Abstract

Plagioclase phenocrysts from a series of andesitic basalt flows, pyroclastics, and epiclas-tics in the Western Santa Monica Mountains of Los Angeles County, California, characteristically exhibit an unusual textural feature referred to here as “convolute zoning.” Phenocrysts exhibiting this texture are embayed, the compositional zoning being thinner and “deflected inward following Lhe contours of the embayments. A possible explanation of this feature is that the embayments represent response by growing crystals to semiporous obstructions encountered during crystallization. This suggests that similar resorption textures reported for unzoned crystals, which may not have left a clear record of events, may not in all cases represent magmatic corrosion.

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