PRECISE ZIRCON U-PB AGES FROM THE MARRA MAMBA IRON-FORMATION AND WITTENOOM FORMATION, HAMERSLEY GROUP, WESTERN-AUSTRALIA

The Wit Bruce Supergroup of the Pilbara Craton of Western Australia wa s laid down in the Hamersley Basin, unconformably over a basement of g ranite-greenstone terrane. and consists of the Fortescue, Hamersley an d Turee Creek Groups. The base of the similar to 230 m-thick Marra Mam ba Iron Formation, the lowest formation of the Hamersley Group. marks the onset of the major banded iron-formation deposition that character ises the group. ion microprobe U-Pb isotope analyses of zircons from a tuff band (NS3) within a high-grade iron orebody in the uppermost (Mt Newman) member show two distinct, and non-overlapping, age population s. Fifteen grains from the younger group of 16 have a pooled age of 25 97+/-5 Ma (95% confidence), interpreted as the age of syndepositional volcanism: individual grains of the older group of 8, with ages betwee n ca 2950 Ma and ca 2820 Ma. are interpreted as xenocrysts derived fro m basement rocks transected by the rising magma. The conformably overl ying Wittenoom Formation, similar to 300-600 m thick. consists largely of dolomitic rocks. Zircons from a submarine fallout tuff within the uppermost (Bee Gorge) member also have two non-overlapping zircon age populations. Most of the 23 zircons analysed belong to an older popula tion with near-concordant ages between ca 2750 and ca 2650 Ma, but 12 analysed spots from 7 grains have a pooled age of 2561+/-8 Ma. This is interpreted to be the age of volcanism coeval with deposition. The ol der zircons were probably derived from tuffs of Fortescue Group age, e ntrained with the Crystal-rich Tuff ejectamenta during its explosive, probably phreatoplinian, eruption.