Geochemistry of volcaniclastic layers of the Lau Basin and Tonga Platform@en

A series of six holes in the Lau Basin and two in the Tonga Platform were drilled during Leg 135 of the Ocean Drilling Program. The volcaniclastic sands found within the sedimentary cover are typically dominated by fragments of dacite glass, although basaltic andesite glass, calcic plagioclase, and ortho- and clinopyroxene grains are also identified. Total silica contents of individual glass grains indicate a bimodal spread of values at sites close to the Eastern Lau Spreading Center (ELSC), whereas those adjacent to the remnant arc (Lau Ridge) showed a complete spread of compositions. Analyses from the Tonga Platform (Site 840) indicate a prerift phase of arc volcanism (7.0-5.0 Ma) when a complete spectrum of silica values was erupted before backarc basin rifting occurred and the locus of volcanism switched to intrabasinal seamounts producing very similar island-arc tholeiites and their differentiates. […]