In an investigation of the origin of porphyritic and glomeroporphyritic plagioclases in basic rocks, thin sections of rocks from Skye, Mull, and Ardnamurchan in western Scotland, and Slieve Gullion and Carlingford in Ireland were examined. It is shown that thermal metamorphism of zeolite amygdales produces an aggregate of individually homogeneous calcic plagioclase crystals. Original feldspar phenocrysts show little or no change.

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