Inception, growth and collision of the Ordovician Macquarie Arc, Lachlan Orogen, Eastern Australia

The Ordovician intra-oceanic Macquarie Arc is preserved in a tectonostratigraphic terrane, faulted to the west and east against coeval, quartz-rich turbidites of the Adaminaby Group, within the Lachlan Orogen of eastern Australia. Debates exist concerning the allochthoneity of the Macquarie Arc, the polarity of its related subduction and the nature and exact timing of collision with Gondwana. These key problems are addressed by the integrated application of field observations, petrography, zircon U-Pb-Hf isotopes and whole rock geochemistry of key units within the Macquarie Arc stratigraphy. By these approaches, it has been possible to answer (i) the timing and juvenility of the arc initiation, (ii) the timing of arc-continent collision, and (iii) the allochthoneity and emplacement mechanism of the Macquarie Arc onto the eastern edge of Gondwana. A major problem in establishing this information is the low silica, mostly basaltic to andesitic character of the Macquarie arc rocks, meaning low yields of small zircons and thereby hampering radiometric dating.