Anders Scherstén

Detrital zircon age and Hf isotope patterns for the Cahill Formation and Nourlangie Schist are distinctly different from other Paleoproterozoic successions in the North Australian Craton. The Cahill Formation and Nourlangie Schist comprise the bulk of the Paleoproterozoic strata in the Nimbuwah Domain, the easternmost part of the Pine Creek Orogen on the northern margin of the North Australian Craton. They comprise micaceous and quartzofeldspathic schist, carbonaceous schist, calc-silicate rock, amphibolite and quartzite, deformed and metamorphosed during emplacement of the granitic to dioritic Nimbuwah Complex at 1867-1857 Ma. The Cahill Formation hosts several world-class uranium deposits including Ranger, Jabiluka and Nabarlek. U-Pb SHRIMP and LA-SF-ICPMS detrital zircon spectra for four samples of the Cahill Formation and six samples of the Nourlangie Schist show a similar broad spectrum of ages mainly in the range 3300-1900 Ma. A ubiquitous dominant peak at 2530-2470 Ma matches the age of underlying Neoarchean basement, but is distinct in its dominantly mantle-like Hf and O zircon isotopic character, which shows closer similarity with possible source rocks from the Gawler Craton or alternatively from the Dharwar Craton. Common smaller age peaks occur at 2180 Ma, 2080 Ma and 2020 Ma. The first two have no known magmatic age correlatives in the North Australian Craton. Zircons of the 2020 Ma peak have distinctively unradiogenic Hf and elevated delta O-18, at variance with local rocks of this age but similar to detrital zircon of the same age from the Gawler Craton. In contrast to younger Proterozoic sedimentary successions within the Pine Creek Orogen, which contain ubiquitous ca. 1870 Ma detritus, the detrital spectra for the Cahill Formation and Nourlangie Schist contain almost no ca. 1870 Ma detritus. A maximum deposition age of ca. 1866 Ma indicates deposition within error of intrusion of the Nimbuwah Complex. We propose that the Cahill Formation and Nourlangie Schist were deposited at a plate margin immediately prior to convergent tectonism. This resulted in their burial, deformation and amphibolite facies metamorphism during orogenesis associated with the Nimbuwah Event. These findings have implications for understanding the Paleoproterozoic evolution of the Pine Creek Orogen within the context of northern Australia. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.