Myrmekite as a marker between preaqueous and postaqueous phase saturation in granitic systems

A nonreplacive, nonexsolution model of myrmekite growth is based on textural relationships in the Sand Springs porphyritic granodiorite, west-central Nevada. A sequence of crystallization is divided into (1) a preaqueous-phase saturation stage, characterized by major growth of plagioclase (zoned)., quartz, and K-feldspar (phenocrysts), and (2) a postaqueous-phase saturation stage characterized by myrmekite, final euhedral growth of plagioclase and quartz, and final growth of K-feldspar phenocrysts and most K-feldspar of the matrix, including some crystals with adularia-habit characteristics. Myrmekite results from micropressure quenching during the separation of an aqueous phase as crystallization progresses. The occurrence of myrmekite as lobate units on plagioclase, extending into K-feldspar, results from precipitation of oligoclase (the basic ingredient of myrmekite) as local continuations of plagioclase growth from a melt that simultaneously expels an aqueous-rich fluid enriched in K-feldspar component. Late K-feldspar crystallizes from the aqueous-rich fluid, filling in around the myrmekite. Quartz in myrmekite represents the inability of silica to diffuse from the quenched melt and occurs as vermicules chiefly in accord with the principles of binary eutectic crystallization.

The Sand Springs myrmekite model is tested by evaluating its occurrences in aplite-pegmatite systems, in granitic gneisses, and in the hydrothermal secondary K-feldspar environment. Myrmekite commonly occurs in all but the hydrothermal environment, which is postmyrmekite, and a fundamentally magmatic origin can be reasoned for the other rock types if the tectonic environment during crystallization is also considered.