CONSTRAINTS ON THE AGE OF THE WARRAWOONA GROUP, EASTERN PILBARA BLOCK, WESTERN-AUSTRALIA

Zircons from mafic and felsic volcanic rocks in the type area of the Warrawoona Group, the basal Archaean greenstone succession of the eastern Pilbara Block, have been dated precisely using the ion-microprobe SHRIMP. The results allow two alternative time-frames for the duration of the Warrawoona Group dependent on how the dated zircons are considered to relate to the volcanic rocks. Our favoured interpretation requires a hiatus of 135 +/- 5 Ma between the Duffer Formation at 3.46 Ga and the overlying felsic volcanic rocks of the Wyman Formation, and a hydrothermal or later magmatic origin for zircons of age 3.33 Ga within one Duffer Formation sample and the underlying metabasalts. The alternative time-frame requires a short time for deposition of the entire Group. less than 15 Ma at approximately 3.33 Ga, and a xenocrystic origin for the 3.46 Ga zircons of the Duffer Formation. Outside the type area of the Warrawoona Group, the age of an intrusive granodiorite requires that greenstones be older than 3.43 Ga and the Group formed over an interval of > 120 Ma. Visibly different zircons within one of the Duffer Formation samples were found to be Palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock al low temperature. Similar zircons were found within samples from other rock units but with a spread of Proterozoic ages.