In the Willyama Supergroup of the Curnamona Province, the intensity of granulite facies Olarian metamorphism at c. 1600 Ma has long hampered efforts to elucidate the timing and nature of preceding lower-grade metamorphic events. Previous dating of pre-peak monazite inclusions in aluminous metapelites has been complicated by potentially incomplete isotopic "shielding" by their host porphyroblasts, owing to pervasive deformation and recrystallisation of these rock types during the subsequent c. 1600 Ma event. Near the Broken Hill Pb-Zn orebody, quartz-feldspar-biotite-garnet "Potosi-type" gneiss proved more resistant to granulite facies metamorphism: new Sensitive High Resolution Ion Micro Probe (SHRIMP) U-Pb analyses of monazite delineated a population of relatively Y-rich (median 0.96 wt%) cores at 1636.8 ± 4.4 Ma (95% confidence), which are overprinted by Y-poor (median 0.10 wt%) rims and domains at 1597.3 ± 3.3 Ma. The role of garnet as a sink for Y in metamorphic rocks suggests that Y-rich c. 1637 Ma monazite growth predated garnet formation, and possibly occurred at relatively low metamorphic grade; conversely, the Y-poor nature of the c. 1597 Ma rims and domains indicates that monazite dissolution and reprecipitation during Olarian metamorphism postdated porphyroblastic garnet growth. Coexisting low-Th/U metamorphic zircon rims are dominated by a population at 1600.7 ± 3.6 Ma, emphasising that the event responsible for c. 1637 Ma monazite growth was largely invisible to zircon. A concordant mafic granulite body 70 m away yielded partially recrystallised zircons which feature dark-CL internal domains with elevated U and Th/U, which establish a minimum age of 1639 ± 8 Ma for magmatic crystallisation of the igneous precursor. Recrystallisation was accompanied by actinide depletion, with bright-CL, U-poor and low-Th/U domains dated at 1596 ± 8 Ma. This pair of 1640–1635 Ma dates constitutes robust isotopic evidence for pre-Olarian mafic magmatism and metamorphism within the lower Willyama Supergroup. We infer an extensional environment in the upper mid-crust for this tectonothermal activity, coeval with ongoing sedimentation and deposition of the upper Paragon Group at the surface. Extension of the eastern Curnamona Province at c. 1640 Ma is coeval with sediment-hosted base-metal mineralisation in the Mount Isa Inlier and the McArthur Basin, and is consistent with tectonic regimes prevailing throughout Proterozoic northeastern Australia at that time.
Citation: S. Bodorkos, J.A. Fitzherbert, Timing and significance of pre-Olarian metamorphism in the Willyama Supergroup, Broken Hill, Australia, Precambrian Research, Volume 378, 2022, 106772, ISSN 0301-9268, https://doi.org/10.1016/j.precamres.2022.106772.
