Age and isotopic evidence for the origin of the Archaean granitoid intrusives of the Johannesburg Dome, South Africa

Results of Rb-Sr, Ph-Ph and Sm-Nd whole rock, Rb-Sr biotite and Pb-Pb zirco n evaporation analyses are presented for certain granitoid rocks from the J ohannesburg Dome. These data indicate that the granodiorite, granite and le ucosome from migmatite were emplaced similar to 3090 Ma ago, were genetical ly related and were derived primarily from a source between similar to 3300 and similar to 3500 Ma old. A portion of the granodiorite and granite migh t have been derived from a source between similar to 4000 and similar to 43 00 Ma old. The tonalite was emplaced similar to 3170 Ma ago and was derived from a source between 3.3 and 3.5 Ga old. Rb-Sr biotite-whole rock ages, r anging between about similar to 2614 and similar to 2080 Ma, probably refle ct complete resetting during differential uplift, erosion and cooling of th e granitoid rocks in the Neoarchaean and Palaeoproterozoic. If so, they app arently were not influenced by the emplacement of the similar to 2060 Ma Bu shveld Igneous Complex or the similar to 2000 Ma Vredefort event. The grano diorite, granite and leucosome were emplaced coeval with and may be genetic ally related to compositionally similar plutonic and volcanic rocks in the Barberton area, Vredefort structure and Dominion Group. (C) 1999 Elsevier S cience Limited. All rights reserved.