The timescales of subduction initiation and subsequent evolution of an oceanic island arc

The Bonin Ridge and trench slope preserves the geological record of subduction initiation and subsequent evolution of the Izu–Bonin–Mariana (IBM) arc. Diving and dredging in this region has revealed a bottom to top stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas, 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This forearc stratigraphy is remarkably similar to that found in other IBM forearc localities and many ophiolites. Zircon U–Pb ages obtained here for gabbros are 51.6–51.7 Ma. The overlying basalts have 40Ar/39Ar ages of 48–52 Ma. A forearc basalt from the Mariana forearc near Guam produced a similar 40Ar/39Ar age of 51.1 Ma. The collective geochronology of igneous rocks from throughout the IBM system now indicates that the first basaltic magmatism at subduction initiation was produced by decompression melting of the mantle and took place at 51–52 Ma. The change to flux melting and boninitic volcanism took 2–4 m.y., and the change to flux melting in counterflowing mantle and “Normal” arc magmatism took 7–8 m.y. This evolution from subduction initiation to arc normalcy occurred nearly simultaneously along the entire length of the IBM subduction system. Mesozoic rocks found in the deep Bonin forearc suggest that the overriding plate at subduction initiation consisted of Mesozoic terranes and subduction preceded the opening of most or all of the Philippine Sea basins. The contemporaneousness of IBM forearc magmatism with the major change in plate motion in Western Pacific at ca. 50 Ma suggests that the two events are intimately linked.