Pre-emplacement Re-Os ages from ultramafic inclusions in the sublayer of the Sudbury Igneous Complex, Ontario

A S Cohen, O M Burnham, Chris Hawkesworth, P C Lightfoot

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    20 Citations (Scopus)

    Abstract

    Re-Os isotope analyses have been performed on a suite of 14 melanorite inclusions from the Sublayer of the Sudbury Igneous Complex (SIC) at Whistle Mine, 30 km NNE of Sudbury, Ontario, Canada. Six S-poor olivine melanorites have relatively unradiogenic present day Os-187/Os-188 ratios of 0.16-1.2 and define an imprecise age of 1.97 Ga. Their Os-187/Os-188((1.85)) ratios of 0.107-0.237 are compatible with derivation either from the mantle at the time of SIC formation 1.85 Ga ago, or from pre-existing crustal rocks with low Re/Os ratios and unradiogenic Os isotope compositions. In contrast, the relatively S-rich inclusions define much older ages of 2.46 Ga (melanorites) and 1.67 Ga (olivine melanorites), while Os-187/Os-188((1.85)) ratios of 0.41-1.80 unequivocally demonstrate that their Os was largely derived from pre-existing crustal lithologies. The Os isotope compositions acid S abundances of the inclusions are consistent with mixing at 1.85 Ga between an unradiogenic silicate fraction defined by the S-poor suite and a radiogenic sulphide-rich component with Re/Os ratios similar to those of pre-existing crustal lithologies. Attempts to evaluate the source of the Ni, Cu, S and PGE in the SIC through mass-balance calculations are not definitive because of the difficulties in estimating accurately both present-day and pre-SIG crustal abundances. However, crude estimates suggest that the overall levels of Ni and Cu in the pre-impact crust were probably sufficient to account for the budget of these elements in the present-day SIG, but do not rule out the possibility of significant late-Archaean/early-Proterozoic sulphide mineralisation in the pre-existing SIC target rocks. (C) 2000 Elsevier Science B.V. All rights reserved.

    Original languageEnglish
    Pages (from-to)37-46
    Number of pages10
    JournalChemical Geology
    Volume165
    Issue number1-2
    Publication statusPublished - 4 Apr 2000

    Keywords

    • Sudbury Igneous Complex
    • rhenium
    • osmium
    • ultramafic inclusions
    • impact
    • platinum group elements
    • ISOTOPE SYSTEMATICS
    • MASS-SPECTROMETRY
    • OSMIUM
    • ORIGIN
    • GEOCHEMISTRY
    • GEOCHRONOLOGY
    • DEPOSITS
    • RHENIUM
    • CANADA
    • ORE

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