Abstract

Removal from a subduction site has resulted in a significant diminution of andesitic volcanism in Fiji during the past 5 m.y. During this period Fiji became stranded on a remnant arc (Lau-Colville Ridge) by opening of an interarc basin (Lau Basin); simultaneously, Fijian volcanism changed from being predominantly andesitic to predominantly basaltic. The early postsubduction basalts shared similar trace-element and isotopic characteristics with preceding andesites, but later ones are more like ocean-island basalts in minor- and trace-element composition. This may reflect return of subarc mantle to physical and chemical conditions unaffected by subduction.

Frontal arc, interarc basin, and remnant arc volcanic rocks older than 5 m.y. differ in composition, but all have Sr87/Sr86 = 0.7032 to 0.7043. Some have Sr87/Sr86 ratios higher than expected from their Rb/Sr ratios. This contrasts with nearby Samoan basalts on the Pacific plate, which are separated from the previously mentioned provinces by an arc-arc transform fault zone and have more radiogenic Sr, lower concentrations of of K and Rb relative to Na, and higher concentrations of highly charged cations such as Ti, Zr, and light rare-earth elements than do basalts from Fiji having similar normative mineral content. These differences presumably reflect the history and present state of the mantle in the respective areas.

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