philippine Trench marks a nascent plate margin where subduction initiation is propagating from north to south. Magma compositions in the East Philippine Arc record thinning of arc lithosphere as it is eroded from below. The southern part of the arc is dominated by a continuous spectrum of compositions from high-Mg andesite to adakitic rhyolite. Isotopic ratios and most incompatible trace element ratios are indistinguishable from contemporaneous calc-alkaline magmatism. These similarities point to derivation from similar sources i.e. metasomatised mantle wedge. Rare earth elements, notably Dy/Yb ratios, demonstrate that high-Mg andesitic and adakitic compositions developed through differentiation of hydrous basaltic magma at more than 30km depth, where garnet is stable. In the Philippines, where arc crust is thin, this represents sub-Moho depths. This is interpreted as the result of relatively thick lithosphere beneath the younger, southern part of the arc causing basaltic magma to stall and fractionate garnet at high pressure. In the mature, northern section basaltic magma differentiates at shallower levels, at pressures where garnet is not stable. The northern, more mature part of the arc has produced lavas that are dominantly calc-alkaline. This reflects the more typical architecture of active arcs where the lithosphere has been thinned from below by ablation. Rare occurrences of rocks with mildly adakitic chemistry indicate that local variations in lithosphere thickness exist and that lithospheric thinning is rapid and may be piecemeal. Fluctuations in arc lithosphere thickness throughout the history of this margin appear to control spatial and temporal variations in magma fluxes into the arc crust. Varying fractionation depths of hydrous basalt may help explain the andesitic composition of bulk continental crust.