Unravelling the Tumut Trough: A Middle Ordovician age for the Brungle Creek Metabasalt, eastern Lachlan Orogen

Bruce, Michael,1 Percival, Ian1, Zhen, Yong Yi1

1Geological Survey of New South Wales, W.B. Clarke Geoscience Centre, Londonderry, Australia

Alpine-type ultramafic bodies are exposed in numerous localities throughout the Lachlan Orogen of New South Wales. Despite the tectonic significance of such oceanic lithosphere to the development of the orogen, few studies on the genesis of these bodies have been documented.

The Coolac Serpentinite is an Alpine-type ultramafic intrusion that marks the eastern edge of the Tumut Trough in the eastern Lachlan Orogen. Recent petrological, geochemical and geochronological studies into the massive harzburgite (Bruce 2018) that makes up most of this body reject any ophiolitic association with rocks of the North Mooney Complex. These rocks are traditionally ‘lumped in’ as part of the proposed Coolac Ophiolite Suite, largely because of their physical location and resemblance to a layered, crustal ophiolitic sequence. Instead, a 2-stage melting model is proposed for the origin of the Coolac harzburgites with a late Cambrian latest melting event inferred from an allochthonous block (501 ± 2.6 Ma; U/Pb zircon) with petrological links to the harzburgites.

The ‘block’ has been incorporated into the Silurian Jackalass Slate within the trough, which was previously thought to be simple sedimentary trough fill, but is now partly interpreted as a sedimentary-matrix melange incorporating much older blocks. This interpretation is supported by blocks of chert found within the Jackalass Slate that contain conodont elements (Periodon aculeatus) of late Darriwilian to early Gisbornian age. Slightly older conodont assemblages with diagnostic elements of Periodon hankensis, indicating a late Dapingian to early Darriwilian age, are also found within chert lenses of the structurally underlying Brungle Creek Metabasalt.

Chert blocks within the Jackass slate and chert lenses within the Brungle Creek Metabasalt show near-identical, REE chondrite normalised abundances and patterns as well as evidence of significant terrigenous mafic volcanic and hydrothermal input. This implies that the Brungle Creek Metabasalt is coeval with chert deposition and is thus early Middle Ordovician in age (465−468 Ma). The presence of Cambrian basement, widespread cataclastite in and around the Brungle Creek Metabasalt and structurally underlying Bullawyarra Schist and identical chert units in the Brungle Creek Metabasalt and younger Jackalass Slate, as well as lenses of chert−volcanic clast conglomerate within the Brungle Creek Metabasalt, all support the structural interpretation of Stuart-Smith (1990), who suggested uplift, collapse and extension along a low-angle detachment fault. In addition, it is suggested that older, collapsed blocks have later been re-sedimented into younger Silurian basin strata.

Bruce M.C. 2018. Petrology, geochemistry and a probable Cambrian age for harzburgites of the Coolac Serpentinite, New South Wales, Australia. Australian Journal of Earth Sciences 65, 335−355.

Stuart-Smith P.G. 1990. Evidence for extensional tectonics in the Tumut Trough, Lachlan Fold Belt, NSW, Australia. Australian Journal of Earth Sciences 37, 147−167.


Michael Bruce currently works for the Geological Survey of NSW, Department of Regional NSW. Michael joined this organisation in 2005, having completed a PhD at the University of Queensland. His interest includes petrology and geochemistry of igneous rocks but is particularly focused on the genesis of mafic and ultramafic rocks.

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