Early Geochemical Evolution of an Oceanic Island Arc and Backarc: Fiji and the South Fiji Basin

The initial 25 to 35 Ma of oceanic arc volcanism in Fiji (the "early arc stage") was dominated by eruption of LREE-depleted, low-K tholeiitic basalts and andesites. The oldest volcanics include the highest-HFSE group, MORB-like in composition, plus high-Mg basalts and basic andesites with flat REE patterns as low as 4x chondritic concentrations. These erupted either within the volcanic arc during its first 5 Ma or in an adjacent backarc during its first Ma of formation. Subsequent volcanics of the early arc stage apparently erupted onto early backarc basin crust. Most are intermediate between earlier extremes in HFSE contents, but at least one volcano produced strongly LREE-depleted island arc tholeiites. In contrast, medium-K tholeiitic and calc-alkaline andesites erupted during the "mature arc stage." Parental basalts were enriched in LREE but depleted in HREE relative to earlier basalts due to a change in source composition. Counterflowing MORB-source asthenosphere within the mantle wedge dominated volcanism throughout the early arc stage. Reduced counterflow later allowed more enriched mantle sources to play a larger role, especially behind the volcanic front. In contrast, the South Fiji Basin experienced the opposite sense of evolution.