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

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