Veined lithosphere supplies incipient magmatism in the southern Taupō intra-arc rift, New Zealand: insights from melt inclusions in primitive olivine antecrysts

Abstract

Arc magmatism is generally attributed to flux melting of the sub-arc mantle previously modified by metasomatism, induced by subduction-derived aqueous fluids and silicate melts. Primitive magmas in supra-subduction settings rarely preserve direct evidence for metasomatic mantle lithologies because such lithologies are typically fusible and melt preferentially at low degrees, producing only small volumes of melt that are readily diluted by mixing with larger volumes of melt generated by subsequent, higher-degree melting of the ambient peridotite. Here, we report a compositionally distinctive suite of melt inclusions hosted in exceptionally magnesian olivine antecrysts (up to Fo94.8) from basaltic andesites of Pukeonake volcano, at the incipient rifting southern tip of the Taupō Volcanic Zone (TVZ), New Zealand. These inclusions record a continuous compositional spectrum, from silica-undersaturated melts with low SiO2 (down to 41.0 wt%), high CaO (up to 18.5 wt%), and high Cl (up to 1.3 wt%) to andesitic compositions (up to 57.9 wt% SiO2). The silica-undersaturated end-member cannot be explained by melting of lherzolite or garnet pyroxenite, but instead, based on major- and trace-element systematics and partial melting models, points to a source enriched in clinopyroxene and amphibole. Highly elevated Ni concentrations (up to 6085 μg/g) and high Fe/Mn ratios (up to 86) in the host olivine antecrysts further suggest derivation from an amphibole-rich clinopyroxenite source. Such lithologies may have formed as metasomatic veins within the lithospheric mantle through reaction and crystallisation of hydrous silicate melts with peridotite, and were subsequently sampled and fluxed by subduction-derived saline fluids during incipient rifting in the southern TVZ, suggesting that a veined lithospheric mantle contributes to the generation of volumetrically minor yet widespread CaO-rich arc magmas. These results demonstrate that magmatism during incipient rifting and rift propagation provides a natural window into cryptic mantle heterogeneity and that antecryst-hosted melt inclusions can archive otherwise inaccessible stages of arc magma generation.