SM-ND MINERAL ISOCHRON AGES OF LATE PROTEROZOIC DYKE SWARMS IN AUSTRALIA - EVIDENCE FOR 2 DISTINCTIVE EVENTS OF MAFIC MAGMATISM AND CRUSTALEXTENSION

Two distinctive events of widespread dyke intrusion have been identifi ed in central-southern Australia. Crystallization ages of the dykes we re determined using Sm-Nd mineral isochrons obtained by splitting clin opyroxene and plagioclase into different magnetic and density fraction s, respectively. The Stuart Dyke Swarm in the southern Arunta Inlier a nd the Kulgera Dyke Swarm in the eastern Musgrave Inlier define essent ially identical crystallization ages of 1076 +/- 33 and 1090 +/- 32 Ma , respectively, marking the first episode of post-orogenic mafic magma tism in the region. Two samples from the Gairdner Dyke Swarm in the St uart Shelf yield Sm-Nd mineral isochron ages of 86 7 +/- 47 and 802 +/ - 3 5 Ma, respectively, whilst two samples from the Amata suite in the central Musgrave Inlier, 790 +/- 40 and 79 7 +/- 49 Ma. These four ag es represent the second episode of mafic magmatism in central-southern Australia. These ages allow new constraints to be placed on the timin g of large-scale crustal extension and formation of intracratic deposi tional basins in the region. Initial epsilon(Nd)-values of -7.8 to -6. 6 for the Stuart dykes and +0.7 for the Kulgera dykes suggest they wer e derived from different mantle sources with the Stuart dykes being pr oduced from strongly enriched sources. In contrast, positive and relat ively uniform initial epsilon(Nd)-values of +2.5 to +4.1 for the young er Amata Suite and Gairdner dykes indicate that they were derived from relatively depleted and homogeneous mantle sources. The 1076 +/- 33-M a Sm-Nd age of the Stuart Dyke Swarm is significantly older than its R b-Sr mineral isochron age of 897 +/- 9 Ma previously reported, suggest ing mobility of Rb and/or Sr and partial resetting of Rb-Sr isotopic s ystematics. This study demonstrates that reliable and relatively preci se Sm-Nd ages can be obtained for Precambrian dykes, despite the limit ed number of mineral phases available in the dykes.