Foden, John1, Tappert, Ralph1, Todd, Angas1, Segui, David1
1Department of Earth Sciences, University of Adelaide, Australia
Stretching from southern Africa to north east Australia the Late Neoproterozoic to Late Cambrian aged Delamerian – Ross Orogen formed at the rifted Rodinia break-up margin, facing the newly opened Pacific. The orogenic history of this margin reflected initiation of subduction of the Pacific plate. At the end of the Cambrian, along the entire belt, active convergent orogenesis was terminated abruptly by rapid exhumation, uplift and cooling. This event is recorded as a widespread regional Upper Cambrian unconformity from southern Africa across Antarctica and into Tasmania. Rapid erosion that resulted from this event produced latest Cambrian to Early Ordovician proximal and distal siliciclastic sediment deposits including conglomerates and fluvial sandstones as well as marine turbidites. These deposits include the South African Cape Supergroup, the Ross Orogen Carryer and Douglas Conglomerates, the Tasmanian Jukes and Owen Conglomerate and the western Victorian turbidites.
In South Australia Jurassic aged kimberlite intruded the Delamerian Orogen and transported an abundant population of mafic xenoliths ranging from garnet-pyroxenite (‘eclogite’) to pyroxenite and mafic granulite. Mineral assemblages include; Cpx-Gt-Rt, Cpx-Gt-Amp-Rt, CPX-Gt-Amp-Ky, Plag-Cpx-Gt± Amp, Ky, Il. These were sampled from lithospheric mantle recording pressures in the range 8 to 25 kbar. Exsolution of garnet and kyanite from Cpx provides evidence for cooling at constant or increasing pressure. Whole rock Nd isotopes yield an imprecise Late Neoproterozoic external isochron and their geochemical composition indicate that parental mafic magma was anorogenic rift-related tholeiite. Importantly the suite of samples forms clear compositional trends that show igneous ‘gabbroic’ pyroxene + plagioclase fractionation control even though many samples are now plagioclase-free.
The conclusion is that these were magmas produced during Rodinian rifting and breakup and formed underplated gabbros at Moho depth. Subsequent cooling to produce plagioclase-free, garnet and pargasite -bearing assemblages lead to increasing density and subsequent delamination resulting in buoyant crustal uplift and probably coupled with slab roll back led to orogenic termination. The common occurrence of high pressure pargasitic amphibole may implicate the role of hydrous flux from the subducting Pacific plate in catalysing high pressure cooling reactions in the mafic underplate. Critical to their density increase, P-T modelling of the pyroxenite bulk compositions indicates that at Moho depths (9kbar) cooling of the Neoproterozoic magmatic underplate would cross the garnet and pargasite-in reactions at 1000oC and the plagioclase-out reaction at 750oC. The time taken for initial mafic magmatic intrusions at 1350oC to cool to cross these important density increasing reactions at a Moho T of ~600oC is of the order of 5-15 m.y. Delamination may also promote local thermal convection leading to anomalous asthenospheric ascent and the production of post-tectonic magmas.
The style of orogenic termination described here forms a distinctive class and reflects subduction at the rift magma-rich margins of continental fragments formed during break-up of earlier continents. This orogenic style seems common to many belts formed during Gondwanan accretion.
Emeritus Professor John Foden is an igneous petrologist and geochemist with a lengthy history of teaching, research and post-graduate supervision at the University of Adelaide . His specialist interests include; magma generation and differentiation, modern subduction magmatism in the Indonesian Sunda Arc, the orogenic history of the early Palaeozoic margin of Australia, the use of Fe-isotopes in the interpretation of high temperature processes and kimberlite and diamonds.