The Izu-Bonin-Mariana (IBM) forearc is characterized by the occurrence of boninite-like lavas. The study of the Cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the Tonga and Fiji regions lead us to define three tectonic settings that favour the formation of boninites in back-arc basins in addition to previous settings that involve the presence of a mantle plume: (1) propagation at low angle between a spreading centre and the associated volcanic

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... arc; (2) intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc; and (3) intersection at a right angle between an active spreading centre and a newly created subduction zone. A geodynamic model of the Philippine Sea Plate shows that boninites in the Bonin Islands are related to the second mechanism mentioned above, whereas Mariana forearc boninites are relevant to the third mechanism. In the early Eocene, the transform plate boundary, bounding the eastern margin of the Philippine Sea Plate at the location of the present-day Mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the West Philippine Basin, somewhere around 43-47 Ma. The presence of a mantle plume in the vicinity of the subduction zone bounding the northern IBM arc explains boninites that erupted in its northern part, but only in early Eocene time. boundary set at CaO/A1203 c. 0.75 (Crawford et aL 1989). Low-Ca boninites are interpreted as being produced from relatively more depleted sources than high-Ca boninites. Variations in CaO/AI203 values and TiO2 content recorded in several boninite suites (e.g.