Magmatic Evolution of the Fossil Melanesian Island Arc: Evidence From Lower Miocene Lavas of Malekula Island (Vanuatu)

The subduction zones in the SW Pacific Ocean are some of the most dynamic plate boundaries on Earth with changes in subduction polarity and subduction initiation processes. The New Hebrides island arc formed some 10 million years ago after the Melanesian island arc was abandoned due to the cessation of subduction of the Pacific Plate after collision of oceanic plateaus with the island arc. Parts of the Melanesian island arc occur within the New Hebrides island arc and we show that Miocene volcanic rocks exposed on Malekula island in the New Hebrides island arc consist of island arc tholeiites to alkaline basalts with variable slab contribution. The lava erupted between 22.5 and 16.3 Ma, similar to the Wainimala Group lavas on Fiji representing the eastern portion of the Melanesian island arc. Partial melts from subducted sediments affected a Pacific MORB-type mantle, but we do not find evidence for a slab component derived from the Ontong Java Plateau or for assimilation of continental crust. Thus, no Miocene subduction of the Ontong Java Plateau occurred beneath Malekula and the basement of the island probably does not consist of continental crust rifted from Australia. The Malekula lava succession resembles that of other portions of the Miocene Melanesian island arc between New Britain and Fiji, indicating continuous subduction of the Pacific plate along this arc. The subduction processes in the SW Pacific Ocean changed polarity some 10 million years ago and little is known about the Melanesian island arc that was active before this time. Volcanic rocks of this Melanesian island arc occur on Malekula island in Vanuatu and these have typical compositions of subduction-related magmas. The lava erupted between 22.5 and 16.3 Ma, similar to lavas on Fiji representing the eastern portion of the Melanesian island arc. There is no chemical or isotopic evidence for subduction of the Ontong Java Plateau or for assimilation of continental crust. Thus, the basement of the island probably does not consist of continental crust rifted from Australia, which was suggested for the neighboring island. Variable lavas formed between 22 and 16 Ma in the Melanesian island arc, probably as small eruptions in an extensional basin Magmas reflect mixing of Pacific-type mantle with melt from subducted pelagic sediment, but no assimilation of continental crust Malekula lava resemble other magmatic rocks of the Melanesian island arc and isotopes do not show subduction of Ontong Java Plateau material