Reworking of Hadean crust in the Acasta gneisses, northwestern Canada: Evidence from in-situ Lu-Hf isotope analysis of zircon

Tsuyoshi Iizuka, Tsuyoshi Komiya, Simon P. Johnson, Yoshiaki Kon, Shigenori Maruyama, Takafumi Hirata

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    Abstract

    The Acasta Gneiss Complex of northwestern Canada comprises early Archean orthogneisses and includes the oldest known terrestrial rocks (3.94-4.03 Ga). Previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4.2 Ga in the oldest rocks, indicating that the source of early Archean granitoids contained Hadean (> 4.03 Ga) crust. In this study, we have determined the Lu-Hf isotopic compositions of zircon grains extracted from the early Archean Acasta gneisses to evaluate the extent of the Hadean crust contribution to the formation of these granitoids. Analyses of the Lu-Hf isotopes were carried out using in-situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains. Two ca. 3.59 Ga granitoids have εHf(T) of - 2.4 ± 2.2 and - 3.2 ± 2.5, suggesting that the source of the granitoids was extracted from the mantle as far back as 3.8 Ga. This is consistent with the presence of abundant zircon xenocrysts with ages up to 3.9 Ga. The Hf isotopic composition of a 3.72 Ga granitoid is significantly less radiogenic, with an εHf(T) of - 6.1 ± 2.5. This indicates that the granitoid formed by remelting of very old crust. The εHf(T) values for the two oldest rocks, the ca. 3.97 Ga tonalites, are - 1.2 ± 3.3 and - 3.3 ± 1.7, respectively, indicating that even the oldest known granitoids contain reworked older crustal materials. These results suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean Acasta granitoids.

    Original languageEnglish
    Pages (from-to)230-239
    Number of pages10
    JournalChemical Geology
    Volume259
    Issue number3-4
    DOIs
    Publication statusPublished - Feb 25 2009

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    Keywords

    • Ancient zircon
    • Crustal evolution
    • Crustal reworking
    • Hadean
    • LA-MC-ICPMS
    • Lu-Hf isotope

    ASJC Scopus subject areas

    • Geology
    • Geochemistry and Petrology

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