Evolution of the Lord Howe Volcano

<p>Lord Howe Island (31° 33°S, 159° 05' E), located on the western edge of the Lord Howe Rise, is the remnant of a Miocene (McDougall <em>et al</em>, 1981), subaerial shield volcano at the southern end of the Lord Howe Seamount chain. Lavas in the north dip 5° to the northwest and are cut by sub vertical dykes that strike to the southeast. The lavas in the southern part of the island dip at approximately 3° to the south. The petrography and magnetic fabric suggests the magma in the dykes in the north flowed at a low angle to the northwest. The vent, as inferred from the strike of the dykes, and the dips of the lavas, is located in the southern part of the island to the northeast of Mt Lidgbird. At the maximum size, the Lord Howe Volcano had an average radius of 18km at the base some 2km below sea-level and had a subaerial base radius of approximately 8km and reached an elevation of approximately 1km above sea-level. The volcano had a maximum volume of approximately 1000km³, with both the subaqueous pillow cone and the subaerial shield each having a volume of about 65km³ each. The volcanogenic sediment deposited on the flanks of the volcano had a volume of approximately 870km³ that represents a volume of erupted of approximately 400km³, assuming basalt weathers to twice the volume of the volcanogenic sediments (Sample and Karig, 1982). The difference in volume between the basalt that formed the volcanogenic sediments, pillow and shield cones and the total volume of the volcano is due to the constant erosion and eruption of material over the time the volcano was active (approximately 1 Ma). Estimates of eruption rates and from age constraints of the Lord Howe Basalts it is inferred that the volcano took approximately 0.1Ma to construct the pillow cone, and almost 1Ma to construct the subaerial shield cone. Significant erosion has resulted in the present-day island representing approximately 1 % of the volume of the original subaeial shield. Coastal erosion has occurred at more than 11cm per Ma, while subaerial erosion at 20m per Ma. Geochemically the lavas and dykes (Mg# ~70, D.I. ~32) from Lord Howe Island are similar. Tholeiitic and transitional basalts are dominant with some samples from the youngest rocks in the south being more alkaline. Clinopyroxene compositions of En 46:Fs11:Wo44 are calcic augites and are present as phenocrysts in both the lavas, dykes and an ankaramite sample. The anakaramite contains cumulates of olivine (Fo80) and clinopyroxene. The basalts are classified as continental, intra-plate basalts and are comparable to the ocean island basaltic signatures of eastern Australia.</p>