STUDYING AN OPHIOLITE IN ITS NATURAL ENVIRONMENT: IODP LEG 352 DRILLING IN THE IZU-BONIN-MARIANA FOREARC (Invited Presentation)

Increasing evidence supports the interpretation that most ophiolites formed during an episode of massive extension and seafloor spreading accompanying the formation of a new subduction zone. Thus, drilling into the submerged crust of forearcs in convergent margins can test and refine both “ophiolite as fossil forearc lithosphere” and subduction initiation hypotheses. Drilling in the Izu-Bonin-Mariana (IBM) fore-arc during IODP Expedition 352 ~28°20’N and DSDP Leg 60 ~18° recovered similar stratigraphic sequences of volcanic rocks. These sequences are like those in many ophiolites, with MORB-like melts early/trench-proximal and arc-like lavas late/trench-distal. IODP 352, in July 30-Sept. 29 2014, cored 1.22 km of igneous basement and 0.46 km of overlying sediment from 4 sites in the forearc east of the Bonin Islands. These cores provided stratigraphically controlled suites of MORB-like fore-arc basalts (FAB) and boninite related to seafloor spreading and earliest arc development above an incipient subduction zone. FAB were recovered at the two bathymetrically-deeper sites (U1440 and U1441; collectively “352-FAB”) and boninites at the two sites (U1439 and U1442; collectively “352-BON”) upslope to the west. The oldest lavas in IBM forearc sections are FAB with ~51.5 Ma ages. Boninites began erupting approximately 2-3 m.y. later and further from the trench. First results from IODP Expedition 352 and preliminary post-cruise data suggest that 352-FAB were generated by decompression melting during near-trench sea-floor spreading, and that fluids from the subducting slab were not significantly involved in their genesis. Geochemical data for the IODP 352 and DSDP 60 reveal that mantle became more depleted and more fluid-enriched though time, as spreading rates waned, the volcanic front retreated from the trench and true subduction began. Ophiolites that preserve the vertical sequence MORB-like at the base and arc-like at the top like that seen in IODP site 352 are almost certainly subduction-initiation-related slabs of forearc lithosphere. We expect that the distribution of such ophiolites with time should reflect episodes of plate reorganization.