Abstract
Abstract The Paleoproterozoic rare‐element pegmatites of the Falun area with the classic localities of Finnbo quarry and KSrarvet are of the NYF (niobium‐yttrium‐fluorine) family. Fractionation trends based on alkali elements in K‐feldspar and muscovite, and Fe‐Mn‐Y in garnet have been defined for the pegmatites and pegmatitic granites of the area. The K/Rb ratio varies from 370 to 59 in blocky K‐feldspar and from 91 to 17.2 in book muscovite; the classic beryl‐topaz‐(Y,REE)‐mineral‐cassiterite‐albite‐bearing Finnbo quarry pegmatite is the most fractionated. Y2O3 is markedly enriched in the cores of garnet crystals, attaining up to 3 wt.% Y2O3. Overall, Y2O3 in garnet initially increases with alkali fractionation but decreases in the most fractionated dykes. The trend in the 100xMnO/(MnO+FeO) ratio is reciprocal. Partitioning of (Y.REE) between garnet and (Y,REE)‐minerals, and variations in the abundance of biotite, acting as a sink for Fe, can explain the apparent differences between the fractionation tr...
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