Middle Eocene boninites were simultaneously generated over a large region during the early history of the Izu-Bonin-Mariana (IBM) arc. However, widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate at the start of the Middle Eocene. These features are similar to those observed \ud in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the Philippine Sea Plate. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present-day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume-track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day
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