Geochemistry and age of magmatic rocks in the unexposed Narromine, Cowal and Fairholme Igneous Complexes in the Ordovician Macquarie Arc, New South Wales

Abstract

Much of the northern and southern sections of the Junee – Narromine Volcanic Belt of the Ordovician Macquarie Arc in central-western New South Wales are buried beneath the sediment cover of the Great Artesian Basin. Exploration drilling of these aeromagnetically defined blocks has provided important new material to assess the temporal and magmatic affinities of the rocks in the Narromine Igneous Complex (northern end) and the Cowal and Fairholme Igneous Complexes (southern end). Basement rocks are representative of Phase 1 magmatism in the Macquarie Arc, and consist of Lower Ordovician basalts and andesites and common volcaniclastic rocks, all with high-K calc-alkaline affinities. These are very similar compositionally to the Lower Ordovician Nelungaloo Volcanics that outcrop in the central part of the Junee – Narromine Volcanic Belt west of Parkes. Intruding the basement volcanic – volcaniclastic package are three distinct igneous suites. The oldest of these, dated at ca 466 – 460 Ma (Middle Ordovician) and representative of Phase 2 magmatism in the Macquarie Arc, consists of stocks of monzogabbro, monzodiorite and monzonite with high-K calc-alkaline affinities, and is well represented in the Narromine and Cowal Igneous Complexes. Phase 2 suite rocks were, in turn, intruded in the Bolindian (445 ± 5 Ma) by kilometre-size stocks, narrow sheets and dykes made up of hornblende gabbro, diorite and granodiorite, and including quartz + plagioclase + hornblende-phyric dacitic porphyries in the Narromine Igneous Complex. These medium-K calc-alkaline rocks, assigned to the Phase 3 Copper Hill Suite that also occurs in the Molong and Rockley – Gulgong Volcanic Belts further east, crystallised from andesitic or more evolved magmas, probably derived via partial melting of low-K rocks in the arc basement, or amphibolites in the subducting slab. The youngest rocks in these igneous complexes are believed to be the high-level shoshonitic intrusives in the Cowal and Fairholme Igneous Complexes, which are correlated on petrographic and compositional grounds with the Macquarie Arc Phase 4 (Bolindian and Llandovery) magmatic products best represented by the Northparkes Igneous Complex in the central part of the Junee – Narromine Volcanic Belt.

key words:

• Cowal Igneous Complex
• Fairholme Igneous Complex
• geochemistry
• geochronology
• Macquarie arc
• Narromine Igneous Complex

Acknowledgements

This work was supported by an ARC SPIRT grant. Considerable assistance in accessing core material was provided by Bruce Mowat and Piers Reynolds (Narromine) and Stuart Smith, John Holliday and Fraser McCorcqudale (Cowal and Fairholme). We acknowledge useful input by Paul McInnes and the staff at Barrick Gold's Cowal exploration office. Phil Robinson, Sarah Gilbert and Katie McGoldrick assisted substantially with obtaining analytical data. Evgeni Bastrakov provided several whole-rock analyses and dates discussed in our study, Dick Glen provided useful comments and editorial assistance, and David Forster and Oliver Raymond made many useful points in their reviews of this paper.