Tracing the evolution of island-arc volcanism in the Tanna–Futuna transect (New Hebrides)

Abstract:

The New Hebrides island arc, located in the southwestern Pacific, is associated with the fast subduction of the Australian Plate under the North Fiji Basin. It extends over 1500 km including the entire Vanuatu archipelago. Several studies dealing with the geochemistry of the most important islands interpret the chemical variability to originate from the heterogeneities in the sub-arc mantle wedge¹ and variable addition of the subduction component along the arc². In order to trace the differences between the source(s) of New Hebrides volcanic arc and back-arc magmatism, five submarine cones (4 of them aligned NE-SW), located in the Futuna Through, were sampled. The lavas range from basalt to andesite with fractionation of olivine being the main magma evolution mechanism until MgO ≈ 6 wt.%. The most primitive lavas have similar fractionation-corrected TiO₂ (0.90–1.18 wt.%) and Na₂O (2.89–3.41 wt.%) contents suggesting comparable degrees of partial melting. The comparison with published data from adjacent islands shows a more important contribution of the slab closer to the trench (Tanna) where the erupted basalts, basaltic trachyandesites and trachyandesites have considerably higher U/Nb and Ba/Nb ratios. Yet, these lavas display significant negative Sr anomalies (PM-normalized). This could provide evidence of input of continental derived sediments or could reflect the role of plagioclase in the source / evolution of these magmas. The first hypothesis is not supported by published data from the Vanuatu trench³ and the second is not supported by the decoupled behavior of Sr and Eu in normalized-diagrams. On the other hand, island crust samples collected along the northern flank of Futuna Island display strong positive anomalies of Sr and, although more modest, the submarine cones show a similar behavior. Based on source chemical tracers, an increasing depletion of the source is observed from east to west, consistent with progressive mantle flow towards the arc front⁴.

¹Greene et al. (1994) Proceedings ODP Scientific Results 134, 19-46.; ²Peate et al. (1997) J Petrol 38, 1331-1358; ³Planck & Langmuir (1998) Chem Geol 145, 325-394, ; ⁴Pearce & Stern (2006) Geophys. Monograph Series 166, 63-86.