SHRIMP baddeleyite age for the Fraser Dyke Swarm, southeast Yilgarn Craton, Western Australia

Recent high-resolution aeromagnetic data have delineated an extensive swarm of undeformed northeast-trending dolerite dykes in the southeastern Yilgar n Craton, known previously only from isolated exposures in surface mining o perations. Owing to parallelism of the dykes to the Fraser Mobile Belt, the eastern segment of the Albany-Fraser Orogen, the swarm Is referred to here as the Fraser Dyke Swarm, lon-microprobe dating of baddeleyite from a gran ophyric segregation in the centre of one dyke yields a mean Pb-207/Pb-206 a ge of 1212 +/-10 Ma (95% confidence limits). The location of the Fraser Dyk e Swarm, adjacent and parallel to the Fraser Mobile Belt, suggests that the dykes may have been emplaced into lines of weakness that originated during tectonic loading and downwards flexure of the craton margin. This is the f irst evidence of ca 1210 Ma mafic dykes and associated crustal-scale extens ion in the southeast Yilgarn Craton, although the age is similar to those r eported recently for dolerite and quartz diorite dykes in the central and s outhern part of the craton, suggesting that a genetic relationship may exis t between intrusions in the two areas.