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.

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