In the southern New Hebrides arc, magmatic and tectonic processes are closely linked. Between 21°S and 22°S, a “normal” broadly arc tholeiitic magmatic suite is essentially similar to those occurring in the main part of the arc, whereas south of 22°S, at the southern termination of the arc, a high-Mg andesite suite appears, with the more mafic endmembers having mineralogical and compositional affinities to high-Ca boninites. During the last 2 Ma, this southern termination propagated southwards and played a continuing role in the transient transform junction between the southern tip of the trench and the N-S spreading axis of the North Fiji Basin. In this region, which has been dominated for several million years by fast transform motions, the low magma production rates in this section of the arc, and the unusual boninite-related affinities of the arc volcanics may be due to a combination of a subducting slab torn by hinge zones, abnormally small depth-to-slab distances beneath volcanoes, and an unusually hot ambient geotherm due to rising diapirs supplying the intersecting back-arc spreading axis. The on-going collision between the Loyalty Ridge and the arc may also contribute to these unusual magmatic characteristics, as presently only a very small amount of convergence occurs along the southernmost segment of the trench.

In such a complex tectonic setting, with a spreading axis propagating into an intra-oceanic arc, and major transform plate motions, generation of high-Ca boninitic magmas occurs by melting of a refractory hydrated mantle at a shallow level under the Hunter Ridge, the necessary extra heat being supplied by the rising diapirs supplying magmas to the intersecting spreading ridge axis. The high-Mg andesite suite probably results via a combination of fractional crystallization from such high-Ca boninitic parental magmas and subsequent magma mixing processes, operative all along the southern termination of the arc.