Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks,Ellsworth-Whitmore Mountains block,Antarctica

摘要

U-Pb detrital zircon geochronology from the upper Cambrian to Devonian part of the Ellsworth Mountains succession,Antarctica,yields dominant late Mesoproterozoic and late Neoproterozoic-Cambrian age populations that are consistent with a provenance from within Gondwana.Hf isotope compositions reveal a source predominantly within west Gondwana and identify a change in provenance up-stratigraphy that coincides with the change of sedimentation setting from active rift to passive margin,which has been independently determined by stratigraphic,structural,and geochemical arguments.For the Late Cambrian Frasier Ridge Formation,late Mesoproterozoic grains have positive epsilon_(Hf),values,suggesting derivation from juvenile crust,and late Neoproterozoic-Cambrian grains have epsilon_(Hf) values greater than-5,consistent with remelting of similar juvenile late Mesoproterozoic crust during the Pan Afri-can-Ross orogenies.Provenance during rifting was from proximal sources from within west Gondwana,most likely,southernmost Africa and basement to the Ellsworth-Whitmore Mountains block.At higher strati-graphic levels where deposition occurred along a passive margin,in the early Ordovi-cian Mount Twiss Member and middle Devonian Mount Wyatt Earp Formation,late Neoproterozoic-Cambrian grains have epsilon_(Hf) values less than-5;this means that early Meso-proterozoic-Archean crust was remelted to generate these zircons.Provenance was from a more expansive source region within west Gondwana,and probably included the Kaapvaal and Congo cratons of south and west Africa.Isolated outcrops of sedimentary rock of uncertain age at Mount Woollard and the Whitmore Mountains have detrital zircon signatures similar to the Frasier Ridge Formation,suggesting correlation with these Late Cambrian deposits.Sedimentary rock from the Stewart Hills contains some late Mesoproterozoic grains with lower epsilon_(Hf) values than the previously mentioned samples.This suggests that the Stewart Hills sample has a provenance from within east Gondwana and was possibly deposited on the East Antarctic craton prior to the Ross orogeny and is not part of the displaced Ellsworth-Whitmore Mountains crustal block.