New rock magnetic and paleomagnetic results for the 1.64 Ga Suomenniemi dyke swarm, SE Finland

Abstract

This study represents new petrophysical and paleomagnetic data obtained for the 1.64 Ga Suomenniemi diabase and quartz porphyry dyke swarm associated with Suomenniemi rapakivi granite. We also present petrophysical data for the coeval dykes of the Sipoo (1.63 Ga) and Häme (1.642 ± 0.002 Ga; 1.647 ± 0.014 Ga) swarms in order to address: (1) the discrepancy between the coeval Sipoo and Häme poles and (2) the proposed severe overprint in paleomagnetic data for rapakivi related dyke swarms in Fennoscandia.Demagnetization of Suomenniemi dyke samples revealed dominantly normal polarity magnetization carried by magnetite. For the first time, a few samples from the few sites have shown a trace of a reversed magnetization. The combined normal (N) polarity direction from nine dykes (=sites) is at D = 031.2°, I = 07.2°, (with α95 = 10.6°, k = 21.8) yielding a paleomagnetic pole at Plat = 27.8°N; Plong = 171.7°E (with A95 = 7.9°, K = 38.4). The Suomenniemi pole fulfils four of the seven quality criteria for paleomagnetic poles (Van der Voo, 1990). The new pole for the Suomenniemi swarm is similar to the coeval Sipoo swarm pole, but distinctly different from the nearly coeval Häme swarm pole. The relative position of the poles compared to other well-defined paleomagnetic poles for Baltica indicates that the Suomenniemi and the Sipoo poles are slightly younger than the Häme pole.We show here that the original magnetization in some of the 1.64 Ga dykes in Suomenniemi have been overprinted with a secondary magnetization component, but several dykes of the swarm lack this overprint. Moreover, the presumably primary Subjotnian remanence component for the Suomenniemi dykes can be clearly isolated due to its different coercivity and/or unblocking temperature from the overprinted component. The same applies to the Sipoo and Häme dyke swarms.