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

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 487Ma 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 1.4Ga. Large-scale Pb isotopic homogenization, as proposed for the Lachlan fold belt did not occur in the Adelaide fold belt.