IMPLICATIONS OF BASEMENT ROCK GEOCHEMISTRY AT IODP SITE U1438 FOR INITIATION AND EARLY GROWTH OF THE IBM ARC (Invited Presentation)

Drilling at Site U1438, west of Kyushu-Palau Ridge (KPR), the site of IBM subduction initiation, penetrated 1460 m of volcaniclastic sedimentary rock over 150 m of basement. Biostratigraphic controls indicate a probable age for the oldest sedimentary rocks at approximately 55 Ma (Arculus et al., 2015 – Nat Geosci). This is close to the 48-52 Ma time period of IBM subduction initiation based on studies in the forearc where the first products of IBM volcanism are tholeiitic basalts termed FAB (forearc basalt – Reagan et al., 2010 - GCubed). Hf-Nd isotopes for Site U1438 basement show a range of compositions from ε_Nd of 7.0 to 9.5 and ε_Hf of 14.5 to 19.8 (present-day values). The data define a well-correlated and steep array in Hf-Nd isotope space with relatively radiogenic Hf, indicating an Indian-type MORB source. The dominant signature, however, with ε_Hf >16.5, is more radiogenic than most Indian MORB. The combined new data indicate that the mantle source of basement basalts in all of the drill sites west of the KPR (1438, 1201, 447) are closely similar to FAB, including new results for northern IBM forearc sites (DeBari et al., 1999 – Earth Planet Sc Lett). As a group, these rocks are more depleted than more than 90% of global MORB with respect to key trace element ratios such as Lu/Hf and Nd/Lu. Because of their uniquely depleted character and close association near the KPR during the Eocene, it is impossible to avoid the conclusion that the FABs and all of the drill site basement rocks were derived from the same distinctive mantle source. This source was sampled widely around the time of IBM initiation, and continues to be sampled today by modern IBM volcanism. These results indicate that initial products of IBM volcanism (the protoarc) were formed in an extensional setting over a broad area and above a subducting plate immediately prior to, and contemporaneous with, initial construction of the KPR. In this model for subduction zone initiation, the foundation of the arc is formed in the first stage of arc magmatism, so the arc is not constructed upon pre-existing oceanic lithosphere.