Replacement antiperthites in gneisses of the Babbitt-Embarrass area, Minnesota, U. S. A

Zoned and antiperthitic plagioclase is abundant in the amphibolitefacies gneisses of the Babbitt-Embarrass area. Concentrations of K-feldspar blebs within antiperthite grains are surrounded by haloes of plagioclase that is either more sodic or more calcic than the host grain. Plagioclase twinning is commonly obliterated in such zones. Microprobe analyses show that the composition of the halo and of the host plagioclase varies from grain to grain within a single sample. Potassium depletion in the haloes is quantitatively insufficient to supply the potassium in the antiperthitic K-feldpar blebs, suggesting that an origin by exsolution is improbable. However, the relations between zoning and antiperthitic K-feldspar are generally in accord with those predicted by Hofmann (1967) for alkali-infiltration metasomatism of plagioclase-bearing rocks. The presence of wide sodic or calcic haloes around K-feldspar blebs in antiperthites may probably be regarded as evidence of origin by replacement.