Volcanology, geochemistry and structure of the Ordovician Cargo Volcanics in the Cargo – Walli region, central New South Wales

Abstract

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclase + clinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (>25) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P₂O₅ contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu – Au mineralised, plagioclase + hornblende + quartz-phyric dacites with medium-K calc-alkaline affinities. These dacites have lower TiO₂ and higher MgO contents at any SiO₂ level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites are locally abundant in volcaniclastic conglomerate near the top of the Cargo Volcanics, indicating that the dacites were intruded and exhumed prior to deposition of the Barrajin Group limestones, which commenced at ca 454 Ma in this area. The dacites at Cargo are intruded by small, apparently unmineralised monzonitic intrusions with shoshonitic affinities, which also appear to pre-date deposition of the Barrajin Group. LA-ICPMS U – Pb dating of detrital zircons from the Ranch Member at the base of the Daylesford Limestone (basal Barrajin Group) revealed a dominant population with an average age of 453.0 ± 4.1 Ma (identical within error to the mid-Ea1 age for the host sediments based on fossil assemblages) and subordinate populations with average ages of 480 Ma and 505 Ma. The 453 Ma zircons were most probably derived from either the intrusive dacites or monzonites, suggesting that the Cargo region was rapidly exhumed following emplacement of the felsic intrusions. The angular unconformities near and at the top of the Cargo Volcanics and the broadly coincident change in magma chemistry suggest that this sector of the Macquarie Arc underwent major tectonic upheaval commencing in the latest Gisbornian – early Eastonian. The Cargo Volcanics are interpreted to have undergone major eastward translation at this time (from an original position now buried beneath the Cowra Trough). Their present juxtaposition with the contiguous Molong Volcanic Belt probably did not occur until the late Early Silurian during the final stages of the Benambran Orogeny.

Key words:

• Cargo Volcanics
• geochemistry
• Macquarie Arc
• Ordovician
• stratigraphy
• stratovolcano

Acknowledgements

This project was funded by the Ordovician Macquarie Arc SPIRT project. In particular, we would like to thank Newcrest Exploration and Golden Cross for access to drillcore and field data, and Amy Libbiter for her BSc (Hons) work mapping the Canomodine Creek area. We also thank the owners of properties at Cargo who allowed us access, and John Shelley, Stuart Hayward and John Holliday for helpful discussions, and accessing data and maps. We thank Newcrest for providing accommodation, Pete Hollings and Chris Woodfull for helping with sampling the Walli Volcanics and Cargo Volcanics, respectively, and Robin Offler, David Foster and Ian Percival for useful discussion. Thorough reviews by Gordon Packham and Dick Glen substantially improved this paper.