Spinel facies dunite, harzburgite, lherzolite and wehrlite mantle xenoliths from a cluster of Miocene volcanoes in southern New Zealand record evidence for the complex evolution of the underlying mantle lithosphere. Spinel Cr# records melt extraction with some values indicative of near complete removal of clinopyroxene. LREE-enriched, low-Ti/Eu and Al2O3 clinopyroxene and rare F-rich apatite reflect subsequent interaction between peridotite and carbonatite. The clearest metasomatic overprint occurs in the formerly depleted samples because there was little or no pre-existing clinopyroxene to dilute the carbonatite. For the same reason, the isotopic character of the metasomatising agent is best indicated by clinopyroxene in the formerly depleted peridotites (⁸⁷Sr/⁸⁶Sr = 0.7028-0.7031; ¹⁴³Nd/¹⁴⁴Nd = 0.5129; ²⁰⁶Pb/²⁰⁴Pb = 20.2-20.3). These isotope ratios are very close to, but slightly less radiogenic than, the HIMU mantle reservoir. Metasomatism appears to pre-date ubiquitous pyroxene core to rim Al diffusion zoning, which may have resulted from cooling of the lithospheric mantle following cessation of Late Cretaceous-Eocene rifting of Zealandia from Gondwana. Nd isotope data, however, suggest that metasomatism likely occurred within the last several hundred million years. This is significantly younger than ancient Re-depletion ages of ~2 Ga and indicates decoupling of peridotite isotope systems.