Magmatic age of rare-earth element and zirconium mineralisation at the Norra Kärr alkaline complex, southern Sweden, determined by U–Pb and Lu–Hf isotope analyses of metasomatic zircon and eudialyte

Abstract

The agpaitic Norra Kärr alkaline complex in southern Sweden is rich in heavy rare-earth elements and zirconium. Despite generally containing high concentrations of Zr, agpaitic rocks sensu stricto are devoid of igneous zircon. During the late stages of magmatic activity at Norra Kärr, metasomatic Na- and F-rich fluids transporting Zr complexes caused fenitisation (syn-magmatic alkali metasomatism) of the granitic wall rocks, which formed new metasomatic zircon. Fenite zircon was dated by LA-MC-ICP-MS with the U–Pb method at 1.49±0.01Ga, while the unaltered country rock granite was dated at 1.79±0.01Ga. Zircon in the fenites exhibits εHf+6.58±0.36 at 1.49Ga; significantly above the expected value for Svecofennian crust at that time (ca. −5±2), but identical, within error, of that measured in eudialyte from Norra Kärr. This suggests a common source of Hf for fenite zircon and eudialyte from Norra Kärr, which isotopically strengthens the genetic link between fenite and nepheline syenite. The 1.49Ga age dates not only the fenitisation, but also the relatively small agpaitic intrusion, which caused the fenitisation. This also dates the origin of the rare-earth element and Zr ore within the agpaitic nepheline syenite, which probably formed from an evolved magma derived from the mantle. A few zircon xenocrysts, which survived in the agpaitic nepheline syenite magma, may indicate the presence of an Archaean component in the basement.