9:15 AM - 9:30 AM
★ [SIT05-06] Overview of IODP Expedition 352 - Testing subduction initiation and ophiolite models by drilling the outer IBM fore-arcs
Keywords:IODP, Fore-arc basalt, boninite, subduction zone, ophiolite
Two sites (U1440 and U1441) located nearer to the trench, recovered igneous rocks at the basement that are mostly fore-arc basalts (FABs) manifest as pillow lavas, sheet flows and hyaloclastites. At the lowermost part of Hole U1440B, FABs are overlain by dolerites, which are interpreted as feeder dikes for the upper FAB lava units. Compositions of FABs are similar to those of mid oceanic ridge basalts, and exhibit little evidence of subduction influence.
From the two sites (U1439 and U1442) located ~15 km west from U1440 and U1441, pillow lavas, massive lavas, hyaloclastites and pyroclastic flow deposits of boninite were recovered. Boninites with doleritic texture were also recovered from the lowermost part of Hole U1439C, which may represent a dike complex. No FAB was found beneath boninite in these sites. Boninites are chemically distinct from FAB by virtue of higher SiO2, MgO and K2O and lower TiO2. These chemical criteria dictate that boninites formed by partial melting of a more depleted mantle source enriched in slab-derived subduction components.
The presence of feeder dikes at the bases of FAB and boninite holes (U1440B and U1439C, respectively) indicates that the occurrence of boninitic and FAB lavas was offset horizontally as opposed to vertically. At a relatively early stage of subduction formation, conditions of magma genesis changed drastically from predominantly decompression melting (formation of FAB) to flux melting (formation of boninite).