The age of the Kambalda greenstones resolved by ion-microprobe - Implications for Archaean dating methods

Details The age of the Kambalda greenstones resolved by ion-microprobe - Implications for Archaean dating methods The age of the Kambalda greenstones resolved by ion-microprobe - Implications for Archaean dating methods

Jul 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988e%26psl..89..239c&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 89, no. 2, July 1988, p. 239-259.

Mathematics

Logic

35

Basalt, Geochronology, Magma, Petrology, Sediments, Australia, Geological Surveys, Ion Probes, Lead Isotopes, Uranium Isotopes

Scientific paper

A 'chert' horizon within the Archaean basalt pile at Kambalda, Western Australia, contains zircons from air-fall tuffs erupted as the sediment was laid down. Ion-microprobe analyses of these magmatic zircons constrain the long-sought age of the Kambalda lavas to 2692 + or - 4 Ma (2 sigma), and show that regional metamorphism and deformation of the greenstone pile occurred within 30 Ma of deposition. Rare zircon xenocrysts are evidence of pregreenstone crust as old as 3450 Ma, exposed remnants of which are yet to be identified. Subsets of both the magmatic and xenocryst zircons show evidence of hitherto-unsuspected low-temperature U and/or Pb mobility in the Proterozoic, long after stabilization of the craton. The ion-microprobe data unravel complexities of isotopic inheritance and alteration, illustrating the hazards of using bulk or whole-rock samples in Archaean isotopic investigations. Identification of interflow 'cherts' as a precisely dateable rock type opens the way to direct ion-probe dating of mafic-ultramafic piles, and close numerical age control of the complex, but ill-exposed, stratigraphic and structural relations in this richly-mineralized greenstone belt.

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