The Nature and Origin of the Groundmass Textures in Felsites and Granophyres from Rhum, Inverness-shire

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Abstract
Chemical and mineralogical data are presented to demonstrate the overall similarity between granophyre and felsite from Rhum. However, although the compositions of the groundmasses of the two rock-types are very similar their characteristic textures are quite different. The felsitic texture is shown to be the result of devitrification of a glass, whilst the granophyric texture is attributed to the simultaneous crystallization of quartz and feldspar on the cotectic line in the system Ab–Or–SiO2–H2O. An explanation of the formation of the different textures is given in terms of the degree of super-cooling experienced by separate intrusions of the same magma in contrasted environments.