Site U1439: Expedition 352

Abstract

The Izu-Bonin-Mariana (IBM) fore arc is believed to carry one of the best records worldwide of processes associated with the initiation of subduction. If current models are correct, subduction began with a period of rapid rollback and sinking of a newly subducting Pacific plate, continued through a transitional period of reorganization, and ended with the stable trench-parallel subduction that we see today. In geological terms, it evolved from seafloor spreading through proto-arc volcanism to normal arc volcanism. Reconstruction of the IBM volcanic stratigraphy is key to understanding and dating this evolutionary sequence. At the base of the volcanic section is a distinctive magma type known as fore-arc basalt (FAB). This FAB is overlain by lavas with compositions that are transitional between FAB and boninite, then by boninite lavas themselves, and finally by members of the tholeiitic and calc-alkaline series typical of normal island arcs. This stratigraphy is speculative, however, having been pieced together from a series of partial sections that are typically a considerable distance apart. Site U1439 provides an important test of the stratigraphy of the middle part of this sequence, namely the period that records the transition from spreading to arc volcanism. It thus records the birth of an island arc. Information on this part of the volcanic stratigraphy is currently limited and dispersed. Whereas FAB from both the Izu-Bonin and Mariana fore arcs typically is dated between 52 and 51 Ma, compositions that are transitional between FAB and boninite have only been dated (~49 Ma) at Deep Sea Drilling Project (DSDP) Site 458 in the Mariana fore arc. Boninite series lavas erupted either above these transitional lavas and/or farther to the west, both on the Bonin Islands and offshore Guam, give ages between ~48 and 43 Ma. These lavas vary from low-calcium to high-calcium boninites, the former being particularly distinct in their high MgO and high (andesitic) SiO2concentrations, low high-field-strength element concentrations (e.g., TiO2 < 0.2), low Sm/Zr, low rare earth element (REE) contents, and U-shaped REE patterns. The basal lavas are considered to be low-pressure remelts of depleted mantle (harzburgite) in response to a slab flux comprising fluids and/or melts from subducted pelagic sediment and oceanic crust. The low-calcium boninites are distinct from most reported high-magnesium andesites (HMA) and “boninite-series” lavas, many of which have chemical compositions that are transitional between boninite and arc basalts. The oldest lavas of this type are ~46 Ma, slightly younger than the oldest low-calcium boninites.