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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.

Artikel i vetenskaplig tidskrift
Författare Axel S.L. Sjöqvist
David H. Cornell
Tom Andersen
Ulf I. Christensson
Johan T. Berg
Publicerad i Lithos
Volym 294–295
Sidor 73-86
ISSN 0024-4937
Publiceringsår 2017
Publicerad vid Institutionen för geovetenskaper
Sidor 73-86
Språk en
Länkar https://doi.org/10.1016/j.lithos.20...
Ämnesord agpaitic rocks, rare-earth elements, Norra Kärr alkaline complex, zircon, eudialyte, fenitization
Ämneskategorier Geovetenskap och miljövetenskap

Sammanfattning

The agpaitic Norra Kärr alkaline complex in southern Sweden is rich in heavy rare-earth elements and zirconium. Despite generally hosting high concentrations of Zr, agpaitic rocks sensu stricto are devoid of igneous zircon. During the late stages of magmatic intrusion, 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 ± 0.01 Ga. The unaltered country rock granite was dated at 1.79 ± 0.01 Ga. Zircon in the fenites exhibits εHf +6.58 ± 0.36 at 1.49 Ga; 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 link between fenite and nepheline syenite. The 1.49 Ga age dates not only the fenitisation, but also the agpaitic intrusion, which caused the fenitisation, and with it the origin of the rare-earth element and Zr ore within the agpaites, which probably formed from an evolved magma derived from the mantle. A few zircon xenocrysts, which survived in the agpaitic magma, may indicate the presence of an Archaean component in the basement.

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