40Ar/39Ar and K-Ar ages: early Paleozoic metamorphism and deformation in the Narooma accretionary complex, NSW

E. Prendergast, R. Offler, D. Phillips, Horst Zwingmann

    Research output: Contribution to journalArticle

    8 Citations (Scopus)

    Abstract

    White micas in slates from an imbricated sequence of deformed turbidites, and clast-in-matrix-melanges that form the Narooma accretionary complex at Batemans Bay, record apparent 40Ar/39Ar whole rock ages in the range 450 to 500 Ma, with intermediate-temperature average ages of 471 +/- 13 Ma and 476 +/- 11 Ma (2 standard deviations). By contrast, two K-Ar whole rock ages are 430.9 +/- 8.6 Ma and 449.2 +/- 9.0 Ma. Fractions (2, 3-5 and 10-20m) from the same samples show a progressive increase in age with grainsize (2m=413.1 +/- 8.3-404.7 +/- 8.1 Ma; 3-5m=417.1 +/- 8.3 Ma; 10-20m=434 +/- 8.6-437.5 +/- 8.8 Ma 2 standard deviations). The older whole rock 40Ar/39Ar ages are attributed to recoil loss of 39Ar. The cleavage forming event is believed to have occurred at 460-440 Ma (Upper Ordovician to Lower Silurian). The finest fractions possibly record a younger thermal event at approximately 413 Ma. Kubler Index values obtained from the Wagonga and Adaminaby Groups indicate lower epizonal (greenschist facies) metamorphic conditions. b0 lattice parameter values (x=9.033; n=21) are characteristic of the intermediate pressure facies series. Assuming a temperature of approximately 300 degrees C and P=4 kb suggests a depth of burial of approximately 15km was reached by these rocks and that a geothermal gradient of 20 degrees C/km operated during subduction.
    Original languageEnglish
    Pages (from-to)21-32
    JournalAustralian Journal of Earth Sciences
    Volume58
    DOIs
    Publication statusPublished - 2011

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    metamorphism
    Paleozoic
    rock
    melange
    geothermal gradient
    greenschist facies
    clast
    Silurian
    cleavage
    Ordovician
    subduction
    temperature
    matrix

    Cite this

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    title = "40Ar/39Ar and K-Ar ages: early Paleozoic metamorphism and deformation in the Narooma accretionary complex, NSW",
    abstract = "White micas in slates from an imbricated sequence of deformed turbidites, and clast-in-matrix-melanges that form the Narooma accretionary complex at Batemans Bay, record apparent 40Ar/39Ar whole rock ages in the range 450 to 500 Ma, with intermediate-temperature average ages of 471 +/- 13 Ma and 476 +/- 11 Ma (2 standard deviations). By contrast, two K-Ar whole rock ages are 430.9 +/- 8.6 Ma and 449.2 +/- 9.0 Ma. Fractions (2, 3-5 and 10-20m) from the same samples show a progressive increase in age with grainsize (2m=413.1 +/- 8.3-404.7 +/- 8.1 Ma; 3-5m=417.1 +/- 8.3 Ma; 10-20m=434 +/- 8.6-437.5 +/- 8.8 Ma 2 standard deviations). The older whole rock 40Ar/39Ar ages are attributed to recoil loss of 39Ar. The cleavage forming event is believed to have occurred at 460-440 Ma (Upper Ordovician to Lower Silurian). The finest fractions possibly record a younger thermal event at approximately 413 Ma. Kubler Index values obtained from the Wagonga and Adaminaby Groups indicate lower epizonal (greenschist facies) metamorphic conditions. b0 lattice parameter values (x=9.033; n=21) are characteristic of the intermediate pressure facies series. Assuming a temperature of approximately 300 degrees C and P=4 kb suggests a depth of burial of approximately 15km was reached by these rocks and that a geothermal gradient of 20 degrees C/km operated during subduction.",
    author = "E. Prendergast and R. Offler and D. Phillips and Horst Zwingmann",
    year = "2011",
    doi = "10.1080/08120099.2011.534506",
    language = "English",
    volume = "58",
    pages = "21--32",
    journal = "Australian Journal of Earth Sciences",
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    40Ar/39Ar and K-Ar ages: early Paleozoic metamorphism and deformation in the Narooma accretionary complex, NSW. / Prendergast, E.; Offler, R.; Phillips, D.; Zwingmann, Horst.

    In: Australian Journal of Earth Sciences, Vol. 58, 2011, p. 21-32.

    Research output: Contribution to journalArticle

    TY - JOUR

    T1 - 40Ar/39Ar and K-Ar ages: early Paleozoic metamorphism and deformation in the Narooma accretionary complex, NSW

    AU - Prendergast, E.

    AU - Offler, R.

    AU - Phillips, D.

    AU - Zwingmann, Horst

    PY - 2011

    Y1 - 2011

    N2 - White micas in slates from an imbricated sequence of deformed turbidites, and clast-in-matrix-melanges that form the Narooma accretionary complex at Batemans Bay, record apparent 40Ar/39Ar whole rock ages in the range 450 to 500 Ma, with intermediate-temperature average ages of 471 +/- 13 Ma and 476 +/- 11 Ma (2 standard deviations). By contrast, two K-Ar whole rock ages are 430.9 +/- 8.6 Ma and 449.2 +/- 9.0 Ma. Fractions (2, 3-5 and 10-20m) from the same samples show a progressive increase in age with grainsize (2m=413.1 +/- 8.3-404.7 +/- 8.1 Ma; 3-5m=417.1 +/- 8.3 Ma; 10-20m=434 +/- 8.6-437.5 +/- 8.8 Ma 2 standard deviations). The older whole rock 40Ar/39Ar ages are attributed to recoil loss of 39Ar. The cleavage forming event is believed to have occurred at 460-440 Ma (Upper Ordovician to Lower Silurian). The finest fractions possibly record a younger thermal event at approximately 413 Ma. Kubler Index values obtained from the Wagonga and Adaminaby Groups indicate lower epizonal (greenschist facies) metamorphic conditions. b0 lattice parameter values (x=9.033; n=21) are characteristic of the intermediate pressure facies series. Assuming a temperature of approximately 300 degrees C and P=4 kb suggests a depth of burial of approximately 15km was reached by these rocks and that a geothermal gradient of 20 degrees C/km operated during subduction.

    AB - White micas in slates from an imbricated sequence of deformed turbidites, and clast-in-matrix-melanges that form the Narooma accretionary complex at Batemans Bay, record apparent 40Ar/39Ar whole rock ages in the range 450 to 500 Ma, with intermediate-temperature average ages of 471 +/- 13 Ma and 476 +/- 11 Ma (2 standard deviations). By contrast, two K-Ar whole rock ages are 430.9 +/- 8.6 Ma and 449.2 +/- 9.0 Ma. Fractions (2, 3-5 and 10-20m) from the same samples show a progressive increase in age with grainsize (2m=413.1 +/- 8.3-404.7 +/- 8.1 Ma; 3-5m=417.1 +/- 8.3 Ma; 10-20m=434 +/- 8.6-437.5 +/- 8.8 Ma 2 standard deviations). The older whole rock 40Ar/39Ar ages are attributed to recoil loss of 39Ar. The cleavage forming event is believed to have occurred at 460-440 Ma (Upper Ordovician to Lower Silurian). The finest fractions possibly record a younger thermal event at approximately 413 Ma. Kubler Index values obtained from the Wagonga and Adaminaby Groups indicate lower epizonal (greenschist facies) metamorphic conditions. b0 lattice parameter values (x=9.033; n=21) are characteristic of the intermediate pressure facies series. Assuming a temperature of approximately 300 degrees C and P=4 kb suggests a depth of burial of approximately 15km was reached by these rocks and that a geothermal gradient of 20 degrees C/km operated during subduction.

    U2 - 10.1080/08120099.2011.534506

    DO - 10.1080/08120099.2011.534506

    M3 - Article

    VL - 58

    SP - 21

    EP - 32

    JO - Australian Journal of Earth Sciences

    JF - Australian Journal of Earth Sciences

    SN - 0812-0099

    ER -