Proving the age of a paleomagnetic pole: The case of the Ntonya Ring Structure, Malawi

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Abstract
Geological application of paleomagnetic data generally relies on equating radiometric or biostratigraphic age with that of the characteristic remanence. Yet the standard of proof of paleomagnetic age is often unsatisfactory and is extremely difficult to achieve where contact tests or conglomerate tests are unavailable. In this study of the Ntonya igneous ring complex the thermoremanent (TRM) origin of one component of natural remanent magnetization (NRM) is confirmed by paleointensity determination and its carrier is identified mineralogically, and the age of this TRM is estimated by extrapolation from 40Ar/39Ar mineral ages. The characteristic remanence in the Ntonya Ring Structure, which was determined by Briden (1968) as D = 311° I = +43° α95 = 1.9°, pole 28°N 15°W, is shown to be a TRM in single‐domain/pseudo‐single‐domain magnetite, acquired in a field of approximately half the present geomagnetic field strength. The 40Ar/39Ar mineral ages (biotites 471.5 ± 7 Ma; hornblendes 510 ± 7 Ma) and the high precision of the paleomagnetic directions combine in a model of steady cooling (∼ 4.6° m.y. −1) that leads us to the conclusion that the thermoremanence was acquired over approximately 13 m.y., 522 ± 13 m.y. ago. In passing it is shown that the isothermal (IRM) component that dominates the remanence is carried by multidomain magnetite. The study shows how remanence age may be determined with precision in the absence of geological constraints. It makes the Ntonya pole a key point in the African (and Gondwanan) apparent polar wander path, correlating with Middle Cambrian glacial deposits in the Sahara at about 80°S paleolatitude.

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