Zircon geochronology and partial structural re-interpretation of the late Archaean Mashaba Igneous Complex, south-central Zimbabwe

M. D. Prendergast, Michael T D Wingate

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    Abstract

    Ion microprobe (SHRIMP) U-Pb zircon dating has determined the age of a gabbroic enclave within the supraregional intrusive komatiitic Mashaba Igneous Complex (MIC) to be 2743 ± 11 Ma and that of an interpreted comagmatic thick differentiated komatiitic flow within the adjacent Mashava-Masvingo greenstone belt to be 2754 ± 13 Ma. Recent geological observations support a revised structural interpretation of the MIC as an assemblage of three separate horizontal layered sills emplaced almost coevally at two different crustal depths: the chromititiferous Prince Sill intruded concordantly, and prior to regional folding, into the basal sedimentary rocks of the late Archaean Upper Bulawayan Supergroup, the relatively thin and evolved West Sill within early Archaean greenstone and gneissic basement, and the major Northwest Arm-Main Sill mainly within adjacent basement gneiss. The sill assemblage was vertically linked via ultramafic dykes and, as previously proposed, was fed through the dyke-like Northeast Arm, whose overlying layered rocks are here interpreted as correlatives of the Northwest Arm-Main Sill. The new precise age for the MIC is 50 Ma years older than the former geological estimate of ∼2.7 Ga. The comparable date for the volcanic phase supports the interpretation of both as component phases of a putative Masvingo W komatiitic sill-flow complex but is incompatible with interpretation of the late Archaean lithostratigraphy of the Mashava-Masvingo belt as a thrust-stacked package of unrelated continental and oceanic rocks.

    Original languageEnglish
    Pages (from-to)585-596
    Number of pages12
    JournalSouth African Journal of Geology
    Volume110
    Issue number4
    DOIs
    Publication statusPublished - Dec 2007

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    sill
    geochronology
    Archean
    zircon
    lithostratigraphy
    greenstone
    ion microprobe
    enclave
    greenstone belt
    rock
    gneiss
    folding
    dike
    sedimentary rock
    thrust

    Cite this

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    title = "Zircon geochronology and partial structural re-interpretation of the late Archaean Mashaba Igneous Complex, south-central Zimbabwe",
    abstract = "Ion microprobe (SHRIMP) U-Pb zircon dating has determined the age of a gabbroic enclave within the supraregional intrusive komatiitic Mashaba Igneous Complex (MIC) to be 2743 ± 11 Ma and that of an interpreted comagmatic thick differentiated komatiitic flow within the adjacent Mashava-Masvingo greenstone belt to be 2754 ± 13 Ma. Recent geological observations support a revised structural interpretation of the MIC as an assemblage of three separate horizontal layered sills emplaced almost coevally at two different crustal depths: the chromititiferous Prince Sill intruded concordantly, and prior to regional folding, into the basal sedimentary rocks of the late Archaean Upper Bulawayan Supergroup, the relatively thin and evolved West Sill within early Archaean greenstone and gneissic basement, and the major Northwest Arm-Main Sill mainly within adjacent basement gneiss. The sill assemblage was vertically linked via ultramafic dykes and, as previously proposed, was fed through the dyke-like Northeast Arm, whose overlying layered rocks are here interpreted as correlatives of the Northwest Arm-Main Sill. The new precise age for the MIC is 50 Ma years older than the former geological estimate of ∼2.7 Ga. The comparable date for the volcanic phase supports the interpretation of both as component phases of a putative Masvingo W komatiitic sill-flow complex but is incompatible with interpretation of the late Archaean lithostratigraphy of the Mashava-Masvingo belt as a thrust-stacked package of unrelated continental and oceanic rocks.",
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    N2 - Ion microprobe (SHRIMP) U-Pb zircon dating has determined the age of a gabbroic enclave within the supraregional intrusive komatiitic Mashaba Igneous Complex (MIC) to be 2743 ± 11 Ma and that of an interpreted comagmatic thick differentiated komatiitic flow within the adjacent Mashava-Masvingo greenstone belt to be 2754 ± 13 Ma. Recent geological observations support a revised structural interpretation of the MIC as an assemblage of three separate horizontal layered sills emplaced almost coevally at two different crustal depths: the chromititiferous Prince Sill intruded concordantly, and prior to regional folding, into the basal sedimentary rocks of the late Archaean Upper Bulawayan Supergroup, the relatively thin and evolved West Sill within early Archaean greenstone and gneissic basement, and the major Northwest Arm-Main Sill mainly within adjacent basement gneiss. The sill assemblage was vertically linked via ultramafic dykes and, as previously proposed, was fed through the dyke-like Northeast Arm, whose overlying layered rocks are here interpreted as correlatives of the Northwest Arm-Main Sill. The new precise age for the MIC is 50 Ma years older than the former geological estimate of ∼2.7 Ga. The comparable date for the volcanic phase supports the interpretation of both as component phases of a putative Masvingo W komatiitic sill-flow complex but is incompatible with interpretation of the late Archaean lithostratigraphy of the Mashava-Masvingo belt as a thrust-stacked package of unrelated continental and oceanic rocks.

    AB - Ion microprobe (SHRIMP) U-Pb zircon dating has determined the age of a gabbroic enclave within the supraregional intrusive komatiitic Mashaba Igneous Complex (MIC) to be 2743 ± 11 Ma and that of an interpreted comagmatic thick differentiated komatiitic flow within the adjacent Mashava-Masvingo greenstone belt to be 2754 ± 13 Ma. Recent geological observations support a revised structural interpretation of the MIC as an assemblage of three separate horizontal layered sills emplaced almost coevally at two different crustal depths: the chromititiferous Prince Sill intruded concordantly, and prior to regional folding, into the basal sedimentary rocks of the late Archaean Upper Bulawayan Supergroup, the relatively thin and evolved West Sill within early Archaean greenstone and gneissic basement, and the major Northwest Arm-Main Sill mainly within adjacent basement gneiss. The sill assemblage was vertically linked via ultramafic dykes and, as previously proposed, was fed through the dyke-like Northeast Arm, whose overlying layered rocks are here interpreted as correlatives of the Northwest Arm-Main Sill. The new precise age for the MIC is 50 Ma years older than the former geological estimate of ∼2.7 Ga. The comparable date for the volcanic phase supports the interpretation of both as component phases of a putative Masvingo W komatiitic sill-flow complex but is incompatible with interpretation of the late Archaean lithostratigraphy of the Mashava-Masvingo belt as a thrust-stacked package of unrelated continental and oceanic rocks.

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