Nature and record of igneous activity in the Tonga arc, SW Pacific, deduced from the phase chemistry of derived detrital grains

Sedimentary rocks in dredge samples from the Tonga arc, SW Pacific, are simple two-component admixtures of volcanogenic detritus and bioclastic material. The volcanic detritus consists largely of lithic and plagioclase grains, but also includes, in decreasing order of abundance, significant minor amounts of clinopyroxene, orthopyroxene and pigeonite. In addition, amphibole, olivine and opaque oxide minerals are accessory phases and trace amounts of monocrystalline quartz occur in a few samples. Plagioclase grains have high An and low Or contents and fall largely in the bytownite field. Pyroxene grains show progressive Fe enrichment and low Ti contents. Volcanic glasses also show Fe enrichment and have low K ₂O contents. These features indicate derivation of the volcanic detritus from a low-K tholeiite source. The uniform compositional range and Oligocene to Holocene age range of the detritus is consistent with their derivation from the adjacent Tongan arc, and indicate that Pliocene rifting of the arc and associated back-arc basin formation did not affect the composition and affinities of the magma source.