Cooling history and age of magnetization of a deep intrusion : A new 1.7 Ga key pole and Svecofennian-post Svecofennian APWP for Baltica
Summary, in English
A paleomagnetic and chronogical study has been performed on the Turinge gabbro-diabase formation and on a cross cutting basic dyke in central Sweden and on the Joulovaara gabbro intrusion in northern Sweden in the Fennoscandian Shield. U-Pb age of baddeleyite and 40Ar/39Ar ages of hornblende and biotite reveal a cooling history of the deep gabbro-diabase intrusion in Turinge. The cooling is suggested to have taken place in two stages, one related to the time of intrusion in temperature down to ca 500 °C with a cooling rate up to 46–59°/Ma and another at a lower rate of ca 2.9 °C/Ma, which is suggested to be related with uplift. From this cooling history it can be concluded that the magnetization age of the diabase, ca 1695–1700 Ma, is close to the crystallization age and the 40Ar/39Ar age of hornblende. Applying a similar cooling history for the other studied deep intrusion, the ca 1800 Ma gabbro of Joulovaara gabbro, it is estimated that the magnetization age of the gabbro should be close to that of the U-Pb age of the formation, although the pole of the Joulovaara gabbro is less reliable. The cooling history presented here for the Turinge gabbro-diabase has implications for estimations of magnetization ages also for other deep intrusions. The new pole (Plat. = 51.6°, Plon. = 220.2°; A95 = 4.8°) of the Turinge gabbro-diabase passes most of the reliability criteria and is considered a new key pole for Fennoscandia. The Basic dyke that cuts the Turinge gabbro-diabase was here dated at ca 1200 Ma (whole rock, 40Ar/39Ar) and the virtual geomagnetic pole calculated from its primary magnetization falls into the expected trend of APWP for Baltica. The new Turinge key pole prolong the time of overlapping poles for Fennoscandia, indicating only small movements of the shield between ca. 1870 to 1700 Ma.
- ISSN: 0301-9268