Subduction initiation ophiolites of the SW Pacific II: second-stage melts of boninites, high-Mg andesites, and related rocks

Abstract

ABSTRACT Boninites (BONs), high-Mg andesites (HMAs), and related 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 subsequent to forearc basalt (FAB) during subduction initiation (SI) are investigated to determine whether these are similar to second-stage lavas and dikes of other SI ophiolites and the IBM forearc. SW Pacific second-stage lavas and dikes are mostly high-Si BONs, but range to HMAs with the latter appearing to have preceded the formation of the former. Partial melting was due to flux-melting of a depleted source with the addition of hydrous and LREE-, Sr-, and Zr-enriched sediment melts to the mantle wedge subsequent to extraction of first-formed FAB. New Caledonia second-stage BONs are mostly consistent with ~5–15% partial melting of a source, which had undergone two previous melt extraction events and in New Zealand, second-stage HMA dikes are consistent as the result of 10–20% partial melting of a depleted source having undergone initial FAB extraction followed by a maximum of ~25% partial melting of a source produced subsequent to the first extraction event. Thermobarometry calculations of primitive second-stage BON and HMA lavas and dikes record potential temperatures generally similar to but pressures lower than MORB. Although most PUB ophiolite and both New Zealand HMA samples plot within the field of MORB with TP of about 1280–1350°C and pressures of ~0.8–1.80 GPa, all remaining second-stage BONs and HMA exhibit TP and pressures lower than MORB. A clear demarcation exists between New Caledonia BONs and PUB HMA, the latter of which records much higher TP and lower pressures than the former and range from ~1330 to 1420°C and from 0.7 to 1.40 GPa. These anomalously high temperatures for the PUB may infer a plume-induced SI event.