Paleomagnetism of 2.44 Ga mafic dykes in Russian Karelia, eastern Fennoscandian Shield — implications for continental reconstructions

Abstract

Results of a paleomagnetic study of Paleoproterozoic dykes and Archean host rocks in the eastern Fennoscandian shield are reported here. Most of the dykes are related to ca 2440 Ma layered intrusions and have isotopic ages between 2349 and 2476 Ma. The dykes form an extensive NW–SE and E–W trending swarm. The majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature. The high temperature remanence component (D), which is thought to represent the primary 2.44 Ga magnetization of the dykes, has a paleomagnetic pole at 20°S, 278.8°E (A95=6.0°, N=11 sites). Another D-like component (D′) with shallower inclination (VGP 9.6°N, 256.2°E, eight samples), at one site from baked Archean host rock may also represent the primary remanence of the dykes. The commonest remanence component (A) records an extensive Svecofennian partial overprint and is interpreted to be ca 1.84 Ga old. It yields a paleomagnetic pole at 52.6°N, 226.8°E (A95=2.7°, N=23). The fourth component, B, has a paleopole at 53.9°N, 169.6°E (A95=6.3°, N=16) and is inferred to be ca 1.75 Ga on the basis of isotopic dating, although ages of 2.1, 1.3 and 0.3 Ga are also possible. Baked contact tests for the 2.44 Ga dykes are negative, as shown by the occurrence of component D in Archean host rocks beyond one dyke width. It is proposed that the negative baked contact test indicates regional reheating and remagnetization of the Archean basement at ca 2.44 Ga ago due to the spatially vast occurrence of coeval 2.44 Ga mafic dykes, layered intrusions, volcanics and granites in the area. Based on this hypothesis, it is suggested that component D in the dykes may be primary and acquired at ca 2.44 Ga ago. The similarity of pole D with poles from coeval dykes and layered intrusions elsewhere in the Karelian Province and Kola Peninsula suggest that, within the Archean domain of the Fennoscandian shield, no large scale block movements have taken place since 2.44 Ga. At 2.44 Ga the Fennoscandian Shield was located at the latitude of ca 30°S which is different to that at 1.84 and 1.75 Ga, when Fennoscandia was located at the latitudes of ca 30°N. A continental reconstruction based on pole D and the pole from the 2.45 Ga Matachewan dykes of the Superior craton in Laurentia, places the Kola Peninsula adjacent to southern Greenland. Reconstructions based on recent geological interpretations and a reconstruction yielding pole D′, are also discussed.