Lu-Hf isotope systematics of the Hadean-Eoarchean Acasta Gneiss Complex (Northwest Territories, Canada)

Martin Guitreau, Janne Blichert-Toft, S. Mojzsis, Antoine S G Roth, Bernard Bourdon, Nicole L. Cates, Wouter Bleeker

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Abstract

The Acasta Gneiss Complex (AGC) is a remnant Hadean-Eoarchean terrane composed of strongly deformed polyphase mafic to felsic gneisses which preserve a multi-stage history of magmatic emplacement, inheritance, and subsequent tectono-thermal modifications. The complexities encountered in such an old terrane fragment have been documented in previous geochronological studies of the AGC (e.g. zircon U-Pb, 147Sm-143Nd), and are evident also in its Lu-Hf isotope systematics. Here, we report new Lu-Hf isotope whole-rock measurements which show that some AGC gneisses were severely disturbed by migmatization and associated mineral segregation, while others preserve their Lu-Hf isotope systematics relatively intact with mostly near- to sub-chondritic initial 176Hf/177Hf ratios. Results reveal identifiable Eoarchean and later (Paleoarchean) magmatic events at around 3960Ma and again at 3600Ma, with a major metamorphism of the complex at 3750Ma. The oldest and least disturbed gneisses have a Lu-Hf regression age of 3946±87Ma, in good agreement with U-Pb zircon geochronology. A role of yet older crust (4000-4200Ma) in the formation of the AGC is also evident, but seems not to have influenced to first order the Lu-Hf isotope systematics of the 3960Ma group. The ca. 3960Ma group is proposed to be representative of its mantle source based on the absence of correlation between εHf(t) and Ce/Pb. It is further suggested that these two parameters show that the ca. 3600Ma gneisses were sourced in part from a mafic lithology belonging to the 3960Ma group, and that multiple sources (mantle and crust) were involved in AGC formation. The identification of preserved Lu-Hf isotope systematics in AGC gneisses means that complementary geochemical and isotopic studies bearing on the petrogenesis of pre-3900Ma rocks are possible. Despite its history of strong deformation and alteration, carefully selected domains within the AGC carry surviving information about the evolution of the mantle-crust system at the Eoarchean-Hadean boundary.

Original languageEnglish
Pages (from-to)251-269
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume135
DOIs
Publication statusPublished - Jun 15 2014

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Hadean
Isotopes
gneiss
isotope
Bearings (structural)
crust
Rocks
Geochronology
mantle source
terrane
Lithology
zircon
migmatization
Minerals
history
petrogenesis
geochronology
rock
lithology
emplacement

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

Lu-Hf isotope systematics of the Hadean-Eoarchean Acasta Gneiss Complex (Northwest Territories, Canada). / Guitreau, Martin; Blichert-Toft, Janne; Mojzsis, S.; Roth, Antoine S G; Bourdon, Bernard; Cates, Nicole L.; Bleeker, Wouter.

In: Geochimica et Cosmochimica Acta, Vol. 135, 15.06.2014, p. 251-269.

Research output: Contribution to journalArticle

Guitreau, Martin ; Blichert-Toft, Janne ; Mojzsis, S. ; Roth, Antoine S G ; Bourdon, Bernard ; Cates, Nicole L. ; Bleeker, Wouter. / Lu-Hf isotope systematics of the Hadean-Eoarchean Acasta Gneiss Complex (Northwest Territories, Canada). In: Geochimica et Cosmochimica Acta. 2014 ; Vol. 135. pp. 251-269.
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