Scott, James M.3; Hodgkinson, A.3; Palin, J.M.3; Waight, Tod Earle4; van der Meer, Quinten4; Cooper, A.F.3
1 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet2 University of Otago3 University of Otago4 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet
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.
Contributions To Mineralogy and Petrology, 2014, Vol 167