Xenoliths and metamorphosed rocks associated with the Auckland basalts

Abstract A variety of thermally metamorphosed rocks from xenolithic and similar material associated with the alkali olivine basalts of Auckland is described. Rock types recognised include: hypersthene‐plagioclase hornfels; mullite and sillimanite buchite; cordierite hornfels and buchite; pyroxene sanidinite. Assimilation of predominantly argillaceous sediment by the basalt is shown to have formed hypersthene basalt and to have initiated an olivine — hypersthene reaction with phenocrystic olivine transforming the under‐saturated basalt into one of tholeiitic type.