Geochemistry vs. seismo-tectonics along the volcanic New Hebrides Central Chain (Southwest Pacific)

Compositional variations (major- and trace-element) of Pleistocene to Present rocks along the volcanic New Hebrides Central Chain (Southwest Pacific) are presented and discussed, using a new set of analyses. We focus in particular on the anomalous Mg-, K-, LILE- and LREE-rich and Al and Si-poor character of the basalts from Aoba and Santa Maria volcanoes (and, to a lesser extent, from Ambrym volcano), all located in the area where the D'Entrecasteaux Zone collides with the arc. In addition, we note a compositional transition from 'normal' arc basalts to boninite-related high-Mg andesites at the southern termination of the NHCC. The peculiar composition of the rocks of the volcanoes facing the D'Entrecasteaux Zone has traditionally been related, more or less directly, to the collision-subduction occurring in this area. We present an alternative hypothesis, of a strong geochmeical anomaly under Santa Maria and Aoba volcanoes, unrelated to this collision. In this model, a westward and upward invasion of DUPAL-type enriched mantle, corroborated by the pattern of intermediate seismicity, occurs under this part of the arc, in strong contrast to the usual Pacific-type MORB mantle source present under all other volcanoes of the NHCC. Such a DUPAL-type source may explain the anomalous geochemistry of the resulting volcanic products. (Résumé d'auteur)