In the New Hebrides, an Eastern and a Western Belt of mainly Neogene volcanic and sedimentary rocks are bisected by a longer chain of active and recently extinct volcanoes consisting of primary and reworked volcanic rocks.
The Miocene volcanic rocks of the Western Belt are predominantly andesitic, though they range in composition from high-alumina basalt to dacite. Basalt and hornblende-bearing andesite are cut by intrusions of microgabbro, hornblende microdiorite, and gabbro-diorite. The dominantly calc-alkaline andesitic rocks of the Western Belt possibly originated through partial melting of the mantle in the Benioff zone.
The Miocene volcanic rocks of the Eastern Belt are more tholeiitic and basic than those of the Western Belt. Hornblende is absent and orthopyroxene is only present in andesite, which is mainly restricted to Pentecost Island. Tectonically remobilized serpentinite on Pentecost is the only ultrabasic plutonic rock found in the New Hebrides. Small gabbro, norite, and die rite intrusions are also present. Primitive tholeiite of the Eastern Belt probably originated at a shallower depth in the Benioff zone than the calc-alkaline volcanic rocks of the Western Belt.
The volcanic rocks of the Central Chain include an older sequence of upper Miocene(?) or Pliocene silica- and alkali-rich submarine pyroclastics.
Polarity reversal during this period might explain a subsequent switch to tholeiitic magma during the latest volcanic cycle through Quaternary into Holocene time. Associated derivatives, which occur in smaller quantities and range in composition from ultramafic cumulates to dacite, were produced by crystal fractionation from the parent magma. Though this process gave rise to temporal compositional variation at individual volcanic centers, there is no conclusive evidence of lateral spatial compositional zoning across the narrow active arc that might reflect derivation of magma from different depths in the Benioff zone.
The active volcanoes within the northern interarc basin are younger than those in the south of the chain; they are predominantly or exclusively basaltic and often produced some mildly alkaline basic rocks. These features, together with a high heat flow and bathymetric evidence, are consistent with recent concepts on the formation of such expanding basins by crustal extension with addition of fresh oceanic crust along the active volcanic axis.