Also found in: Dictionary.
(also porphyry), an extrusive and paleotypal quartz-free rock resembling andesite. It has large crystals of andesine phenocrysts or, more rarely, pyroxene phenocrysts, imbedded in a glassy ground mass. Porphyrites can form lava flows, dikes, stocks, and sills. Unlike andesites and basalts, they bear traces of intensive postmagmatic changes in initially glassy groundmasses resulting from such processes as chloritization, epidotization, and carbonatization. Partial or complete replacement of phenocrysts with secondary minerals can also result.
Porphyrite varieties are determined by their composition; several examples are plagioclase porphyrite, hornblende porphyrite and pyroxene porphyrite. Porphyrite deposits from various geological periods are widely distributed in fold and platform regions.