Equilibration History of the Basal Alpine-Type

The Red Mountain alpine peridotite forms the basal, dominantly harzburgitic tectonite portion of an ophiolite suite in South Island, New Zealand. Olivine and pyroxene Mg/Fe compositions are constant for individual lithologies, but generally increase through the series harzburgite, orthopyroxenite, harzburgitic dunite, dunite. An olivine-clinopyroxene dominated transitional peridotite along the western margin of the mass has more Fe-rich silicates than in the harzburgitic suite. Fe-Mg silicate-spinel relationships and the distribu-tion of AJ between coexisting pyroxenes and spinel indicate nearly complete post-layering equilibration. A partial re-equilibration is suggested by narrow compositional rims on pyroxenes and spinel. Relative to the mineral cores, the rims show enhanced partitioning of Al into spinel relative to pyroxene. The Fe-Mg relationships between silicates and spinel, and the compositional variations from cores to rims of pyroxenes and spinels indicate that the rims formed at lower temperatures than the mineral cores. This conclusion is supported by the application of several geothermometers, which give average temperatures of equilibra-tion and partial re-equilibration of 1000-1070 °C and 920-1030 °C, respectively. Pyroxene overgrowths on olivine probably represent pre-equilibration cooling phenomena. Equilibra-tion pressures cannot be estimated with precision because Red Mountain pyroxenes have Al contents that vary as a function of whole-rock AI2O3, and other compositional variables, as well as of T and P of equilibration. The lack of plagioclase in the harzburgite tectonites, and the wide range of (Al/Cr)Ipinei indicate equilibration at fairly high pressures, probably at depths within the 25-80 km range. The transitional peridotite probably formed by re-equili-bration of residual crystals with basaltic melt at shallower (<25 km) depths, and is evidence supporting the conclusion that the ultramafic and mafic parts of the ophiolite suite at Red Mountain represent complementary parts of the same melting event