Petrology and geochemistry of basalts from norfolk island

The basalts of Norfolk Island were erupted in three main episodes from 3.0 to 2.3 m.y. ago. Both tholeiites and alkali olivine basalts occur, and subordinate, more evolved rocks range into tholeiitic andesites and hawalites. Chemically the rocks show no regular change in composition with time, over the 0.7 m.y. eruptive period, nor any compositional features peculiar to one period of eruption. They are characterized by high Na₂o, moderately high Al₂O₃, uniform TiO₂, and high K/Rb ratios. These chemical features are similar to Pliocene basalts in Northland, New Zealand, and southwest Victoria, but are distinctly different from other basalts in weil documented oceanic and continental provinces. The Norfolk Island rocks plot in a unique field on a Ti-Zr-Y diagram. This is possibly attributable to the unique tectonic setting of Norfolk Island, on a ridge 20 km thick, about midway between the Australian continent and the active Tonga-Kermadee trench-arc system.