Abstract
Although amphibole has long been predicted to be a major component in the mantle source of intraplate volcanic rocks in New Zealand, the physical evidence for this phase, despite inspection of hundreds of peridotite mantle xenoliths from across the country, is limited to a handful of samples in which it is mostly a sparse component. Here, we present a suite of ultra-refractory peridotite xenoliths in an ultramafic lamprophyre from the intraplate Alpine Dike Swarm, New Zealand, that contain abundant MARID-like veins of mica (phlogopite), amphibole (pargasite) and clinopyroxene, with minor to trace rutile, ilmenite, carbonate and apatite. A key feature of inspecting these formerly refractory peridotites (olivine Mg# commonly >92) is that the chemistry of the precipitated metasomatic agent is less diluted than in cases where an invading melt reacts with fertile lithosphere. The bulk compositions of the precipitated veins vary between sub-alkaline basalt and alkaline basanite, with the peridotite wallrock hosting the basanite veins commonly enriched in Fe ± K, Ti and Al. Amphibole in all veins has high Nb, and the 24 Ma age-corrected 87Sr/86Sr values for diopside and amphibole (0.7027 to 0.7031) are indicative of derivation from a depleted mantle free of crustal material, and thus not related to subduction and arc magmatism. The Sr isotope ratios also overlap with the composition of many primitive intraplate magmas across Zealandia. Melt modelling indicates that partial melting of the veins could generate magmas with geochemical aspects of the known intraplate rocks. These rare veined peridotite occurrences therefore confirm that the Zealandia mantle lithosphere contains fusible zones that, if melted, could have played an important role in the source of intraplate basaltic magma.
•Amphibole has long been predicted in the mantle source of intraplate volcanic rocks in New Zealand.•A new set of ultra-refractory peridotite xenoliths contain abundant veins comprising phlogopite, amphibole and clinopyroxene.•Amphibole Nb and 87Sr/86Sr indicate fluid derivation from a depleted mantle unrelated to subduction.•Sr isotope ratios overlap with many primitive intraplate magmas across Zealandia.•Melt models indicates that vein partial melting could generate geochemical aspects of the intraplate magmas.