Résumé
The Izu-Bonin-Mariana (IBM) forearc is the preeminent location to study subduction initiation and, by inference, the origin of most ophiolites. Subduction in the IBM system began circa 55 Ma on a long-offset fracture zone that juxtaposed crust of significantly different ages. IODP Expedition 352 cored four holes in the forearc near Chichi Jima in order to document the physical, petrologic, and chemical stratigraphy of a nascent subduction zone, and to constrain the tectonic processes involved. Holes U1440 and U1441, drilled closest to the trench, sampled forearc basalt (FAB), which are generally older than associated boninites. U1439 and U1442, drilled farther upslope (stratigraphically upsection), sampled boninite and high-Mg andesite. FAB are characterized by MORB-like compositions overall, with relatively constant Ti, Zr, and Ti/Zr, with the exception of one unit with exceptionally depleted compositions. FAB have lower Ti and Zr, lower Ti/V ratios, and are LREE-depleted relative to MORB, documenting a more refractory source than MORB, while the increase in K and Rb upsection show they formed in a suprasubduction environment. Three types of boninite are present in U1439 and U1442: high silica boninite (HSB), low silica boninite (LSB), and basaltic boninite (BB), as well as high Mg andesites (HMA). HSB, the youngest unit in both drill holes, is underlain by LSB-BB-HMA lavas, which often occur in mixed magma zones. Boninite types are distinguished by their SiO (sub 2) -MgO and TiO (sub 2) -MgO covariations and their Ti/Zr ratios, which increase from HSB through LSB to BB. HSB, LSB and define parallel trends in TiO (sub 2) -MgO space: a low Ti trend represented by LSB and BB, and a lower Ti trend represented by HSB. All of the boninite suite rocks are slightly LREE-rich relative to MORB. LSB and BB have flat REE patterns relative to primitive mantle, whereas HSB are slightly LREE-rich. These trends require distinct source compositions in HSB relative LSB/BB. Like FAB, the boninite suites increase in K (sub 2) O and Rb upsection, suggesting progressive increase in fluid flux over time. The decrease in Ti/Zr from BB to HSB suggests a slab melt component (low Ti, high Zr). These data require a heterogeneous source during subduction initiation, tapping progressively more refractory mantle through time, and showing progressive enrichment in slab components.