VARIATIONS OF K/RB RATIO IN MIGMATITIC PARAGNEISSES OF NORTHWEST ADIRONDACKS

Data for the migmatitic paragneisses of the Northwest Adirondacks indicate fractionation of potassium and rubidium during metamorphism and migmatization; total rock K Rb ratios are controlled to a significant degree by modal composition. Biotite-poor granitic veins and lenses show K Rb ratios averaging 431 in contrast to the host gneisses which have an average K Rb ratio of 271. Paragneisses from the zone of granulite facies metamorphism show significantly higher (mean 307) K Rb ratios than those from the amphibolite facies (231), indicating the possibility of regional depletion in Rb relative to K under intensive metamorphic conditions. © 1969.