Ion microprobe baddeleyite and zircon ages for Late Archaean mafic dykes of the Pilbara Craton, Western Australia

Ion microprobe U-Th-Pb analyses of baddeleyite and zircon yield precise age s for several mafic intrusions in the Pilbara Craton of Western Australia. Baddeleyite was dated from four dolerite dykes of the north-northeast-trend ing Black Range swarm intruded into granitoid-greenstone basement in the no rthern part of the craton. The mean Pb-207*/Pb-206* age of 2772 +/- 2 Ma, i nterpreted as an unambiguous age of emplacement for the dykes, is within er ror of previous ion microprobe U-Pb zircon ages for the Mt Roe flood basalt s and confirms that the dykes acted as feeders to the volcanic rocks. The S ylvania Inlier, in the southeastern Pilbara Craton, also contains north-nor theast-trending dykes that were correlated previously with the Black Range swarm. Based on concordant and discordant zircon analyses from samples of t wo dykes, the best estimate of the age of the Sylvania dykes is 2747 +/- 4 Ma. The Sylvania dykes thus appear to be significantly younger than, and he nce unrelated to, the Black Range swarm, but may have acted as feeders to y ounger volcanic units in the Fortescue Group such as the Kylena Formation.