ISOTOPIC GRADIENT ALONG THE NEW HEBRIDES ARC (VANUATU, SW PACIFIC) - COLLISION OF THE DENTRECASTEAUX ZONE AND HETEROGENEITY OF MANTLE SOURCES

The recent volcanic activity of the New Hebrides are (Vanuatu) has bee n influenced by the collision with the d'Entrecasteaux Zone, the compo sition of which remains uncertain. New trace elements and isotopic dat a from these lavas allow previous interpretations of compositional gra dients along the are to be qualified. The geochemical compositions of lavas display abrupt changes from calc-alkaline compositions straight in front of the collision zone, to arc-tholeiite on both sides. In con trast, isotopic compositions display a progressive variation related t o less and less enriched sources away from the collision zone, evolvin g toward 'normal' MORE-type sources. Such a trend suggests that two ty pes of contamination have occurred in front of the collision zone, a m agmatic contamination due to the subduction of an older lithosphere (P b-206/Pb-204 trend) and contamination by an 'enriched-type' mantle com ponent (Dupal feature). (C) Academie des sciences/Elsevier, Paris.