Subduction initiation ophiolites of the SW Pacific I: first-formed MORB-like lavas

Data collated from subalkaline lavas and dikes of Palaeogene ophiolites the SW Pacific in Papua, New Guinea, Caledonia, North Island, Zealand interpreted as having formed first upon subduction initiation, are investigated to determine whether these MORB like, similar other initiation IBM forearc. ophiolite mostly tholeiitic basalt; however, calc-alkaline comprise ~ 25% samples. Major element abundances first-formed broadly MORB, however incompatible range MORB-like more depleted than most enriched forearc basalt (FAB). The reason for incompatible-element nature relative FAB may be that sources former, unlike latter, based on recent studies were not subjected a pre-subduction melt extraction event. Partial melting estimates 12–23% an initial source maximum 18% partial this after 20% extraction. was due decompression received little no slab-derived contributions. Thermobarometry calculations primitive with ≥ 8 wt.% MgO Mg# 60 show mantle records potential temperatures pressures 1341–1431°C 1.1–1.8 GPa, MORB. These at least 30–70°C lower those required Central American supporting previous suggesting plume-induced origin America IBM.