Geochronology of the Norra Kärr alkaline complex, southern Sweden

• The Norra Kärr alkaline complex is a small intrusion of agpaitic nepheline syenite, located approximately 10 km north of Grän- na, southern Sweden, that is being explored for REE and Zr. Agpaitic nepheline syenites per definition do not contain simple Zr minerals like zircon and baddeleyite, despite generally high Zr contents (up to 1–2 wt.% ZrO2). Instead, Zr is mainly hosted in rock-forming complex Na-Ca-Zr silicate minerals such as members of the catapleiite, eudialyte, rosenbuschite, and wöhlerite groups. U-Pb zircon geochronology is thus highly impeded by the agpaitic nature. We established a new, reliable igneous age for the Norra Kärr alkaline complex at 1489±8 Ma (MSWD = 0.95) by dating zircons (U-Pb) in the country rocks that were affected by the magmatism-related alkaline alteration (fenitisation) by LA-MC-ICP-MS. Zircons from a satellite body of non-agpaitic syenite gave intercept ages within error of the age of fenitisation. This is an improvement upon an imprecise whole-rock Rb-Sr age of 1545±61 Ma (Blaxland 1977; Welin 1980). The non-fenitised country granite itself is dated at 1781±8 Ma (MSWD = 0.59), and thus belongs to the TIB1 (1.81–1.76 Ga) episode of the Transscandinavian Igneous Belt (TIB). Rare zircon xenocrysts extracted from the nepheline syenite show ages corresponding to 1.5 Ga lower crustal intrusives (rapakivi?), TIB, Svecofennian, and one Archaean zircon, which suggests the possibility for a remnant of Archaean basement below the TIB. There has historically been a vivid discussion about wheth- er or not Norra Kärr has been deformed and metamorphosed. New Ar-Ar step heating ages on sodic amphibole from Norra Kärr and muscovite and biotite from the country rocks give plateau ages at 1.1 Ga and 0.94 Ga, which correspond to ages derived for Sveconorwegian shear zones in the area. Together with textural and crystal chemical evidence, these ages make a compelling argument for some form of Sveconorwegian overprint of the Norra Kärr alkaline complex.

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NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

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