A multi-mineral U-(Th)-Pb dating study of the Stetind pegmatite of the Tysfjord region, Norway, and implications for production of NYF-rare element pegmatites during orogenic collapse

Field relationships, textures, compositions and U-(Th)-Pb dates for minerals from the niobium‑yttrium‑fluorine (NYF-) Stetind pegmatite in the 1.798 Ga Tysfjord granitic gneiss in Norway are described. The pegmatite has a sub-horizontal orientation and anisotropic mineral layering oriented sub-parallel to contacts with the host-rock and oblique to its foliation. Epidote-group minerals have allanite cores and interstitial REE-rich epidote. Zircon, thorite, uraninite, fergusonite and xenotime, as well as apatite, fluorite, plagioclase and orthoclase are compositionally homogenous and display relict albite twinning and ternary junctions. Feldspars lack grain boundary recrystallisation textures, and no undulatory extinction is observed in quartz. Isotope UPb TIMS analysis of zircon, allanite, fergusonite and uraninite give dates between 400 and 410 Ma that have a high degree of concordance and no evidence of inheritance. A F-rich fluid dissolved fergusonite (YNbO4) and reacted with apatite to produce xenotime (YPO4) mantles around apatite. The reaction was dated by electron-probe micro-analysis total U-Th-Pb methods to 393 ± 12 Ma. The field and petrographic observations, coupled with mineral dates are interpreted to reflect primary emplacement and crystallisation of the Stetind NYF pegmatite after the peak of Scandian orogenesis and metamorphism during exhumation of the Tysfjord granitic gneiss. The xenotime forming event reflects a late magmato-hydrothermal reaction during the final stages of crystallisation. The compositional affinity of the Stetind pegmatite to its host rock suggests that the pegmatite-magma was produced by partial melting of the Tysfjord granitic gneiss or a rock with similar characteristics. The pegmatite is one of many rare-element pegmatites in the Tysfjord-Hamarøy pegmatite field, which may indicate a Scandian phase of pegmatite emplacement into the uppermost allochthon of the Norwegian Caledonides. The results validate models that propose the production of NYF-type pegmatites by anatexis during orogenic collapse.