Proterozoic crustal growth in the southeastern Gawler Craton: the development of the Barossa Complex, and an assessment of the detrital zircon method

Abstract

The Barossa Complex, southeast Gawler Craton, South Australia, forms the southeastern-most exposure of pre-Neoproterozoic crust in Australia. Understanding the geodynamic evolution of this area can improve paleogeographic reconstructions of the economically significant Gawler Craton, as well as global reconstructions in the Proterozoic. The first part of this thesis addresses the geological development of the Barossa Complex during the Palaeo-Mesoproterozoic. The Barossa Complex is composed of metasedimentary and metaigneous gneisses. These include calcsilicate, quartzofeldspathic, psammopelitic, and pelitic gneisses. In the northern inliers, the protoliths to these gneisses are indicative of a progressively deepening basin. Syndepositional felsic orthogneisses and mafic amphibolites indicate a tectonically active basin. Deposition of the metasedimentary protoliths to the Barossa Complex occurred between 1730-1655 Ma, synchronous to the onset of the Kimban Orogeny in the Gawler Craton and the deposition of the Willyama Supergroup in the Curnamona Province. U-Pb and Hf isotopic analyses from detrital zircon indicates sediment was largely derived from the Gawler Craton. Syndepositional granite intrusions occurred in the northern extent of the Barossa Complex at 1717 ±7 Ma. Metamorphism initiated in the Barossa Complex at c. 1630 Ma with the development of a low angle metamorphic fabric. Peak granulite conditions of approximately 8-9 kbar and 800-850 °C occurred at c. 1590 Ma in the southern Barossa Complex. The northern Barossa Complex preserves lower grade metamorphic features and c. 1600 Ma zircon with hydrothermal Rare Earth Element (REE) signatures, which are potentially linked to the Hiltaba event in the Gawler Craton. Post peak metamorphism continued until c. 1550 Ma and is associated with retrograde shear zones in the southern Barossa Complex, and late pegmatites in the northern inliers. The Barossa Complex shares a depositional and metamorphic history with the Willyama Supergroup in the Curnamona Province and Mt. Isa Inlier basin sequences, and was part of a transcontinental plate margin system during the Late Palaeo- Early Mesoproterozoic. East dipping subduction was the likely driver for extensive rift basin development across the eastern margin of Proterozoic Australia before the Isan-Olarian Orogeny inverted these basins. The Barossa Complex is the southern-most exposure of this system. The second part of this thesis addresses the use of detrital zircon in modern sediment as a means of characterising the bedrock of a catchment area, which has been used previously in the Gawler Craton and Curnamona Province. In the Broken Hill area of the Curnamona Province, stream sediments were sampled from drainage pathways with catchments that have stratigraphically and chronologically well characterised bedrock lithologies. Zircon ages from the modern sediment found up to 30% of the zircons were significantly younger than what expected from the bedrock sources (>1.6 Ga). Aeolian dune sands from the Strzelecki Desert to the north of the study area are found to contain zircon with U-Pb and Lu-Hf isotopic compositions matching the ‘exotic’ zircon populations in Broken Hill. Aeolian detritus is considered to have contributed zircon to the stream sediments in Broken Hill, and should be considered in any study utilising modern detritus in arid environments. Detrital zircon provenance studies of the geological record should be interpreted cautiously if aeolian input may have occurred.