THE DISTRIBUTION OF 3.0-GA AND 2.7-GA VOLCANIC EPISODES IN THE YILGARN CRATON OF WESTERN-AUSTRALIA

The results of a conventional zircon U-Pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the Yilgarn Craton of Western Australia indicate that volcanism took place during two distinct episodes at ca. 3.0 and ca. 2.7 Ga. Each episode spans a time period of about 75 Ma and the two ages of volcanism are spatially separated. 3.0 Ga volcanic rocks are found in greenstone belts in the northern part of the Western Gneiss Terrain and in the Murchison Province, whereas 2.7 Ga volcanic rocks are present in the southern part of the Western Gneiss Terrain and in the Southern Cross and Eastern Goldfields Provinces. There is no evidence of inheritance in the zircon U-Pb systems which would support an older crustal origin for these rocks. Present results, together with previous studies of the ages of granites and gneisses, suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism. It is suggested that the evolution of the Yilgarn Craton took place in three main stages. The first was the development of a stable 3.4 Ga granitic nucleus, which incorporated earlier generations of granitoid, a 3.7 Ga layered complex and possibly 4.2 Ga primitive crust. This was followed by the two separate granite-greenstone episodes, at ca. 3.0 and ca. 2.7 Ga., involving the addition of significant amounts of new crust. Each episode was characterised by the accretion of extensive granite-greenstone terranes at the margins of the stable craton, the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems. © 1990.