Ancient melt depletion overprinted by young carbonatitic metasomatism in the New Zealand lithospheric mantle |
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| Authors: | Email author" target="_blank">J?M?ScottEmail author A?Hodgkinson J?M?Palin T?E?Waight Q?H?A?Van der?Meer A?F?Cooper |
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| Institution: | 1.Department of Geology,University of Otago,Dunedin,New Zealand;2.Department of Geosciences and Natural Resource Management, Geology Section,Copenhagen University,Copenhagen,Denmark |
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| Abstract: | Spinel facies dunite, harzburgite, lherzolite and wehrlite mantle xenoliths from a cluster of Miocene volcanoes in southern New Zealand preserve evidence of the complex evolution of the underlying continental mantle lithosphere. Spinel Cr# records melt extraction with some values indicative of near complete removal of clinopyroxene. LREE-enriched, low Ti/Eu and low Al2O3 clinopyroxene and rare F-, LREE-rich apatite indicates subsequent interaction between peridotite and a metasomatising carbonatitic melt. The clearest metasomatic signature occurs in the formerly highly depleted samples because there was little or no pre-existing clinopyroxene to dilute the carbonatite signature. For the same reason, the isotopic character of the metasomatising agent is best observed in the formerly highly 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 end-member mantle reservoir. Nd isotope data imply carbonatite metasomatism occurred within the last several hundred million years, with ubiquitous pyroxene core-to-rim Al diffusion zoning indicating that it must pre-date cooling of the lithospheric mantle following Late Cretaceous–Eocene rifting of Zealandia from Gondwana. Metasomatism was significantly younger than ancient Re-depletion ages of ~2 Ga and shows that decoupling of peridotite isotope systems has occurred. |
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