Rapakivi granite and basic dykes in the Fennoscandian shield a palaeomagnetic analysis

Abstract: Rapakivi rocks and basic dykes in the central part of Sweden have been subject to a palaeomagnetic study with the aim of putting the formation of these rocks into a plate tectonic context. Virtual geomagnetic poles were calculated and found to be distributed around the Jotnian Loop of the APWP of Fennoscandia. Paleolatitudes for the shield at the time of the intrusions were determied to a range from the equator to a 30 degrees North and a hot spot origin was tested as a model for the origin of the rapakivi rocks. Palaeomagnetic data from a basic dyke swarm in central Sweden suggest the dykes to be related to rapakivi intrusions, which means that this Neoproterozoic magmatism seems more widespread than was earlier suggested. From very detailed palaeomagnetic studies of basic dykes and the baked host rocks the ambient temperature of the rock at the time of intrusion of the dykes was determined at 376 degrees Celsius. A palaeothermal gradient was estimated and the depth of burial of the present erosion surface at 1.5 Ga ago was determined at 12 km.