The origin and petrogenetic significance of hour-glass zoning in titaniferous clinopyroxenes

Abstract

SummaryTertiary alkali basaltic rocks from the Southern Highlands, New South Wales, contain two types of clinopyroxene crystals, which exhibit excellent morphological development of hour-glass zoning. These are small, quench clinopyroxene crystals and large, abundant, euhedral phenocrysts. Electron microprobe analyses show that the different zone sectors derive from compositional differences, with variation in TiO2and Al2O3contents inversely with SiO2content being most significant. Increased depth of colour of titanaugites depends on increased coupled substitution of Al for Si in tetrahedral sites and of Ti3−in anM1site, while pleochroism is enhanced by increasing Fe content. It is suggested that the formation of hour-glass zoning is determined by an environment of crystallization where initial crystal growth is rapid relative to ionic diffusion in the melt, resulting in different chemistry for different growth directions within the crystal. This implies that, under certain conditions, kinetic factors, as well as the chemical composition of the host magma, may be significant in determining the chemical composition of phases precipitated, particularly where more than one crystallographic direction of growth is possible.