Early Ordovician development of the Macquarie Arc, Lachlan Orogen, New South Wales

Abstract

The first recorded igneous activity of the now-fragmented Macquarie Arc in central New South Wales began in Early Ordovician (Lancefieldian – Bendigonian) times. In the westernmost Junee – Narromine Volcanic Belt, mafic to intermediate lavas of the Nelungaloo Volcanics, intruded by monzonite at 481 Ma, pass up into sandstone, conglomerate and siliceous siltstone of the Lancefieldian to early Bendigonian Yarrimbah Formation. Further east, in the northern Molong Volcanic Belt, the Mitchell Formation consists of volcaniclastic conglomerate, sandstone and siltstone with minor lavas and is succeeded conformably by volcaniclastic granule conglomerate, sandstone and siltstone of the middle – late Bendigonian Hensleigh Siltstone. This earliest phase of volcanism in the Macquarie Arc is dominated by high-K calc-alkaline to shoshonitic evolved basalt, basaltic andesite and andesite, but felsic lavas with SiO₂ >63% are absent. Age-corrected ϵ _Nd values for five lavas range from +6.2 to +7.8, indicating an absence of any old continental crustal component in the petrogenesis of these volcanic rocks. Together, these data suggest formation of the Macquarie Arc as an intra-oceanic arc, probably built on Cambrian oceanic crust.

Key words:

• geochemistry
• Lachlan Orogen
• Macquarie Arc
• Ordovician
• petrology
• stratigraphy

Acknowledgements

We thank the landholders of the Parkes – Forbes district for access to their land; to sponsoring companies for access to data, ideas and drillcore, and Lawrie Sherwin for graptolite identification. Journal reviews by Ollie Raymond and Carol Simpson assisted in clarification of our ideas and improved the final version of the paper. LMB, RAG and IGP publish with permission of the Deputy Director-General, New South Wales Department of Primary Industries – Mineral Resources.