PALEOMAGNETISM, MAFIC DYKES AND THE PROTOGINE ZONE, SOUTHERN SWEDEN

The Protogine Zone (PZ) forms the border between the Svecofennian and Sveconorwegian domains of southern Sweden. Intruded in the Zone are three generations of mafic dykes dated 1510 Ma, 1180 Ma and 930 Ma. A palaeomagnetic study of these dykes in the central and southern parts of the PZ yields two main directions of magnetization: 'A' (D = 291-degrees, I = -78-degrees, N = 31) and 'B' (D = 129-degrees, I = 40-degrees, N = 13). These directions give palaeopoles on the Sveconorwegian Loop of the Fennoscandian apparent polar wander path (APWP). A third, poorly defined group, 'C', yields a mean declination of 185-degrees and inclination of 9-degrees, with the corresponding palaeopole situated on the c. 1500-1600 Ma part of the APWP. Two other sites yield palaeopoles located in the 1200 Ma part of the path. The data imply that the Sveconorwegian orogeny caused a remagnetization of the two older dyke generations and that this remagnetization was complete in the southern part of the PZ, while a few dykes retained their original magnetization in the central part of the Zone. Either the difference between the 'A' and 'B' directions record appreciable APW movement during differential cooling of the PZ, or the 'A' and 'B' directions may represent an asymmetric reversal of the Earth's magnetic field. In the latter case they may be contemporary and have implications for the size and form of the Sveconorwegian Loop of the Fennoscandian APWP.