The anita peridotite, New Zealand: ultra-depletion and subtle enrichment in sub-arc mantle

The orogenic Anita Peridotite in Fiordland, SW New Zealand, provides an opportunity to examine the composition of a large block of upper mantle exhumed from beneath a Cretaceous arc. This little-studied 1 km × 20 km massif is dominated by spinel-facies harzburgite and dunite. Olivine Mg# of 92-93, spinel Cr# of ~70, orthopyroxene with low Al₂O₃, and extremely depleted whole-rock geochemical characteristics indicate that the peridotite body experienced >30% melt extraction, probably within the spinel facies. Mineral compositions show some similarity to those of cratonic peridotitic mantle. Rare Cr-rich amphibole suggests that the peridotite has been subsequently re-enriched. Distinctive, coupled Eu and Sr anomalies in the amphiboles, which can be subdivided into three groups, are interpreted to show that they formed by hydration of metasomatic clinopyroxene-plagioclase aggregates. Measured amphibole ⁸⁷Sr/⁸⁶Sr (~0·705-0·706), εNd (~ +6·3 to + 11·1), ²⁰⁸Pb/²⁰⁴Pb (~37·8-38·9) and εHf (~ +5·6 to 36·9) indicate that the metasomatic agent, which caused crystallization of clinopyroxene and plagioclase, had an isotopic composition similar to ocean island basalt. On the basis of isotopic data and mineral chemistry, the enriched nature of the peridotite is interpreted to have been caused by percolation of small volumes of a mafic silicate melt. Additional evidence for the passage of such melts is the rare occurrence of hornblendite veins and orthopyroxene hornblendite dykes. This peridotite body therefore preserves evidence of extreme melt depletion and the passage of silicate melts and hydrous fluids within the sub-arc mantle.