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Elements of the Archean thermal history and apparent polar wander of the eastern Kaapvaal Craton, Swaziland, from single grain dating and paleomagnetism

Abstract:

Single grains of zircon, hornblende, biotite and feldspar have been dated to define the thermal history of the Archean Mbabane Pluton, Swaziland. Coincident 207Pb 206Pb zircon and 40Ar 39Ar hornblende ages suggest that the pluton underwent rapid cooling to about 450°C at the time of emplacement at ∼ 2690 Ma. Because the Mbabane Pluton is one of a suite of granites which represent the last major Archean intrusive event in the eastern Kaapvaal Craton, this time of emplacement marks the end of cratonization. Much younger biotite and feldspar ages indicate argon loss, perhaps due to several low-temperature events related to dike intrusion at 2000-2300 Ma. From such a thermal history, the age of magnetization of a paleomagnetic pole (pole latitude = 19.7°N, pole longitude = 105.7°E, A95 = 9.4°, K = 67) obtained from 5 sites can be determined. Based on the unblocking temperature of the magnetic carrier and the consistent direction seen in several sites, it appears that the hornblende age (and coincidently the 207Pb 206Pb zircon age) most accurately represents this age of magnetization. Thus, the Mbabane Pluton yields a precisely dated Archean (2687 ± 6 Ma (2σ), hornblende plateau age) paleomagnetic pole which is different than other Archean poles such as that from the nearby Usushwana Complex (2875 ± 40 Ma). Although the two intrusions are separated by less than 12 km and there is no evidence of relative structural rotation between them, the paleomagnetic poles from the two bodies are 65° apart. This implies that there was significant apparent polar wander during the late Archean. © 1989.