AN EXSOLUTION SILICA-PUMP MODEL FOR THE ORIGIN OF MYRMEKITE

Myrmekite, as defined here, is the microscopic intergrowth between ver micular quartz and modestly anorthitic plagioclase (calcic albite-olig oclase), intimately associated with potassium feldspar in plutonic roc ks of granitic composition. Hypotheses previously invoked in explanati on of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its ab sence in rocks devoid of discrete grains of potassium feldspar challen ge those hypotheses based on direct crystallization or replacement. Ho wever, several lines of evidence indicate that myrmekite may in fact o riginate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagio clase phases. Exsolution of potassium feldspar and plagioclase from an initially calcic alkali feldspar system projected from [AlSi2O8] invo lves the exchange CaAlK-1Si-1, in which the AlSi-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary felds par system is open to excess silica, the exchange reaction that produc es potassium feldspar in the ternary plane is aided by the net-transfe r reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is c oncomitantly incorporated in this same phase. Thus silica is ''pumped' ' into the reaction volume from a ''silica reservoir,'' a process that enhances redistribution of both Si and Al through the exsolving terna ry feldspar.