Alkali-F-Rich Albite Zones in Evolved NYF Pegmatites: The Product of Melt–melt Immiscibility

Albite zones, which are common in many evolved granitic pegmatites, often appear to be at least partly replacive in origin and are therefore widely thought to form by late-stage metasomatic processes. However, based on paragenetic relations and chemical and melt inclusion studies, albite zones in evolved Proterozoic niobium-yttrium-fluorine (NYF) rare metal pegmatites from Evje-Iveland, southern Norway, cannot be metasomatic in a strict sense and are likely to have crystallized from late-stage silicate melts. Analysis of silicate melt inclusions in topaz and albite (the platy variety, "cleavelandite") from the albite zones shows that the melts were F-rich with H2O contents of >= 34 wt.%. In addition, distinctive mineral compositions ("cleavelandite", white and pink mica, spessartine, columbite-group minerals, topaz, and fluorite) indicate that the silicate melts must have been strongly alkaline (Na-dominated) with enrichments in F (1.5 to 3.6 wt.%), Cs, Rb, Ta, Nb, Mn, Ge, Bi, As, and in some cases also Li compared with host pegmatites. For the melt to acquire this composition, the original Li-Cs-Ta-poor EvjeIveland pegmatite-forming melt must have undergone extreme internal chemical differentiation resulting in melt/melt and additional melt/fluid immiscibility. The resulting major pegmatite-forming, alkali-H2O-poor melt, minor albite zone-forming alkali-H2O-F-rich melt, and aqueous fluid would have had very different viscosities and densities, and therefore physical flow/transport properties, resulting in discordant contacts between the rocks which formed from them. The mineralogy, zoning, and melt inclusion data from the Evje-Iveland pegmatites document a difference in crystallization temperature within the investigated pegmatite bodies: similar to 680 degrees C for the intermediate zone (the transitional zone between the wall and core of zoned pegmatites) and similar to 600 degrees C for the albite zone. A protracted crystallization period for the deeply emplaced pegmatites and albite zones of Evje-Iveland (15 to 19 km) is suggested. We argue that the protracted crystallization is due to similar temperatures of the host rocks (at least 550 to 600 degrees C) and the solidifying pegmatite (similar to 600 degrees C of the albite zone). Moreover, crystallization ages of Evje-Iveland pegmatites, ranging from 912 +/- 2 Ma to 904 +/- 1 Ma, imply that the longlasting high-temperature event generated and emplaced anatectic pegmatite melts over a period of at least 5 million years.