Magma flow directions of post Jotnian dolerite sills in central-east Sweden: a magnetic fabric and paleomagnetic survey

An anisotropy of magnetic susceptibility (AMS) and paleomagnetic study has been performed on post-Jotnian basic sills of the Ulvö complex, of the Central Scandinavian Dolerite Group (CSDG). The CSDG intruded some 1.27 Ga ago in the central part of the Fennoscandian Shield and extends over an area of c. 150 000 km2. Previously presented AMS data of the CSDG sills from south-central and northern Sweden indicated a NW or SE directed magma flow. In this study new measurements of the AMS reveal a fairly uniform pattern of NW-SE lineation directions also for the dolerites of the Ulvö complex. While the magma is flowing in a dyke or a sill, elongated particles become imbricated against the chilled margins, and the AMS can be used to determine not only the lineation but also the true direction of the magma flow. An imbrication pattern defined for one of the sills suggests that the magma propagated from the present NW towards SE. The established paleomagnetic pole position (Plat=−3° and Plon=157°) is in very good agreement with other paleomagnetic data from the Ulvö complex and from other parts of the CSDG. The new AMS data from the Ulvö complex presented in this study fit well in to the magnetic fabric pattern previously presented for the CSDG sills and support a plate tectonic model suggesting that the CSDG is a related to the break up of Baltica from Laurentia at c. 1.27 Ga ago.