The miarolitic pegmatites from the Konigshain: a contribution to understanding the genesis of pegmatites
Thomas, R and Davidson, P and Rhede, D and Leh, M, The miarolitic pegmatites from the Konigshain: a contribution to understanding the genesis of pegmatites, Contributions to Mineralogy and Petrology, 157, (5) pp. 505-523. ISSN 0010-7999 (2009) [Refereed Article]
In this paper, we show that the crystallization of miarolitic pegmatites at Konigshain started at about 700A degrees 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 600A degrees 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 CO3 (2-) in the melt, to molecular water and CO2. We note two generations of smoky quartz, one crystallized above the beta-I +/--transition of quartz (a parts per thousand 573A degrees 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 Konigshain miarolitic pegmatites are part of the broad spectrum of pegmatite types, and the processes active at Konigshain 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.