Abstract
Many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase. Inclusions of this sodic plagioclase within the corroded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures. Textural evidence indicates that patchy zoning is the result of a two-stage replacement process involving partial resorption of early plagioclase crystals followed by crystallization of a more sodic plagioclase. Poikilitic inclusions commonly present within the patches of sodic plagioclase are believed to have crystallized from the melt trapped in the cores after corrosion. Patchy zoning is interpreted here in terms of decrease in confining pressure on water-deficient magma during its rise in the crust. The sequence of development is visualized as follows: (1) crystallization of plagioclase at depth; (2) partial resorption of the plagioclase related to fall in pressure; (3) renewed crystalliza...
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