East Antarctic sources of extensive Lower–Middle Ordovician turbidites in the Lachlan Orogen, southern Tasmanides, eastern Australia

ABSTRACT

Lower to upper Middle Ordovician quartz-rich turbidites form the bedrock of the Lachlan Orogen in the southern Tasmanides of eastern Australia and occupy a present-day deformed volume of ∼2–3 million km³. We have used U–Pb and Hf-isotope analyses of detrital zircons in biostratigraphically constrained turbiditic sandstones from three separate terranes of the Lachlan Orogen to investigate possible source regions and to compare similarities and differences in zircon populations. Comparison with shallow-water Lower Ordovician sandstones deposited on the subsiding margin of the Gondwana craton suggests different source regions, with Grenvillian zircons in shelf sandstones derived from the Musgrave Province in central Australia, and Panafrican sources in shelf sandstones possibly locally derived. All Ordovician turbiditic sandstone samples in the Lachlan Orogen are dominated by ca 490–620 Ma (late Panafrican) and ca 950–1120 Ma (late Grenvillian) zircons that are sourced mainly from East Antarctica. Subtle differences between samples point to different sources. In particular, the age consistency of late Panafrican zircon data from the most inboard of our terranes (Castlemaine Group, Bendigo Terrane) suggests they may have emanated directly from late Grenvillian East Antarctic belts, such as in Dronning Maud Land and subglacial extensions that were reworked in the late Panafrican. Changes in zircon data in the more outboard Hermidale and Albury-Bega terranes are more consistent with derivation from the youngest of four sedimentary sequences of the Ross Orogen of Antarctica (Cambrian–Ordovician upper Byrd Group, Liv Group and correlatives referred to here as sequence 4) and/or from the same mixture of sources that supplied that sequence. These sources include uncommon ca 650 Ma rift volcanics, late Panafrican Ross arc volcanics, now largely eroded, and some <545 Ma Granite Harbour Intrusives, representing the roots of the Ross Orogen continental-margin arc. Unlike farther north, Granite Harbour Intrusives between the Queen Maud and Pensacola mountains of the southern Ross Orogen contain late Grenvillian zircon xenocrysts (derived from underlying relatively juvenile basement), as well as late Panafrican magmatic zircons, and are thus able to supply sequence 4 and the Lachlan Ordovician turbidites with both these populations. Other zircons and detrital muscovites in the Lachlan Ordovician turbidites were derived from relatively juvenile inland Antarctic sources external to the orogen (e.g. Dronning Maud Land, Sør Rondane and a possible extension of the Pinjarra Orogen) either directly or recycled through older sedimentary sequences 2 (Beardmore and Skelton groups) and 3 (e.g. Hannah Ridge Formation) in the Ross Orogen. Shallow-water, forearc basin sequence 4 sediments (or their sources) fed turbidity currents into outboard, deeper-water parts of the forearc basin and led to deposition of the Ordovician turbidites ∼2500–3400 km to the north in backarc-basin settings of the Lachlan Orogen.

KEYWORDS:

• Detrital zircon dating
• U–Pb
• Hf isotopes
• turbidites
• sediment sources
• Antarctica
• Lachlan Orogen
• Tasmanides

Acknowledgements

We thank Elena Belousova, Cameron Quinn, Ian Percival and Michael Bruce for discussions and assistance. Sample analysis was funded by a Macquarie University External Collaborative Research Grant in a collaborative project with the Geological Survey of NSW, New South Wales Trade and Investment, where Glen worked before his retirement. Use of QEMSCAN scanning technology, now owned by FEI, was facilitated by Alan Butcher in a trial survey with the GSNSW. U–Pb data from zircons in Mum1 were kindly analysed by Yoann Greau. Zircons from Murrawombie1 and TrittonNE were collected with Straits Resources and Phil Jones, and analysed by Elena Belousova, with permission to use the data kindly given by Straits Resources through Chris Raymond. Samples from the Wagga Group and the Castlemaine Group were collected with Jeff Vassallo, the latter with assistance from Fons VandenBerg. The Gnalta Shelf samples were collected with Kingsley Mills, while sample Te2 was located by Steve Trigg. Collection of the BUN samples was based on mapping by Trangie Johnston and paleontology by Ian Percival. We thank Ross Cayley and Gordon Packham for their helpful reviews, especially Gordon who in his painstaking first review asked questions and suggested changes, which encouraged us to restructure our paper and to assemble more detailed U–Pb and Hf data from potential source regions. This is contribution 875 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and contribution 1125 from the GEMOC National Key Centre (www.gemoc.mq.edu.au). It is also a contribution to IGCP project 648 on ‘Supercontinent Cycles and Geodynamics.’ The analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS/AuScope, industry partners and Macquarie University.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary Papers

Appendix 1. Additional information on zircon populations.

Appendix 2. Analytical procedures and methods.

Appendix 3. T_DMc graphic plots plots for different samples and clusters.

Appendix 4. Terminology of zircon-age populations.

Appendix 5. Supplements the text summary of published detrital zircon data from sedimentary sequences in the Ross Orogen.

Appendix 6. U–Pb data: Excel table.

Appendix 7. ϵ_Hf data: Excel table.

Additional information

Funding

Macquarie University External Collaborative Research Grant [grant number A005405], and a collaborative project with the Geological Survey of New South Wales, NSW Trade and Investment.