Alkali Metasomatism and the Petrology of Some Keratophyres

Abstract

AbstractPetrographic and chemical study of New Zealand keratophyres, occurring in a typical spilite-keratophyre association, shows that they were originally partly glassy flow-rocks and breccias comparable with rhyolites of young volcanic areas. They are now composed of a low temperature mineral assemblage inconsistent with textural evidence of high temperature extrusive origin, and alkali metasomatism has enriched some rocks in soda and others in potash, so that their compositions are symmetrically disposed about that of a primary rhyolite magma with roughly equal amounts of the two alkalis.Data from other occurrences suggest that this condition is common amongst described keratophyres.Though the rocks do not originate from any special magma, the term keratophyre should be retained, with appropriate qualifiers, for old rhyolites with low-temperature mineralogy, which, incidentally, will often be found to have been affected by alkali metasomatism.