Abstract

The magmatic garnet of seven granitic NYF (niobium–yttrium–fluorine-enriched) granitic pegmatites from the Froland and Evje–Iveland areas in southern Norway were studied with respect to their major- and trace-element composition and intracrystalline distribution of major and minor elements. The increasing average MnO/(FeO + MnO) values of the garnet grains investigated reflect the increasing fractionation of pegmatites from abyssal heavy REE to muscovite rare-element REE pegmatites. At a crystal scale, the MnO/(MnO + FeO) values show various trends controlled by coexisting Mn–Fe-consuming minerals. Back-scattered electron imaging revealed a great variety of structures, including large-scale (>100 μm) concentric growth-induced zoning, fine-scale oscillatory growth and replacement overgrowths. The structures predominantly correlate with the distribution of Y and HREE in the crystals. It is presumably the first time that such diversity has been reported from magmatic garnet originating from one area. The average Y and HREE concentrations are related to the bulk composition of the pegmatite-forming melt, whereas the intracrystalline zoning reflects the absence or presence of Y-bearing minerals or, in the case of the Slobrekka pegmatite, diffusion-controlled crystal growth. Sharp drops of the Y and HREE content record the abrupt change of “normal” peraluminous melt composition to a Na-rich aqueous silica-bearing fluid enriched in F, Rb, Cs, Ta, Mn in the case of the Solås and Hovåsen pegmatites. These Na-rich aqueous silica-bearing fluids are responsible for the formation of “amazonite”–“cleavelandite” replacement units. The regional implications are, first, that the Froland pegmatites are characterized by a shorter range of pegmatite fractionation. The parent melts seem to be more primitive with respect to the differentiation of a granitic magma compared to the Evje–Iveland pegmatites, as reflected by the elevated Ca content and the smaller negative Eu anomaly of garnets at Froland. Rough estimates of the bulk composition of the pegmatites could not reveal such a difference. Second, garnet of the Evje–Iveland pegmatites shows a wider range of chemical variability and patterns of Y–HREE zoning compared to the Froland samples. Thus, the gradients of pegmatite fractionation are much stronger within the Evje–Iveland field. The Evje–Iveland melts were probably more enriched in volatiles, being partially responsible for the crystallization of common rare-metal and REE minerals.

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