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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 εNd 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. [ABSTRACT FROM AUTHOR]

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of ∼9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive Nd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee - Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee - Narromine Volcanic Belt have ages that cluster in the 470 - 460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calc-alkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley - Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee - Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley - Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian - Devonian opening of the Hill End Trough. Phase 3 magmatism in the Macquarie Arc is represented by a widespread but relatively small volume magmatic event, dominated by shallow intrusive rocks of the Copper Hill Suite, emplaced in the Eastonian - Bolindian, between 456 and 441 Ma. These distinctive porphyritic dacites and associated holocrystalline diorites and granodiorites show medium-K calc-alkaline compositions, and their emplacement was intimately linked to an episode of regional uplift, erosion and limestone deposition in the Junee - Narromine Volcanic Belt and western Molong Volcanic Belt. Phase 4 magmatism extended from late Eastonian or Bolindian until Early Silurian time, and was dominated by relatively evolved (compared with Phase 2 lavas) shoshonitic lavas until the end of the Bolindian and porphyries in the Early Silurian. Collision-related shut-down of the arc, and initiation of arc extension and dismemberment, occurred around 438 Ma in the latest Ordovician. Post-arc magmatism during the Early Silurian is represented by high-Th, high-Nb lavas of the shoshonitic Nash Hill Volcanics in the Junee - Narromine Volcanic Belt, and Alaskan-type zoned ultramafic intrusions of the Fifield complexes farther west. The latter were emplaced through deformed Ordovician turbidites of the Girilambone Group, and their radiogenic isotope signatures show significant crustal involvement. [ABSTRACT FROM AUTHOR]