Geochemical Evidence for Island Arc Subduction Beneath the New Hebrides Island Arc

The tectonic history of the New Hebrides Island Arc (NHIA) is complex and characterized by collisions of oceanic plateaus, subduction interface rotation, as well as fragmentation and ultimately subduction polarity changes. Despite the intricate tecto-magmatic evolution of the NHIA, geochemical data are sparse and the geodynamic processes governing magmatism in the NHIA are poorly understood.

Between 14° and 17° South, along the New Hebrides Trench, collision and subsequent subduction of the d’Entrecasteaux Zone (DEZ) with the NHIA results in various erosional and accretionary processes. The DEZ encompasses the North d’Entrecasteaux Ridge (NDR) and Bougainville Guyot, which represent the immediate interface of the collision zone. In the vicinity of New Caledonia, the DEZ was previously interpreted as a horst-graben system, while the Bougainville Guyot is commonly referred to as part of a southern seamount chain.

Here, we present new geochemical and Sr-Nd-Hf-Pb isotopic data on volcanic rocks from drill sites 831 (Bougainville Guyot), 828 (NDR), 829 (NHIA fore-arc), as well as from the island of Espiritu Santo which formed in the Miocene Melanesian island arc. Drill site and volcanic arc samples differ distinctly in Nd-Hf isotopic records, indicating that fore-arc samples from drill site 829 comprise accreted material from the subducting plate, while the island arc samples exhibit a mantle source consistent with previous arc formation above Indian MORB-like mantle.

In addition, our new data suggest a strong slab-derived fluid influence on the chemical composition of samples from all locations. Relatively radiogenic Sr isotopic records together with negative Nb-Ta anomalies and positive Pb anomalies in samples originating the d’Entrecasteaux Zone, support the model that the DEZ represents a fossil island arc.

We refine the understanding of the tectonic evolution of the NHIA by providing further geochemical constraints on the mantle composition and magma genesis of arc, fore-arc as well as of the subducting DEZ. Isotope and trace element data of the NDR and Bougainville Guyot resemble island arc tholeiites from the Mariana and Kermadec island arcs. Thus, the DEZ probably represents an immature island arc, implying that such magmatically thickened and therefore buoyant structures can be subducted.