Olivine phenocryst origins and mantle magma sources for monogeneticbasalt volcanoes in northern New Zealand from textural, geochemicalandδ18O isotope data

Abstract

Olivine phenocrysts in basalts erupted in the Kaikohe-Bay of Islands volcanicfield (KBIVF), a cluster of Quater-nary monogenetic edifices in northern New Zealand, provide an opportunity to investigate magma source andassembly. The phenocryst population has both cognate and non-cognate origins with respect to the carriermagma. Common large subhedral-euhedral phenocrysts with compositions of Fo80–86are autocrysts, havingnucleated in a melt equivalent of the host whole-rock (~Mg#56–63). A subordinate population of largesubhedral-euhedral phenocrysts that are Mg-rich (NFo86) nucleated in a melt more mafic(NMg#65). These areantecrystic and are likely to have a deep cumulate origin, perhaps the product of fractionation of a primitive pa-rental magma. Smaller anhedral phenocrysts have compositions (~Fo70–75) that would have crystallised in amore evolved melt (~Mg#42–48). These are xenocrysts from stalled and fractionated crystal mush zones or in-trusions in the mid and upper crust. Other small phenocrysts (Fo66–79) are euhedral and reversely zoned withmafic mantle zones (Fo80–85). They also nucleated in a more evolved crustal magma, but subsequently experi-enced magma-mixing with a more mafic magma. The olivine assemblage requires a vertically extensivemagma system through the crust and upper mantle involving deep cognate crystallisation, and recycling ofnon-cognate crystals from deep and shallow intrusive bodies. This in accord with varied origins for the accompa-nying plagioclase and clinopyroxene populations in the rocks demonstrated in previous studies. This study pre-sents thefirstδ18O analyses of olivines from monogenetic volcanoes in New Zealand. SIMS analysis of cognateand cumulate olivine phenocrysts in KBIVF basalts, and those from other volcanoes in northern New Zealand ba-salticfields, show that they are non-zoned (δ18O ± 0.4‰). The inter-crystal range at most volcanoes is narrow(0.5‰), although the olivine population at a few volcanoes are more varied (range up to 1.25‰), requiringmore than one source. The isotopic data for the more northernfields KBIVF (δ18O mean = 5.60‰;range=5.12–6.20‰) and Whangarei volcanicfield (δ18O mean = 5.50‰; range = 4.98–6.12‰) include values higherthanthat ofolivine from MORB-HIMU sources,and overlap those associatedwithEMI/EMII sourcesorsubductionzone magmas. Whole-rock trace element and Pb-isotope data from thesefields have been interpreted as the re-sult of subduction-induced contamination of the mantle wedge, a relic signal from now extinct Miocene-Pliocenearc volcanism. The magma compositions are also consistent with source depths above the garnet facies zone. Thiscould explain the higher olivineδ18O values. In contrast, olivines from more southernfields do not display this O-isotope signature, consistent with postulated greater source depths.