Dredged igneous rocks from the northern termination of the Tofua magmatic arc, Tonga and adjacent Lau Basin

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Abstract
Fresh igneous rocks were dredged from six stations during the 1984 cruise of the Natsushima to the North Tonga Ridge and Lau Basin. Samples were recovered from inner trench wall, forearc, arc ridge and backarc basin settings in water depths >2000 m. The volcanics are unlike those of the subaerial volcanoes of the Tofua magmatic arc, and provide a glimpse of the petrological diversity present in the submarine levels of this intra‐oceanic arc‐backarc system. Amongst the dredged lavas are unusual olivine‐ and pyroxene‐bearing vitrophyres with high‐MgO (13–20 wt%) and SiO2 (52–56 wt%) contents. These primitive lavas are distinct petrographically and geochemically from boninites in their high CaO/Al2O3ratios (0.7–0.9), high‐CaO (>7 wt%) contents, and the presence of abundant olivine phenocrysts, but are similar to low‐Ti ophiolitic basalts such as those of the Upper Pillow Lavas of the Troodos ophiolite, Cyprus. The dredged Tongan high‐Mg lavas are therefore an in situ occurrence of a low‐Ti ophiolitic lava suite, and support an intra‐oceanic island arc origin for the Troodos and other ophiolites with these lavas.