A reappraisal of the evolution of the palaeo-Pacific margin of Gondwana from the Pb and Os isotope systematics of igneous rocks from the southern Adelaide fold belt,South Australia |
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| Affiliation: | 1. Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia;2. Laboratorie de Géologie de Lyon, Ecole Normale Supérieure de Lyon, Université Claude Bernard Lyon 1, CNRS UMR 5276, 46 Allée d''Italie, 69007 Lyon, France;3. School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5000, Australia;1. State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, PR China;2. Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia;3. School of Earth Sciences and Resources, China University of Geosciences Beijing, No. 29, Xueyuan Road, Haidian District, Beijing 100083, PR China;1. Earth and Environmental Sciences, IKBAS, University of British Columbia, Okanagan, 3247 University Way, Kelowna, BC VIV 1V7, Canada;2. National Centre of Excellence in Geology, University of Peshawar, Peshawar 25120, KPK, Pakistan;3. Department of Earth Science, The University of Hong Kong, Hong Kong;4. Department of Earth Science University of California, Santa Barbara, CA 93106-9630, USA;1. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, Jiangsu, China;2. State Key Laboratory of Geological Processes and Mineral Resources, Collaborative Innovation Center for Exploration of Strategic Mineral Resources, Faculty of Earth Resources, China University of Geosciences, Wuhan 430074,China;3. Max-Planck-Institut für Chemie, Hahn-Meitnerweg 1, D-55128 Mainz, Germany;4. Lamont-Doherty Earth Observatory of Columbia University, P.O. Box 1000, Palisades, NY 10964, USA;5. Faculty of Land Resources and Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan, China |
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| Abstract: | We have undertaken Pb and Os isotopic analyses of igneous rocks from the southern Adelaide fold belt to further understanding of the Neoproterozoic – early Palaeozoic evolution of the palaeo-Pacific margin of Gondwana. The new Pb isotope data and a reappraisal of existing geochronological data suggest that layered mafic intrusions at Black Hill and Cambrai may have intruded much earlier than previously thought. Intrusion may even have occurred prior to deposition of the Normanville and Kanmantoo Group sediments and the onset of what is traditionally regarded as the Delamerian Orogeny. We tentatively term this earlier event the Hallett Cove orogeny. However, equivalent ages are common in the formerly adjacent Ross Orogen. When the Pb isotope signatures of the suites of gabbros, and I-, S-, and A-type granites are compared at 487 Ma they all span a very large range (206Pb/204Pbi = 15.4–18.7, 207Pb/204Pbi = 15.4–15.7, 208Pb/204Pbi = 33.0–38.8) compared with other formerly adjacent terranes or the subsequent Lachlan fold belt. Similarly, 187Os/188Osi ratios span 0.122–1.17 but include one gabbro and one rhyolite that have subchondritic 187Os/188Osi ratios. When taken together, the combined data do not support models in which the observed range in initial isotope ratios reflects major and variable mid to upper crustal assimilation of magmas derived from the contemporary asthenospheric mantle. Rather, it appears that magmas parental to the gabbros and A-type rocks were derived from subduction-modified lithospheric mantle that has a median metasomatic age of 1.4 Ga. Large-scale Pb isotopic homogenization, as proposed for the Lachlan fold belt did not occur in the Adelaide fold belt. |
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