Chemically unaltered calcium-bearing plagioclase (An20–An60) of volcanic origin is found in propylitically altered and low-grade metamorphosed volcanic rocks from several areas. Although parts of the relict plagioclase phenocrysts are replaced by albite or by other secondary minerals, perfectly preserved igneous oscillatory zoning shows that the unaltered parts had not reacted chemically with the remainder of the rock nor undergone solid-state diffusion on a microscopically apparent scale.

Universal stage and X-ray-diffraction measurements show that the chemically unaltered plagioclases in propylitically altered rocks and prehnite-pumpellyite metagraywacke facies and lower green-schist facies rocks are in their original high or high-intermediate structural states, whereas relict chemically unaltered plagioclases in middle greenschist facies rocks have inverted to the low structural state.

The data suggest that natural quenched high plagioclase will not begin to invert to the low structural state until certain conditions of temperature and probably of water pressure have been exceeded. Temperatures of from 250° to 450° C appear to be required for inversion to proceed at water pressures of 1000–2000 bars.

Experimental determination of the minimum inversion temperature at various water pressures may provide a method for inferring the temperature-pressure conditions of low-grade metamorphism and hydrothermal alteration and for estimating the depth of emplacement of certain magmatic bodies.

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