The losses of Ca, Mg, Fe, and Al with increasing K also point to an open system instead of a closed system. Progressive compositional changes would be expected, depending upon the extent of deformation of the rocks and the resultant increasing degrees of openness of the system for fluids to move through the rocks. The following four conclusions can be reached from the observations and data presented in this article. Under the pressure, temperature, and relatively dry conditions in which the Popple Hill gneiss was formed, the microcline and myrmekite in this gneiss must have been formed at temperatures below melting. The microcline must have been primary, grew during increasing grades of thermometamorphism to produce megacrysts or augen, and formed by replacement of primary plagioclase instead of by conversion of primary orthoclase, crystallized from a melt.
Further study is needed to assess the significance of myrmekite in the sillimanite-bearing gneisses.
Master's thesis, Stony Brook, New York, State University of New York at Stony Brook. Geological Society of America Special Paper 194, p. 1-55.