A roughly circular 8-sq km exposure of coarse orthoclase augen gneiss occurs with steeply dipping, narrowly gradational contacts in phyllites and schists of greenschist facies metamorphic grade near Ödemis, western Turkey. Structural, stratigraphic, or intrusive explanations of this occurrence of augen gneiss, a rock type typically associated with only the highest grade metamorphic rocks, in such a low-grade environment seem unsatisfactory. Petrographic evidence of the replacement of sodic plagioclase by potassium feldspar at the borders of the body points to a metasomatic origin in which the phyllites and schists have been converted to augen gneiss locally through the effect of steeply ascending fluids.

Compositional and structural state determination of the potassium feldspar augen by optical and X-ray diffraction techniques, employing a least-squares cell refinement computer method, indicates that all the potassium feldspar specimens investigated lie in the orthoclase range, but range in structural state from barely monoclinic to just below the low sanidine limit, and have compositions which fall within 5 percent of 85 mole percent Or.

Implications are drawn from the structural state of the potassium feldspar augen and the mineral paragenesis of gneiss and associated phyllites and schists as to the temperature environment extent during the metasomatic event. A temperature of 500° C ± 50° is estimated. This lies within the 400° C to 550° C range suggested by Winkler (1967, p. 174) for the greenschist facies. Thus the augen gneiss appears to have been produced in the environment encompassed by that facies.

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