Arc and mantle detritus in the post-collisional, Lower Silurian Kabadah Formation, Lachlan Orogen, New South Wales

Abstract

The Kabadah Formation outcrops in central New South Wales as a thrust package 66 km long, interleaved with Lower Silurian Canowindra Volcanics and situated between the Junee – Narromine and Molong Volcanic Belts of the Ordovician Macquarie Arc. The Kabadah Formation contains Early Silurian corals and Llandovery graptolites. Its provenance is complex, with detrital fragments of mafic – intermediate volcanic rocks, free crystals of pyroxene, chromite and ultramafic clasts, detrital volcanic quartz, garnet, and clasts of welded S-type rhyolitic volcanic rocks; and rare clasts from uplifted fold-belt rocks (granite and metamorphosed and deformed sediments). The variety of these clasts suggests that the Kabadah Formation records the Benambran collision of the Macquarie Arc with Ordovician quartz-rich sedimentary rocks, with detritus also derived from coeval Early Silurian mafic and felsic magmatism. The major source of detritus was from the short-lived emergent Fifield arc that formed from the subduction of an older backarc basin. The Kabadah Formation accumulated in an upward-shallowing Early Silurian marine basin between phases of the Benambran Orogeny.

Key words:

• Benambran Orogeny
• chromite
• Kabadah Formation
• Lachlan Orogen
• Macquarie Arc
• Silurian
• supra-subduction zone

Acknowledgements

Ervin Slanksy identified clinochlore by XRD as the dominant mineral in the altered peridotite clasts. This work was initiated as part of the Dubbo mapping and completed as part of the CODES-DMR SPIRT project. The manuscript benefited from comments by A. J. Crawford, G. Packham, L. Sherwin and an anonymous reviewer. LMB and RAG publish with permission of the Deputy Director General Primary Industries—Mineral Resources.