Prolonged high thermal gradient metamorphism in the Curnamona Province, south-central Australia, during the latter stages of Nuna assembly

Abstract

Granulite-facies metasedimentary rocks from the Broken Hill Block of the Curnamona Province, south-central Australia, record a prolonged period (c. 1630–1550 Ma) of high thermal gradient metamorphism and anatexis. We present monazite and zircon U–Pb geochronological and REE geochemical data from garnet-bearing paragneisses of the Willyama Supergroup, coupled with garnet trace element mapping and mineral equilibria modelling, to develop a petrochronological model for the Palaeo–Mesoproterozoic metamorphic evolution of this region. Our results indicate that the Curnamona Province underwent an early phase of high-temperature–low-pressure metamorphism between c. 1630–1610 Ma. Peak metamorphism involving wholesale anatexis subsequently occurred under an apparent thermal gradient of 150–170 °C/kbar at c. 1600–1580 Ma during regional compression. Monazite U–Pb dates reveal that suprasolidus conditions were maintained for at least 30 Myr, with final melt crystallisation potentially occurring as late as c. 1550 Ma, indicating that the system remained hot for an extended duration. We suggest that high thermal gradient metamorphism in the Willyama Supergroup was driven by elevated levels of radiogenic heat production hosted within the Willyama succession. Temperatures may have been initially boosted by low-to-moderate degrees of crustal extension, although there is a dearth of coeval magmatism and structural evidence to support this claim. Peak metamorphism coincides with the timing of compressional orogenesis and related metamorphism in other Proterozoic terranes that are inferred to have occupied the eastern margin of proto-Australia. Many of these terranes also exhibit prolonged metamorphic histories and elevated levels of radiogenic heat production, suggesting that radiogenic heating may have had a major influence in pre-disposing these terranes to be the locus of metamorphism and deformation during the final assembly of Nuna.