Thomas-Davidson_2013a.pdf (7.21 MB)
The missing link between granites and granitic pegmatites
journal contributionposted on 2023-05-26, 09:15 authored by Thomas, R, Paul DavidsonPaul Davidson
In this contribution we provide evidence for the extraction of volatile and incompatible element enriched melts from common granites, which provides a mechanism which show that a large proportion of granitic pegmatites are genetically directly connected to a main granite body. In granites there are often two principal types of melt inclusions: (i) melt inclusions which represent the bulk chemistry of the granite and (ii) melt inclusions with a composition strongly divergent from this composition. In the Variscan Erzgebirge granites this type is characterized by extremely high fluorine concentration. However, in other geodynamic settings inclusions in granites can contain high concentrations of other elements which may take over the function of fluorine. From textural relationships this inclusion type represents intergranular melts enriched in all elements incompatible with the ideal haplogranite system. Due to the high volatile content of such melts the viscosity can be as much as several orders of magnitude lower than the quasi-solid bulk system and can therefore move rapidly through the partially or totally crystallized host, and flow together into a separate system forming pegmatite bodies inside or outside the granite body. Another important effect of the high volatile content is the phase separation resulting from the speciation change from OH- ‚Äövúv¿ H2O or CO32- ‚Äövúv¿ CO2 due to temperature and/or pressure changes at different locations within the granite-intergranular melt system. Since melt inclusions provide a means of conserving original undegassed compositions they are therefore important evidence for closing the gap between granites and granitic pegmatites. The paper is dedicated to two Czech colleagues - Petr ¿ív•ern‚àöŒ© and Milan Nov‚àö¬8k who have devoted their life to the study of granitic pegmatites.
Publication titleJournal of Geosciences
Rights statementCopyright 2013 Czech Geological Society