Postsubduction ocean island alkali basalts in Fiji

The character of alkalic volcanism in Fiji changed at about 3 Ma when the mode of formation of the adjacent Lau back arc basin shifted from rifting to active spreading, leaving Fiji as a remnant arc. The geochemical change occurred throughout Fiji within 500,000 years of the age of the oldest magnetic anomaly in the Lau Basin. The alkalic volcanism changed from shoshonitic during the rifting stage to ocean island basaltic (OIB) during the spreading stage. The Fijian OIBs have Nb/La ratios >1.0 and are divided into two types on the basis of the enrichment levels and ratios of incompatible elements. The less enriched type generally is older and has Dupal isotopic traits. The Fijian OIBs may reflect the introduction of sub‐Pacific mantle from Samoa beneath Fiji and the northern Lau and North Fiji basins since breakup of the arc.