Neoprot Reworking in Capricorn Orogen

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Geochemical evidence for Neoproterozoic metamorphism in the Capricorn Orogen, WA.

Text of Neoprot Reworking in Capricorn Orogen

  • Geological Survey of Western Australia

    RECORD2007/10

    Department of Industry and Resources

    NEOPROTEROZOIC REWORKING IN THE PALEOPROTEROZOIC CAPRICORN

    OROGEN: EVIDENCE FROM 40Ar/39Ar AGES

    by S. A. Occhipinti

  • GEOLOGICAL SURVEY OF WESTERN AUSTRALIA

    Record 2007/10

    NEOPROTEROZOIC REWORKING IN THE PALEOPROTEROZOICCAPRICORN OROGEN:EVIDENCE FROM 40Ar/39Ar AGES

    byS. A. Occhipinti1

    1 Current address: Fugro Airborne Surveys Pty Ltd, 65 Brockway Road, Floreat, W.A. 6014

    Perth 2007

  • MINISTER FOR RESOURCESHon. Francis Logan MLA

    DIRECTOR GENERAL, DEPARTMENT OF INDUSTRY AND RESOURCESJim Limerick

    EXECUTIVE DIRECTOR, GEOLOGICAL SURVEY OF WESTERN AUSTRALIATim Griffi n

    REFERENCEThe recommended reference for this publication is:OCCHIPINTI, S. A., 2007, Neoproterozoic reworking in the Paleoproterozoic Capricorn Orogen: evidence from 40Ar/39Ar ages:

    Western Australia Geological Survey, Record 2007/10, 41p.

    National Library of Australia Card Number and ISBN 978-1-74168-115-4

    Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zone 50. All locations are quoted to at least the nearest 100 m.

    Cover image modifi ed from Landsat data, courtesy of ACRES

    Published 2007 by Geological Survey of Western AustraliaThis Record is published in digital format (PDF) and is available online at www.doir.wa.gov.au/GSWA/publications. Laser-printed copies can be ordered from the Information Centre for the cost of printing and binding.

    Further details of geological publications and maps produced by the Geological Survey of Western Australia are available from:Information CentreDepartment of Industry and Resources100 Plain StreetEAST PERTH, WESTERN AUSTRALIA 6004Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444www.doir.wa.gov.au/GSWA/publications

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    ContentsAbstract ..................................................................................................................................................................1Introduction ............................................................................................................................................................1

    Geological background and geochronological constraints .............................................................................1Yilgarn Craton ..........................................................................................................................................3Gascoyne Complex ..................................................................................................................................3Errabiddy Shear Zone ..............................................................................................................................3Paleoproterozoic to Mesoproterozoic sedimentary rocks ........................................................................5

    Sample descriptions ...............................................................................................................................................6Narryer Terrane (Yilgarn Craton) ....................................................................................................................6Errabiddy Shear Zone .....................................................................................................................................6Gascoyne Complex .........................................................................................................................................8

    40Ar/39Ar dating ......................................................................................................................................................8Sample preparation .........................................................................................................................................8Technical background .....................................................................................................................................9Results .............................................................................................................................................................9

    Narryer Terrane ........................................................................................................................................9Errabiddy Shear Zone ............................................................................................................................12

    Eastern part of the Errabiddy Shear Zone .......................................................................................12Western part of the Errabiddy Shear Zone ......................................................................................12

    IR-laser step-heating data ........................................................................................................13IR-laser total-fusion analyses ...................................................................................................14UV laser-probe analyses of sample SAO_01_67 .....................................................................16

    Gascoyne Complex .........................................................................................................................18IR-laser step-heating data ........................................................................................................18IR-laser total-fusion analyses ...................................................................................................19

    Summary of 40Ar/39Ar age data .....................................................................................................................19Discussion ............................................................................................................................................................21

    Isotopic diffusion, compositional variations, and excess argon ....................................................................21Interpretation of the 40Ar/39Ar ages ...............................................................................................................22

    Tectonic implications ...........................................................................................................................................23Conclusions ..........................................................................................................................................................25References ............................................................................................................................................................26

    Appendices 1. Sample locations, rock unit, fi eld information, and SHRIMP ages ...........................................................28 2. Sample preparation and analytical procedures ...........................................................................................32 3. Supplementary data tables ..........................................................................................................................34

    Figures 1. Location of the southwestern Capricorn Orogen showing simplifi ed regional geology,

    sample locations, and sample analysis types ................................................................................................2 2. Simplifi ed geology of the Glenburgh Terrane and part of the Errabiddy Shear Zone showing the

    distribution of 40Ar/39Ar ages ........................................................................................................................4 3. Simplifi ed geology of the Errabiddy Shear Zone and the northwestern Yilgarn Craton (southern

    Capricorn Orogen) showing the distribution of 40Ar/39Ar ages ....................................................................5 4. Photomicrographs of six samples used in this study showing the textural relationships of muscovite,

    sericite, and biotite to other minerals in the rocks, and tectonic foliations ..................................................7 5. Single- and multiple-grain IR-laser step-heating data for micas from the Yilgarn Craton ........................12 6. Single- and multiple-grain IR-laser step-heating data for biotite from the Errabiddy Shear Zone ............13 7. Single- and multiple-grain IR-laser step-heating data for muscovite from the Errabiddy Shear Zone .........14 8. Single-grain IR-laser step-heating data for muscovite from the Glenburgh Terrane .................................15 9. Single-grain IR-laser step-heating data for micas from the central Gascoyne Complex ...........................16 10. Relationships between grain size and age for muscovite and biotite analysed by IR-laser total-grain

    fusion on mostly single and rarely multiple grains ....................................................................................17 11. Photograph of muscovite grains from sample SAO_01_67, analysed by UV laser ...................................18 12. Combined probability density plots and histograms of ages measured in muscovite and biotite, by

    single-grain total fusion and some multiple-grain fusion of small grains ..................................................20 13. Simplifi ed maps of the West Australian Craton and adjoining terranes .....................................................24

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    Tables 1. Summary of 40Ar/39Ar results .....................................................................................................................10 2. Summary of UV-laser argon isotope analyses on individual spots from grains 14 of

    sample SAO_01_67 ............................................................................