Agpaitic Rocks of the Norra Kärr Alkaline Complex: Chemistry, Origin, and Age of Eudialyte-hosted Zirconium and Rare-earth Element Ore

Abstract: Agpaites are peralkaline nepheline syenites that are denominated by rock-forming complex silicate minerals such as eudialyte. The exotic mineralogy and chemistry of these rocks are not only interesting for research, but globally these rocks also have potential as polymetallic resources of elements such as zirconium, niobium, rare-earth elements, zinc, and uranium. The Norra Kärr alkaline complex is a small agpaitic intrusion in southern Sweden, which in recent years has become recognised as a major zirconium and rare-earth element resource. These rocks have been known for over a century, but have been researched sparingly with the modern petrologist’s toolkit, including in situ analysis techniques and isotope geochemistry. This thesis work focuses on two aspects of Norra Kärr. Firstly, a systematic investigation of the major element composition of a large selection of eudi- alyte samples established an overview of eudialyte geochemistry. Analyses by SEM-EDS and EMP-WDS were compared. Eudialytes from different rock units cluster according to their Mn/(Fe+Mn) ratios, which distinguish- es three groups. Two of these groups, which are the most common types, are rich in rare-earth elements. This broad overview provides a necessary context for more detailed investigations of eudialyte’s complex zoning. Secondly, the age of intrusion and magma source were not precisely determined before this work. We indirectly determined the age of magmatic emplacement by precisely dating the syn-magmatic alkali metasomatic alteration (fenitisation) of the hanging-wall granite. Here, the U–Pb geochronometer in reset and newly-formed zircon records the time of feni- tisation at 1.49 ± 0.01 Ga. Lu–Hf isotope systematics of these zircons and eudialyte from Norra Kärr support the petrographic link between fenitisation and the emplacement of Norra Kärr. Hf isotopes suggest a liquid line of descent from subcontinental lithospheric mantle, which has a significantly higher εHf signature than the Svecofennian and TIB crust.