The miarolitic pegmatites from the Königshain: a contribution to understanding the genesis of pegmatites

Abstract

In this paper we show that the crystallization of miarolitic pegmatites at Königshain started at about 700°C, in melts containing up to 30 mass% water. Such high water concentration at low pressures (1-3 kbar) is only possible if the melts are peralkaline. Such peralkaline melts are highly corrosive, and reacted with the wall rock – here the granite host – forming the graphic granite zone - in part via a magmatic-metasomatic reaction. With cooling, the water concentration in some melt fractions increased up to 50 mass% H2O. The melt-dominated system ends below 600°C and passes into a fluid-dominated system, the beginning of which is characterized by strong pressure fluctuations, caused by the change of OH and CO32- in the melt, to molecular water and CO2. We note two generations of smoky quartz, one crystallized above the -transition of quartz (≈ 573°C), and one below, both of which contain melt inclusions. This indicates that some melt fraction remains during at least the higher-temperature portion of the growth of minerals into the miarolitic cavity, contradicting the view that minerals growing into a pegmatite chamber only do so from aqueous fluids. We show that the Königshain miarolitic pegmatites are part of the broad spectrum of pegmatite types, and the processes active at Königshain are representative of processes found in most granitic pegmatites, and are thus instructive in the understanding of pegmatite formation in general, and constraining the composition and characteristics of pegmatite-forming melts.