A case of Ampferer-type subduction beneath the New Caledonia arc: Evidence for inefficient subduction of hydrated lithologies into the upper mantle

Abstract

New Caledonia is one of the world’s best-exposed subduction/obduction complexes and is central to understanding the geodynamic evolution of the southwest Pacific region. We present new geochemical and Ar/Ar age dates from the in situ eruptive sequences of the La Conception basaltic-andesite lavas (ca. 29.12 Ma) and correlate the generation of these lavas with the generation of the Saint Louis and Koum/Borindi Massifs (ca. 24 Ma) to provide information on the magmatic processes operating within the mantle wedge over time and to indicate the direction of the subducted slab during the Oligocene. The La Conception basaltic-andesite lavas were emplaced in an arc-to-trench direction to the southwest of New Caledonia due to the partial melting of metasomatized, amphibole-bearing garnet peridotites at the base of mantle wedge (∼112−118 km). However, both the Saint Louis and Koum/Borindi granodiorite massifs were derived from melting within the lower crust of an island arc overlying a mantle wedge. Such temporal and spatial variations of magmatism in New Caledonia are consistent with a northeast subduction zone during the Oligocene. The absence of voluminous arc magmatism related to the Oligocene subduction is a consequence of the low Tp temperatures of the mantle wedge, the old age and the high sinking velocity of the downgoing slab, and the absence of stress within the overriding plate. The Oligocene subduction beneath New Caledonia, therefore, is an ideal example of Ampferer-type subduction, which is an amagmatic closure area due to the inefficient subduction of hydrated lithologies into the convective upper mantle. Once the Paleocene to Miocene contraction in the southwest Pacific region ceased, the subduction west of New Caledonia shut down, the system relaxed, and no more melting occurred during the Oligocene.