Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen

The Ordovician Macquarie Arc is faulted against and surrounded by coeval Gondwana-derived quartz turbidites, in the Lachlan Orogen of southeastern Australia. How these juvenile, island arc rocks were emplaced and structurally interleaved with continental margin sequences of eastern Gondwana is an ongoing debate. Understanding the inception of this arc is critical in building an accurate Early Paleozoic reconstruction for eastern Gondwana. However, the arc's inception and early evolution are poorly-constrained due to the lack of reliable geochronological data. Integrated field observations, zircon U-Pb-Hf, mineral and whole rock geochemistry are presented here for the oldest units of the Molong Volcanic belt, which is a central strand of the dissected Macquarie arc. Geochemistry indicates that these are calc-alkaline rocks with high-K (locally shoshonitic) to medium-K affinities. The stratigraphically-lowest mafic volcanic and volcano-sedimentary rocks of the Mitchell Formation have a unimodal earliest Ordovician zircon U–Pb age of 479.8 ± 3.8 Ma, with uniform depleted mantle like initial zircon εHf values of +12 to +13. The lack of pre-Ordovician zircons and the uniform positive initial εHf value indicate that the Macquarie arc was initiated in an intra-oceanic setting, far from the influence of eastern Gondwana. The stratigraphically overlying Fairbridge Volcanics includes some more evolved volcano-sedimentary components and has a polymodal, but mostly Ordovician, zircon U–Pb age population, with a youngest component at 444.3 ± 2.4 Ma. Overall, its Ordovician grains show a greater spread in initial εHf values from +14 down to +8 than the Mitchell Formation, which suggests some continental influence, at least 35 million years after the arc was initiated. The lack of any significant Gondwanan inheritance in Early Ordovician volcaniclastic rocks of the Macquarie Arc along with geochemical comparisons with modern island arcs, suggest that the arc evolved outboard of Gondwana probably as a result of steep, easterly-directed subduction. As the Macquarie arc approached eastern Gondwana there would have been increasing quantities of continent derived sediment subducted beneath the arc which may account for the decreasing and more variable zircon initial εHf values and the appearance of sparse Precambrian continental detritus at the arc's moribund stage. The collision of the Macquarie island arc with Gondwana initiated the Benambran Orogeny and reflects an important mechanism of episodic continental growth involving the addition of juvenile oceanic crust to a continental margin, which contrasts and alternates with periods of purely accretionary growth.