Age and paleomagnetic signature of the Alnø carbonatite complex (NE Sweden): Additional controversy for the Neoproterozoic paleoposition of Baltica

Abstract

A paleomagnetic investigation of the Alnø carbonatite complex dikes was undertaken in an attempt to refine the apparent polar wander path for Baltica. The currently available paleomagnetic database for the Ediacaran–Cambrian segments of the Baltica APWP is marked by disparate (and often supposedly) coeval poles. This has led to remarkable conclusions about rapid rates of drift or true polar wander. Our study shows that the Alnø Complex (584 ± 7 Ma) is coeval with the Fen Carbonatite Complex (583 ± 15 Ma) via 40Ar/ 39Ar dating of biotite and K-feldspar. We identify three components of remanent magnetization in the Alnø Complex. The first is a low unblocking/coercivity component with a mean declination of 51.2° and inclination of +70.2° ( k = 22, a95 = 8.3°). A paleopole calculated from this direction falls at 62.7°N, 101°E. The high coercivity and unblocking components show a large spread in both declination and inclination. A somewhat artificial grouping of shallow–intermediate vectors (inclinations less than 60°) yields a mean direction with a declination of 108.1° and an inclination of 10.5° ( k = 5.3, a95 = 32.1°). This direction is statistically indistinguishable from that obtained by Piper [Piper, J.D.A., 1981. Magnetic properties of the Alnøn complex. Geol. Foren. Stock. Forhandlingar, 103, 9–15 (Part 1)] and yields a paleomagnetic pole at 3.5°N, 269°E. Conversely, the remaining high unblocking/coercivity components (inclinations > 60°) yields a second grouping with a mean declination of 28° and inclination of 76.8° ( k = 24; a95 = 16.1°; Fig. 5E). This direction is indistinguishable from the present Earth's field at the site. We conclude that the Alnø Complex poles (and indeed many of the Ediacaran poles for Baltica) should be viewed with scepticism when used for paleogeographic/geodynamic models.