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 SiO 2 463% are absent. Age-corrected e Nd values for five lavas range from þ6.2 to þ7.8, 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.

Tectonic evolution of the Ordovician Macquarie Arc, central New South Wales: arguments for subduction polarity and anticlockwise rotation

“…They are restricted to the Goonumbla region west and northwest of Parkes, in the Goonumbla-Trangie Volcanic Belt, and the Bakers Swamp area, mid-way between Molong and Wellington, in the Molong Volcanic Belt (Figure 2). These units are characterised by mainly high-K calc-alkaline igneous rocks but include shoshonites (Glen et al 2007b). Murray and Stewart (2001) also documented Early Ordovician chert in the southern Rockley-Gulgong Volcanic Belt that indicates deep-marine conditions and are overlain by undated mafic to intermediate volcaniclastic rocks.…”

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

“…This is consistent with new SHRIMP dates of ca 465 Ma (Middle Ordovician) for monzodiorites intruding the volcanicvolcaniclastic package in the Narromine Igneous Complex (see below). A trachyandesite clast from drillhole NACD20, and two probable dykes from NACD28 and NACD96, show strong compositional affinities with the trachyte -trachyandesite suites from the main Cowal mine development at E42 (Golden Lava Unit) and from a drill prospect in the south-central part of the block at E43 (Glen et al 2007b). This is true for both diagnostic trace-element ratios and REE patterns (Figure 4a).…”

Junee – Narromine Volcanic Belt, Macquarie Arc, Lachlan Orogen, New South Wales: components and structure

“…The listric fault in the top-right corner of Figure 10c (at *0.5 s under station 658) lies approximately downdip from the surface trace of the Parkes Thrust and is labelled accordingly. The seismic data thus allow the suggestion that there may be major stacking of Ordovician volcanic strata in the core of the Forbes Anticline: if so, this answers the question as to what lies below the basal Ordovician Nelungaloo Volcanics (Glen et al 2007a;Percival & Glen 2007).…”

“…They were most recently described by Simpson et al (2005) and are further described by Glen et al (2007a) and Percival and Glen (2007). They are referred to in Appendix 2 but not discussed further.…”