THE AGE OF THE FORTESCUE GROUP, HAMERSLEY BASIN, WESTERN-AUSTRALIA, FROM ION MICROPROBE ZIRCON U-PB RESULTS

U-Pb zircon ages are reported for nine samples from the volcano-sedimentary Fortescue Group, the lowest of the three groups (Fortescue, Hamersley, Turee Creek) in the Hamersley Basin, and for a 10th sample from the Munni Munni Complex, which immediately underlies the basal unconformity of the Fortescue Group. Three widely separated felsic lava or tuff samples close to the base of the group give ages of 2775 +/- 10,2764 +/- 8 and 2763 +/- 13 Ma (all errors 2-sigma). Two other stratigraphically higher samples give ages of 2760 +/- 10 and 2756 +/- 8 Ma. The latter, from the Bamboo Creek Porphyry, is close to Pidgeon's (1984) published result of 2768 +/- 16 Ma for the related Spinaway Porphyry. The mean of all six results, 2765 Ma, is the best estimate of the age of an abrupt initiating volcanic event in the Fortescue Group; the (short) duration of this bimodal volcanic event is not precisely defined by these results, although an age of 2715 +/- 6 Ma for the youngest zircon population of the volcaniclastic Pillingini Tuff sets an upper stratigraphic limit. The age of 2925 +/- 16 Ma obtained from magnetite ferrogabbro pegmatite of the Munni Munni Complex is consistent with a major non-depositional interval immediately preceding Fortescue Group deposition at approximately 2765 Ma in the western part of the Pilbara Craton. Two widely separated samples of contemporaneous volcaniclastic rocks from the Jeerinah Formation, the uppermost stratigraphic unit of the Fortescue Group, have ages of 2690 +/- 16 and 2684 +/- 6 Ma; the mean of these, 2687 Ma, provides a good estimate of the age of the top of the group. From these results, the integrated depositional rate for the whole of the Fortescue Group lies in the range 40-120 m/Ma; during the initial 2765 Ma volcanic event local depositional rates may have been as high as 1000 m/Ma. By contrast, Compston et al.'s (1981) zircon age of 2490 +/- 20 Ma from the Dales Gorge Member of the Hamersley Group, some 650 m stratigraphically higher than the Jeerinah Formation, suggests that the depositional rate of the mainly chemical sediments of the Hamersley Group was between 3 and 4 m/Ma. These results collectively confirm that much of the Fortescue Group is older than the greenstones of the southern part of the Norseman-Wiluna Belt of the Yilgarn Craton, and demonstrate that there is no unique time within the Australian Precambrian marking the transition from granite-greenstone terrane to overlying platform cover.