THERMOCHRONOMETRY AND PALEOMAGNETISM OF THE ARCHEAN NELSHOOGTE PLUTON, SOUTH-AFRICA

Pb-207/Pb-206 single-grain zircon, Ar-40/Ar-39 single-grain hornblende and biotite, and Ar-40/Ar-39 bulk-sample muscovite and biotite ages f rom the Nelshoogte trondhjemite pluton located in eastern Transvaal, S outh Africa, show that this granitoid had a protracted thermal history spanning 3213+/-4 Ma to about 3000 Ma. Whole-rock Ar-30/Ar-39 ages fr om cross-cutting dolerite dykes indicate that these were intruded at a bout 1900 Ma. There is no evidence of this or other, later events sign ificantly affecting the argon systematics of the minerals from the plu ton dated by the Ar-40/Ar-39 method. The pluton has a well-defined pal aeomagnetic pole which is dated at 3179+/-18 (2 sigma) Ma by Ar-40/Ar- 39 dating of hornblende. This pole (18 degrees N, 310 degrees E, A(95) =9 degrees) yields a palaeolatitude of 0 degrees, significantly differ ent from other Archaean poles from the Kaapvaal Craton. The palaeolati tude difference implies that there was significant apparent polar wand er during the Archaean. A second, overprinting magnetization seen in t he pluton is also seen in the lower-Proterozoic dolerite dykes, and is consistent with other lower-Proterozoic (2150-1950 Ma) poles for sout hern Africa.