Abstract

Igneous rocks record the direction of the Earth’s magnetic field as they cool through their Curie temperature. The mafic magmas of the 8-km-thick Bushveld Complex of South Africa took 65 k.y. to be emplaced, 180 k.y. to solidify (to 900 °C), and a further 500 k.y. for the entire intrusion to cool below 580 °C, the Curie temperature of magnetite. Once solid, the cooling of this intrusion occurred mainly from the top downward, with slower cooling through its floor. As a result, the upper rocks cooled through their Curie temperature before those at the base; the portion 6 km below the upper contact was the last to reach the Curie temperature. Thus, the intrusion records a mainly top-down sequence of three paleomagnetic reversals starting with N (normal direction). The last two are also recorded from the base of the mafic sequence upward as it cooled through 580 °C later than the top. The lateral variations in thickness of the Bushveld Complex are important in this interpretation, because thinner sections cooled more quickly. Hence, reversals do not always correlate with stratigraphy. Specific reversals provide a cooling marker horizon that may crosscut the stratigraphic layering. The interpretation of the order and number of paleomagnetic reversals presented here differs from previous interpretations that envisage the oldest paleomagnetic directions to be recorded sequentially from the base upward, and has implications for the interpretation of paleomagnetic results from all thick intrusions, mafic and felsic.

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