The geology and petrology of Niuafo'ou Island, Tonga: subaerial volcanism in an active back-arc basin

Niuafo'ou is the northern-most outlier of the Kingdom of Tonga, located in the northern part of the Lau Basin. Consisting of a broad lava shield capped by the remnants of a composite cone, with the highest being point 213m.a.s.1. The lower slopes which dip at an average of 5° while the upper slopes dip at an average of 14°. The island is almost circular, measuring 8x8.5kms, the centre being occupied by a caldera 14km*.Niuafo'ou is the most active of the Tongan volcanoes with at least 10 reported explosive or effusive eruptions since 1800. Explosive eruptions have produced several cinder/tuff cone complexes within the caldera. The 1886 eruption was a particularly violent event, ejecting a tephra column height of 2.3kms. An extensive tephra layer, with a maximum thickness of 6metres, was deposited along a westerly oriented dispersal axis. Both airfall and surge-like deposits have been recognized. Physical characteristics and pyroclast morphology suggest a phreatomagmatic mechanism. More frequent Hawaiian-type effusive eruptions have produced extensive lava fields on the lower flanks of the island with eruptions in 1853 and 1929 destroying the villages of 'Ahau and Futu. The 1946 eruption was the most significant in the island's recent history. Although a minor effusive event, it resulted in the destruction of the Government village of Angaha and caused the complete evacuation of the island. Niuafo'ou was not permanently resettled until 1958. The most recent period of volcano-seismic activity occurred on 21-22 March 1985 and produced a "crack" on the northeast flank and a small pumice raft on the caldera lake. Few lives have been lost during recent eruptions. The 1853 eruption killed an unknown number in 'Ahau and the 1886 eruption may have in­ directly resulted in the deaths of a small number of natives. Niuafo'ou is considered to be a potentially dangerous volcano and hazard zones of relative risk from future lava flows and tephra fall have been constructed.Stratigraphic relationships indicate that the evolution of Niuafo'ou has involved four main phases. Following an initial Submarine phase, a Pre-caldera phase occurred, initially with effusive activity to form the basal lava shield, the generally massive lava flows being exposed at the base of the caldera walls. Period of alternating effusive/explosive activity followed to form the interbedded tephra and lava layers of the composite cone, exposed in the upper caldera walls. A Caldera-forming phase, involving summit collapse preceeded the recent Post-caldera phase of intra-caldera explosive and extra-caldera effusive activity. The basalts exposed on Niuafo'ou are olivine/quartz normative tholeiitic basalts showing similarities to MORB. They are porphyrtic in plagioclase, olivine and clinopyroxene. Glomeroporphyritic aggregates of the same mineral phases are also present. Geochemically, the rocks are quite distinctive and like the Lau Basin basalts the major element chemistry confirms the MORB-like character. Olivine appears to have been the dominant frationating phase at low pressure. However, trace element and isotope characteristics indicate that the petrogenesis is more complex. The basalts are enriched in LIL and LREE and have elevated *7Sr/"Sr ratios relative to N-MORB. I<3Nd/I4*Nd ratios are bimodal, Pre- caldera ratios are low(<0.5129), while Post-caldera ratios are high(>0.513) being more typical of N-MORB. Slab-derived fluids or altered oceanic crust are not expected to make a direct contribution to the Niuafo'ou melts. High 207Pb/204Pb and 20*Pb/20'Pb relative to 20*Pb/204Pb suggest the involvement of an OIB-related component probably derived from the nearby Samoan source. A peterogenetic model for the Niuafo'ou basalts would thus invoke the presence of a heterogeneous mantle. Following partial melting(20-30%) of a N-MORB peridotitic source, an OIB-related component enriched in LIL/LREE, Sr and Pb has contam­inated the characteristics of the melt. However, it appears that the effect of the OIB component has decreased with time with the Post-caldera basalts showing Nd isotope characteristics consistent with a greater influence of an N-MORB component. Furthermore, the isotope character­istics suggest that the "Dupal anomaly" may also extend to the region of Niuafo'ou.