The growth of the Zimbabwe Craton during the late Archaean: an ion microprobe U-Pb zircon study

U-Pb zircon ages obtained by ion microprobe are presented for late Archaean granitoids from the Zimbabwe Craton and the North Marginal Zone of the Limpopo Belt. Similar ages of crust formation in the felsic gneisses of the Zimbabwe Craton and the felsic granulites of the North Marginal Zone support the view, previously argued on geological grounds, that they represent the middle and lower crustal layers respectively of the Zimbabwe Craton. Zircons from the southern margin of the Zimbabwe Craton and the northern part of the North Marginal Zone contain old cores, confirming the presence of an old (c. 3.5 Ga) nucleus to the Zimbabwe Craton and indicating that the areal extent of this old nucleus is greater than has been hitherto supposed. Thus late Archaean crustal growth involved both the reworking of and the addition of new magmas to this old crustal nucleus. At 2.7 Ga crustal growth was contemporaneous with plume volcanism in the greenstone belts. In contrast, crustal growth at 2.74, 2.67, 2.64 and ? 2.62 Ga was focused at the margins of the 'old craton' and may be related to Andean style continental margin magmatism. Lower crustal melting at 2.61-2.59 Ga led to the emplacement of granitoids in the upper crust.