Palaeomagnetism of Proterozoic dykes from Bornholm, Denmark

• Published in: Geophysical Journal International
• Main Authors: Abrahamsen, N., Lewandowski, M.
• Format: Text
• Language: English
• Published: Oxford University Press 1995
• Subjects: Research Note; Fennoscandia
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/121/3/949; https://doi.org/10.1111/j.1365-246X.1995.tb06450.x

The island of Bornholm (Denmark) is a structural horst situated at a kink on the margin of and linking two major fault zones, the Sorgenfrei-Tornquist Zone (STZ) and the Tornquist-Teisseyre Zone (TTZ), the fault zones of which separate the downfaulted Danish-Polish Basin from the Baltic Shield. Thirteen dolerite dykes, out of a swarm of more than 200, all belonging to the supposed Precambrian N-S to NE-SW striking dyke swarm on Bornholm, have been sampled and palaeomagnetically investigated. The dykes vary in thickness between 0.2 and 60 m and are intruded in Precambrian gneisses and granites of Gothian and/or Subjotnian (1650–1280 Ma) age. Principal component analysis of detailed AF and some thermal demagnetization experiments reveal three groups of stable direction clusterings from the dykes investigated. The stable remanent magnetization of the major dyke Keldseå dyke and the baked gneiss contact corresponds to a palaeomagnetic mean pole (group A). Comparing it with the APW curve for Fennoscandia, an initial cooling age for this dyke of around 1360 Ma (Subjotnian age) is suggested. The result is confirmed by dual polarities and a positive baked contact test. Nine other dykes show a magnetization corresponding to a younger group of poles (group B), which cluster on the APW path around 1200 Ma (Jotnian age). This result is also confirmed by a positive contact test. Two poles (group C) fall on the APW path around 950 Ma (Sveconorwegian age). Remaining directions display a distribution of poles on the sphere, which suggest the presence of either late Proterozoic-Cambrian overprints or injections of some younger dykes. Based upon a total of 16 Bornholm dykes hitherto studied, the polarity frequency ratio N/R is 7/9. The remanent magnetizations and equivalent palaeomagnetic poles found in the Bornholm dyke swarm thus appear to indicate at least three different periods of igneous activity with dyke injections in early Proterozoic time for this otherwise homogeneously looking group of dykes from the margin of the ...