FROM EVOLVED GRANITE TO PEGMATITE: WHAT CAN WE LEARN FROM MELT AND FLUID INCLUSIONS?

Pegmatites are fascinating rocks, which contain often beautifully well-shaped crystals of quartz, feldspars and rare minerals like beryl, topaz, and many others. Some minerals from pegmatites have stimulated scientific research in crystallography, mineralogy, chemistry and physics. For example, the Swedish amateur mineralogist Carl Arrhenius found in the summer of 1787 a new, heavy and black mineral in a quarry near Ytterby. This mineral, originally called ytterbite, now gadolinite, was maybe the most significant single mineral discovery in the history of inorganic chemistry, and opened the history of rare earths exploitations in 1794. Despite the extensive studies on pegmatites in the past their genesis is not resolved completely. The origin of pegmatites, especially those that are complexly zoned, is very controversial. One group of researchers (e.g., Fersmann, 1931; Turner and Verhoogen, 1960) considers them to be of igneous origin, while another one believes that pegmatites are metamorphic or metasomatic (e.g., Barth, 1962; Ramberg, 1952; Gresens, 1967). In this paper evidence is presented in favor of the igneous origin of pegmatites. This conclusion is based mainly on melt and fluid inclusion studies, which show directly that the vast majority of the granite pegmatites are primarily of magmatic origin, but are often affected by both metasomatism and late-stage hydrothermal events. Furthermore, the study of melt and fluid inclusions in pegmatite minerals provides the crucial evidence for liquid compositions, phase relations and phase separation in nature. According to the studies on natural melt and fluid inclusions and also to the experimental work liquid immiscibility and the supercritical stage are of crucial meaning in the melt-dominated stage for the formation of pegmatites. Liquid immiscibility is not a singular event. These processes take place at all times as long as volatile-rich melts are present. The cause for this peculiar behavior can be seen in the complex interplay of the volatiles H2O, F, and Cl, the semivolatiles B2O3 and P2O5, and fluxing components such as Li2O, Rb2O, and Cs2O along with the SiO2 and Al2O3 in the melt.