Abstract

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 Evje-Iveland 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: ∼680 °C for the intermediate zone (the transitional zone between the wall and core of zoned pegmatites) and ∼600 °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 °C) and the solidifying pegmatite (∼600 °C of the albite zone). Moreover, crystallization ages of Evje-Iveland pegmatites, ranging from 912 ± 2 Ma to 904 ± 1 Ma, imply that the long-lasting high-temperature event generated and emplaced anatectic pegmatite melts over a period of at least 5 million years.

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