Ordovician marginal basin evolution near the palaeo-Pacific east Gondwana margin, Australia

Abstract

Although the Ordovician Macquarie Arc, part of the eastern Lachlan Orogen of southeastern Australia, has long been considered to be an intra-oceanic arc within an accretionary orogen, key characteristics contrast with more typical examples of accreted arcs. Significantly, multiple stacked phases of mafic to intermediate volcanic rocks, with suprasubduction-zone chemistry, are flanked to the east and west by extensive, coeval continental margin turbidite, chert and black shale sequences. By analysing stratigraphic and contact relationships within and between the volcanic rocks, ophiolitic components ( sensu lato ) and turbidite sequences, constrained by precise biostratigraphy, we document a repeated cycle of uplift, upper-plate extension and collapse common to all sequences. This cycle is interpreted as resulting from localized extension (rifting or hyper-extension and lower-crustal delamination) within a continent-margin sequence developed upon an already established marginal or back-arc basin of probable middle to late Cambrian age. Our interpretation provides a counter-example to prevailing arc accretion models by inferring extensional tectonics at the palaeo-Pacific east Gondwana margin during the Ordovician with development of alkalic and calc-alkalic Cu–Au porphyry deposits away from an active arc system.

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…12). This suggests that products of Phase 1 volcanism were either mixing with Early Ordovician quartz-rich turbidites, or that these turbidites were being eroded and redeposited around Phase 1 volcanoes, consistent with the discovery of chert breccias on the edge of the volcanics ϳ370 km to the south (Quinn et al 2014). In contrast, Phase 2 lavas and one of the sampled Phase 4 lavas show (virtually) unimodal age peaks that are consistent with inferred stratigraphic age (459 Ma for Phase 2 WAH1, Fairbridge Volcanics, locality 17 and 453 Ma for Phase 4 OR3 locality 18, Fig.…”

“…Several authors (e.g., Foster and Gray 2000;Collins and Hobbs 2001;Fergusson 2003Fergusson , 2014Gray and Foster 2004) regarded the Lachlan Orogen as the site of multiple coeval subduction zones from the Ordovician to the Silurian, an interpretation not favoured by Glen (2013) or Packham and Hubble (2016). Glen et al (1998) argued for Ordovician subduction in the Lachlan Orogen to account for the calcalkaline signature of Early to Late Ordovician volcanics (Macquarie Volcanic Province) that we now regard as inherited (Quinn et al 2014). The New England Orogen is the most outboard of orogens in the Tasmanides (Figs.…”

“…9). Provenance mixing is restricted to ϳ100 m of section in only one small area (Quinn et al 2014) (File S1 2 ).…”

“…13J-13L). However, the youngest concordant age of 432 Ma suggests it may be Silurian (Quinn et al 2014). …”

Proto-Pacific-margin source for the Ordovician turbidite submarine fan, Lachlan Orogen, southeast Australia: Geochemical constraints

“…A major orogenic event in the late Ordovician to mid Silurian, the Benambran Orogeny, resulted in substantial shortening, crustal thickening and calc-alkaline continental arc volcanism (Collins, 2002). However, the tectonic setting in which the turbidites were deposited is much debated, with models involving multiple subduction zones, along strike terrane translations, island arc -passive margin collision, and a complex backarc setting with anomalous mafic volcanism being proposed (Foster and Gray, 2000;Glen et al, 2009;Aitchison and Buckman, 2012;Quinn et al, 2014).…”