A new hypothesis is formulated to explain the development of rapakivi
texture in and around the mafic enclaves of porphyritic granitoids, i.e.
in environments involving magma mixing and mingling. The formation of a
plagioclase mantle around alkali feldspar megacrysts is attributed to
the localized presence of a melt resulting from the reaction of these
megacrysts, with host hybrid magma with which they are in
disequilibrium. This feldspathic melt adheres to the resorbed crystals
and is virtually immiscible with the surrounding magma. Its composition
is modified in terms of the relative proportions of K2O, Na2O, and CaO
through selective diffusion of these elements, thus allowing the
specific crystallization of andesine. With decreasing temperature, the
K-feldspar, again stable, crystallizes along with the plagioclase,
leading to mixed mantle structures.