Basalts from the Efate Island Group, central section of the Vanuatu arc, SW Pacific: geochemistry and petrogenesis

Abstract

Basalts of the Efate Island Group ( less than 0.7 Ma) occur in the central part of the Vanuatu island arc between the normal southern section of the arc and the central part of the arc affected by collision with the Eocene DEntrecasteaux Ridge. These basalts form small cones constructed upon a volcaniclastic apron principally composed of trachydacitic breccias. Temporal and compositional features allow identification of three basalt groups. Subtle compositional differences exist between two of these suites, whereas the third shows pronounced enrichment in K and Rb, with few differences for other elements. These variations are taken to record slight compositional variations in the ambient mantle wedge composition, with the high-K, high-Rb suite deriving from localised zones relatively enriched in phlogopite. Compared with other Vanuatu arc basalts, the Efate basalts show appreciable enrichment in Sr and P2O5. The Pb isotopic characteristics of the Efate basalts are commensurate with the transitional position of this island within the Vanuatu arc, between the southern arc lavas and those erupted in the region affected by DEntrecasteaux Ridge collision. However, 87Sr/86Sr values (0.70387-0.70424) are slightly higher and 143Nd/144Nd values slightly lower than predicted by the spatial change of these values along the arc. These features are taken to indicate derivation of the Efate basalts from a local, unusual mantle source. Two contrasting models are considered to account for the unusual features of the source mantle of these basalts. Model 1 suggests that the high Sr and P2O5 characteristics of the peridotite source of the Efate basalts were produced by subduction of an intraplate volcano similar to ORSTOM seamount, currently subducting immediately northwest of Efate. Recent studies have demonstrated massive P2O5 and Sr enrichments in highly altered formerly glassy seamount lavas and hyaloclastites. Model 2 proposes that the ambient strongly depleted mantle wedge has been locally modified by carbonatite metasomatism. A well-defined tear or gap in the subducted slab beneath the Efate sector of the Vanuatu arc may have provided appropriate conditions for production of an ephemeral carbonatite partial melt from asthenosphere convecting into the hole in the slab. This melt invaded the mantle wedge, stamping enrichment of Sr and P2O5 on a mantle wedge peridotite that had already been metasomatised by fluids derived from sediments on the subducting plate.