Qing Zhang1, 2, Allen Nutman1, Solomon Buckman1
1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
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 to 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. The new results confirm that the Macquarie Arc was initiated far from the continent with no continental contamination, most likely via outboard (eastward) subduction at high dip angle. The arc started colliding with the eastern Gondwana during the Late Ordovician (~456 Ma), indicated by the trench-fill sedimentary protoliths of the Triangle Formation. Preservation of juvenile island arc on continental margins is aided by outboard subduction that results in emplacement of the arc complex as a klippe in an upper plate position on top of the passive margin sequence, instead of an autochthon extending deep to the mantle, amalgamated with the continent through a back-arc closure.
These results enriched the knowledge of continental growth of eastern Gondwana that it involved the episodic addition of juvenile oceanic terranes via east-dipping subduction, and emphasized that the detrital zircon ages could record the process. By establishing the arc chronology via these zircons is a major contribution to understanding the geodynamic setting of this Paleozoic arc-related copper mineralization along the Pacific margin.
Qing Zhang is a postdoc at the Institute of Geology and Geophysics, Chinese Academy of Sciences (2020.7-), working with the SIMS research group. She is interested in understanding the evolution of orogens, with particular interests in using zircon U-Pb ages and Hf-O isotopes methods.