U-PB ZIRCON CHRONOLOGY OF PROGRADE PROTEROZOIC EVENTS IN THE CENTRAL AND SOUTHERN PROVINCES OF THE ARUNTA BLOCK, CENTRAL AUSTRALIA

A series of conventional and ion-microprobe U-Pb zircon ages broadly confirm the timing previously proposed from Rb-Sr dating for three major tectonic episodes that fashioned the Arunta Block of central Australia. Ages of 1763(-1)+2 Ma, 1760 +/- 11 Ma and 1754 +/- 9 Ma are derived for the Strangways Event, the oldest widespread tectonothermal activity to have affected the Central Tectonic Province. The two youngest values constrain the age of the second phase of folding within the Strangways Event in the central western part of the Central Tectonic Province to 1760-1750 Ma. The Southern Tectonic Province is demonstrably younger. Assuming that the analysed sample is representative, most of the currently exposed meta-igneous rocks originally crystallized at 1660 +/- 4 Ma in an event immediately preceding the Chewings phase of the Aileron Event, dated by Rb-Sr at 1590 +/- 70 Ma. This ca 1660 Ma succession of the Southern Arunta Province is younger than the youngest unit (Division 3) in the Reynolds Ranges region of the Northern Arunta Province. In the Reynolds Ranges this youngest unit is overprinted by ca 1730 Ma deformation and metamorphism, and correlates with the Hatches Creek Group dated at ca 1820-1810 Ma. The present study has strengthened the case for a major terrane boundary between the Central and Southern Arunta Provinces. Differences in the geoehronological and deformational histories between the two provinces are confirmed. Furthermore, the dated granite bodies alongside the province boundary appear to have unusual chemical characteristics, unlike those of the dominant granite suites of comparable age in the Proterozoic of northern Australia. The dominantly thermal Ormiston Event, which was centred on the Southern Tectonic Province, occurred at 1137(-7)+9 Ma, possibly in response to the addition of mafic magmas to the crust. A comparison of the U-Pb data obtained by different analytical approaches is instructive. None of the three examples where both techniques have been applied show precise agreement between the ion-microprobe and initial conventional results. This was eventually obtained for one sample by the abrasion of fractions prior to conventional analysis. The differences possibly result from the accidental presence of exotic zircon xenocrysts in bulk fractions used for conventional analyses. The ion-microprobe allows realistic assessment of the accuracy of the generally more precise age estimates produced by conventional analysis.