New geochemical and age constraints (⁴⁰Ar/³⁹Ar and U-Pb) on forearc intrusive rocks from the New Caledonia Ophiolite (SW Pacific): diversity of melts generated at hot subduction inception

The New Caledonia Ophiolite is cross-cut by coarse-to medium-grained pyroxenite and hornblende gabbro/diorite dykes intruded between 55.5 and 50 Ma (U–Pb zircon and40Ar/39Ar hornblende), whereas the finer grained dolerites of tholeiitic affinity are younger (50–47 Ma). The production of hornblende gabbros/diorites was modelled by moderate degrees (20–40%) of partial melting of the high-temperature amphibolites of the metamorphic sole. The end-member compositions (hornblendites and anorthosites) resulted from solid-state phase segregation of crystal mushes within tectonically active magmatic conduits. Cascade reactions of the slab melts with mantle wedge peridotites successively formed clinoenstatite-bearing boninite magmas, which fed gabbronorite cumulate lenses at the mantle–crust transition; in turn, the clinoenstatite-bearing boninite melts reacted with peridotites to form websterites. The youngest magmas (of tholeiitic affinity) appeared c. 6 Myr after the inception of subduction when the cooler subducting slab plunged more steeply. Incipient slab retreat allowed corner flow, triggering low-pressure hydrous melting of the uplifted asthenosphere. The early stages of forearc magmatism were closely associated with transcurrent shear zones, which recorded oblique subduction inception. The early Eocene tectonic and magmatic features of the New Caledonia Ophiolite provide evidence for a north-or NE-dipping hot (forced) subduction zone in the SW Pacific, notably distinct from the slightly younger west-dipping Izu-Bonin–Marianna cold (spontaneous) subduction system.