Age and tectonic significance of Niobium-Yttrium-Fluorine (NYF)-pegmatites in the Tysfjord region, Nordland, Norway
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Niobium-yttrium-fluorine (NYF) pegmatites comprise a subset of the rare-element pegmatite class. Because of their composition and geochemistry, these pegmatites are commonly associated with A-type magmatism in extensional environments. NYF pegmatites exposed in the Nordland area of the Norwegian Caledonides have been proposed to be coeval, and genetically related to their 1.7 Ga A-type Tysfjord granitic-gneiss host. A geochronological study, using U-Pb Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) and Electron-Probe Micro-Analysis (EPMA) total U-Th-Pb techniques applied to zircon, fergusonite and xenotime provide ages of 400 Ma. Petrographic analysis by optical and back-scattered electron (BSE) imaging shows evidence of fergusonite dissolution and growth of xenotime halos around apatite occurring in a fluorine-rich siliceous-melt, and allanite and zircon display primary growth textures attributed to pegmatite emplacement with no evidence of metamorphic overprint. Based on the textural evidence it is proposed that the geochronological data represents the age of primary pegmatite crystallization. This study concludes crystallization of NYF pegmatites through partial melting of their host during late pro-grade metamorphism associated with Caledonian nappe emplacement. Partial melting was likely aided by H2O, made available through local metamorphism, and fluorine, also released by metamorphism of the host. A Caledonian-aged emplacement and crystallization of NYF pegmatites in Nordland challenges the existing model that these pegmatites were part of the host Tysfjord Granite magmatic event. Furthermore, it suggests that the primary model for NYF-pegmatite emplacement that ties them to anorogenic tectonic environments and associated with A-type magmatism may not be appropriate for all cases.