Abstract: The East Antarctic Shield consists of a variety of Archean and Proterozoic to Cambrian high-grade terranes with distinct crustal histories that were amalgamated at various times in the Precambrian to Cambrian. At least six distinct Archean terranes or reworked relics of Archean crust separated by regions of Proterozoic or Cambrian tectonism are preserved around the East Antarctic margin, while the Ruker Terrane of the Southern Prince Charles Mountains occurs inland, again separated from other Archean areas by Proterozoic and Cambrian belts. The oldest Archean crustal relics, with ages greater than c.3300 Ma, are preserved in four terranes. The Ruker Terrane contains tonalite–trondhjemite–granodiorite rocks up to c.3390 Ma old and Nd isotopic evidence may have crustal sources as ancient as c.3900 Ma. The Archean of the Rauer Terrane contains composite layered tonalitic/mafic gneisses, inferred to be the sources to crosscutting tonalitic sheets, with zircon ages up to c.3460 Ma and Nd model ages of up to c.3750 Ma. Reworked Archean gneisses in the Rayner Province of Kemp Land, which most likely correspond to components present in the adjacent Napier Complex, were formed from protoliths at least as old as c.3460 Ma and up to c.3650 Ma in age, with Hf isotopes in zircon indicating crust formation ages as old as c.3900 Ma. The most extensive group of early Archean rocks occur in the Napier Complex, where tonalitic and granitic gneisses from Mount Sones and Gage Ridge retain protolith zircon ages of at least 3650 Ma and possibly c.3850 Ma. These, the oldest rocks found in Antarctica, contain some zircons with apparently near-concordant ages of c.3960–4060 Ma. The veracity of such ancient ages is uncertain given the recent demonstration of Pb clustering and mobility within Eoarchaean zircons from the Napier Complex, which have been strongly affected by its long-lived late Archean ultrahigh-temperature metamorphism. The nano- to micron-scale clustering of ancient Pb and formation, on annealing, of Pb melt nanospheres within such zircons leads to reverse discordance and potentially the overestimation of 207Pb/206Pb ages. The highly positive Hf isotopic compositions (ƐHf(3850) = +2 to +6) of the oldest zircons analyzed from the Napier Complex indicate crystallization in juvenile crust extracted from highly depleted mantle sources if the protolith age is 3850 Ma and would be more extreme if the measured 207Pb/206Pb ages of these zircons (3960–4060 Ma) are correct rather than spurious artifacts of later Pb mobilization. This demonstrates that Hf isotopic signatures of ancient zircons are optimally evaluated within the context of prior ion imaging that can establish the veracity of the associated age information recovered from such ancient Earth materials.
Publication Year: 2018
Publication Date: 2018-09-28
Language: en
Type: book-chapter
Indexed In: ['crossref']
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Cited By Count: 6
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