TY - JOUR
T1 - The Anita Peridotite, New Zealand
T2 - ultra-depletion and subtle enrichment in a supra-subduction setting
AU - Czertowicz, Tom
AU - Scott, James
AU - Waight, Tod Earle
AU - Palin, Mike
AU - van der Meer, Quinten
AU - Le Roux, Petrus
AU - Münker, Carsten
AU - Piazolo, S.
PY - 2016
Y1 - 2016
N2 - 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 1km2 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 reenriched. 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), eNd ( +6.3 to +11.1), 208Pb/204Pb (37.8–38.9) and eHf ( +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.
AB - 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 1km2 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 reenriched. 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), eNd ( +6.3 to +11.1), 208Pb/204Pb (37.8–38.9) and eHf ( +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.
U2 - 10.1093/petrology/egw001
DO - 10.1093/petrology/egw001
M3 - Journal article
SN - 0022-3530
VL - 57
SP - 717
EP - 750
JO - Journal of Petrology
JF - Journal of Petrology
IS - 4
ER -