A SIMS U-Pb (zircon) and Re-Os (molybdenite) isotope study of the early Paleozoic Macquarie Arc,southeastern Australia: Implications for the tectono-magmatic evolution of the paleo-Pacific Gondwana margin |
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| Institution: | 1. Université Côte d''Azur, CNRS, OCA, IRD, Géoazur, 250 rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France;2. EDYTEM, Université de Savoie Mont Blanc– CNRS, UMR, 5204 Le Bourget du Lac, France;3. ISTerre, Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, IFSTTAR, Grenoble, France;4. Ifremer, Geosciences Marines, Centre de Brest, 29280 Plouzané, France;5. Université de Lyon, Université Jean Monnet and UMR-CNRS 6524, Laboratoire Magmas et Volcans, 42023 Saint-Etienne, France;1. Programa de Pós-Graduação em Geologia (PPGL), Instituto de Geociências, Universidade Federal do Rio de Janeiro, Brazil;2. Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil;3. Programa de Pós-Graduação em Geologia, Instituto de Geociências, Universidade de Brasília, Brasília, Brazil;1. School of Earth and Planetary Sciences, Curtin University, GPO Box U1987, Perth, WA 6845, Australia;2. Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa;3. Future Industries Institute, University of South Australia, Mawson Lakes, SA 5125, Australia;4. Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou 510275, China;5. Centre for Exploration Targeting, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;6. Earth Research Institute, University of California, Santa Barbara, CA 93106, USA |
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| Abstract: | New SIMS U-Pb (zircon) data for intrusive rocks of the Macquarie Arc and adjacent granitic batholiths of the Lachlan Orogen (southeastern Australia) provide insight into the magmatic and tectonic evolution of the paleo-Pacific Gondwana margin in the early Paleozoic. These data are augmented by Re-Os dates on molybdenite from four Cu-Au mineralised porphyry systems to place minimum age constraints on igneous crystallization. The simplicity of the zircon age distributions, and absence of older inheritance, stands in contrast to previous geochronological studies. The earliest magmatism within the Macquarie Arc is registered by a ca. 503 Ma gabbro from the Monza igneous complex, whereas a monzodiorite from the same drillhole records the youngest (ca. 432 Ma). Igneous activity in the Macquarie Arc thus overlapped deformation and magmatism in the craton-proximal Delamerian Orogen to the west, and the emplacement of the Lachlan granitic batholiths at 435–430 Ma; the thermal pulse associated with the latter may have triggered the formation of richly mineralised Silurian porphyries in the Macquarie Arc. The juvenile Hf isotope signature of the Monza Gabbro, together with the lack of zircon inheritance and the radiogenic Hf-Nd isotope systematics of Ordovician Macquarie Arc rocks, is consistent with early development of the arc, or a precursor magmatic belt, in an oceanic setting remote from continental influences, and with the arc being built on primitive Cambrian mafic crust. Outboard arc magmatism in the Cambrian may have initiated in response to convergent Delamerian orogenesis adjacent the Gondwana margin. Overlapping radiogenic isotope-time trends are consistent with the evolution of the Macquarie Arc and the Gondwana continental margin being linked from the Cambrian to the Silurian. These data provide further evidence for the growth of continental crust along the southeastern Australian segment of this margin being related to the dynamics of an extensional accretionary orogenic system. |
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