Geochemical systematics of spreading ridge-arc collision – Philippine Sea CBF rift and the Izu-Bonin arc

Triple junctions between fundamental tectonic units are where different compositions and magmatic processes interact. We investigated the intersection of a spreading center with an island arc within the Philippine Sea Plate, i.e. an arc-ridge-arc triple junction, using a combination of submersible sampling and shipboard geophysics. We use Ar Ar dating, elemental concentrations and Sr-Nd-Pb radiogenic isotope ratios to constrain the structure, age and origin of magmatism as spreading interacted with the arc. Between 33 and 31 Ma the tip of the West Philippine Basin spreading center was in the back-arc west of the Kyushu-Palau arc. Spreading volcanism had mantle-like Ce/Pb and Th/Ce indicating a lack of slab flux. Radiogenic isotopes form a trajectory towards low 143 Nd/ 144 Nd and high 20x Pb/ 204 Pb, but extend to Δ 208 Pb/ 204 Pb > 85 which corresponds to an EM-2-like mantle component rather than pelagic sediment input from the slab. The final stage of spreading-related activity occurred between 31 and 28.5 Ma when the spreading axis rotated from NW-SE to E -W and propagated eastwards into the arc. It resulted in magmatism contemporaneous with the adjacent frontal arc. This final magmatism extended the radiogenic isotopic trajectories observed in the earlier spreading towards less radiogenic Pb and higher 143 Nd/ 144 Nd, i.e. towards MORB from the EM-2 component. However, during the final stage magmatism La/Sm, Nb/Zr increased while Pb and Sr isotopes became progressively more radiogenic, demonstrating a progressive decrease in the degree of partial melting and preferential tapping of the EM-2 component. This change in melt generation and composition appears a common process at the end of spreading.