Mantle geochemistry beneath south Westland, New Zealand: A study on peridotite xenoliths from Hidden Rivulet

Crase, Jordan

Peridotite xenoliths entrained in alkaline magmas can preserve a record of processes that affected them in the lithospheric mantle. In Hidden Rivulet in the Haast area, the Alpine Dike Swarm entrained a variety of spinel facies peridotites that are predominantly of harzburgite composition. A large proportion of the spinel phases in these rocks form distinctive orthopyroxeneclinopyroxene symplectites and represent the decompression of garnet peridotite followed by equilibration in the spinel facies at some point in the past. Spinel Cr# and olivine Mg# as well as HREE indicate that high degrees of partial melting removed essentially all of the primary clinopyroxene, and likely some of the orthopyroxene, in these peridotites. However, the mantle has been overprinted by metasomatism, with two types inferred from the geochemistry of secondary clinopyroxene. The first event, only affecting some peridotites, is silicate metasomatism as indicated by high Ti and LREE concentrations in clinopyroxene. The second type, affecting all of the peridotites, is carbonatite metasomatism as indicated by low Ti/Eu, high Th/U and high LREE of the clinopyroxene. As the peridotites were devoid (or very close to devoid) of clinopyroxene prior to metasomatism, the isotopic ratios of these grains should closely represent the composition of the metasomatic agents. Clinopyroxene radiogenic isotopes are comparable to that of the HIMU mantle reservoir. HIMU-like ratios have also been recorded in the host Alpine Dike Swarm, which suggests that the dike swarm could share a similar source to the metasomatic agent, or could be derived from melting of the HIMU-enriched mantle lithosphere. The last process prior to entrainment was the equilibration of major and trace elements at 900-1100degC.

xii, 120 pages A4

2014Crase

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