Palaeomagnetism of the Egersund-Ogna anorthosite, Rogaland, Norway, and the position of Fennoscandia in the Late Proterozoic
The 930-Ma Egersund-Ogna anorthosite is part of the Rogaland igneous complex of massif-type anorthosites intruded into Sveconorwegian basement of southwestern Norway. Average susceptibilities on samples from 13 sites range from 0.03 to 2.24 × 10-3 SI and natural remanent intensities range from 0.004...
| Published in: | Geophysical Journal International |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
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Oxford University Press
2004
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| Subjects: | |
| Online Access: | http://gji.oxfordjournals.org/cgi/content/short/158/2/479 https://doi.org/10.1111/j.1365-246X.2004.02349.x |
| Summary: | The 930-Ma Egersund-Ogna anorthosite is part of the Rogaland igneous complex of massif-type anorthosites intruded into Sveconorwegian basement of southwestern Norway. Average susceptibilities on samples from 13 sites range from 0.03 to 2.24 × 10-3 SI and natural remanent intensities range from 0.004 to 1.54 A m-1. Corresponding Koenigsberger ratios range from 3 to 148 with a mean value of 36, indicating that magnetic field surveys will observe remanent-controlled magnetic anomalies. Optical observations indicate that haemo-ilmenite is the major oxide phase present, with lesser amounts of ilmeno-haematite and rare magnetite. Remanent directions from all samples are characterized by steeply negative inclinations with variable northwest declinations. Thermal demagnetization reveals square-shouldered demagnetization curves with little or no loss of intensity until 550 or 575 °C. Alternating field demagnetization produces a wide range of demagnetization behaviours with mean destructive fields varying from less than 5 mT to greater than 80 mT. There is little evidence for overprinting or secondary components, and all information points to a remanence acquired during initial cooling of the anorthosite at ~ 900 Ma. Mean directional data for the 13 sites are <f> </f> and <f> </f>. The magnetic pole calculated for Egersund-Ogna is at <f> </f> latitude and <f> </f> longitude <f> </f>, in excellent agreement with an earlier pole determined from Egersund, and similar to other poles from Rogaland igneous complex rocks. This work supports Rodinia reconstructions that place Baltica at high (southern) latitude, ~ 70°, at about 900 Ma. Apparent polar wander paths for Baltica at this time are ambiguous and it is difficult to discriminate between proposed clockwise or counter-clockwise loops. |
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