Abstract

A petrographic and x-ray study of the Bedford augen gneiss suggests the temperature during metamorphism ranged from 400°C in the southern part of the body to approximately 700°C in the northern part. The mineral assemblage of sillimanite-almandine-orthoclase, a subfacies of the amphibolite facies (Winkler, 1967, p. 106) that characterizes the northern part of the augen gneiss mass, suggests the temperature of formation was 550° to 700°C. Microcline and abundant muscovite in stable association with the kyanite-almandine-muscovite subfacies of the amphibolite facies characteristic of the southeast half of the mass suggests the temperature here was 400° to 500°C (Steiger and Hart, 1967). X-ray diffraction analysis indicates that two crystallographic forms of potassium feldspar, orthoclase, and maximum microcline, exist in several structural states, further suggesting a temperature gradient increasing from south to north in the body.

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