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The Archean

        The c. 3000-2700 Ma Archean age Superior Structural Province is made  up of east-west trending paired sets of volcanic and sedimentary sub-provinces  (e.g. Uchi-English river; Wapigoon-Quetico/Opatica?, Abitibi-Pontiac), irregular  elongated belts of plutonic rock (e.g. Winipeg River; Berens; Opatica?), and  minor zones of high grade gneisses (e.g. Kapuskasing). Although no mantle- bearing ophiolite or arc units have been recognised in the Archean, ultramafic  rocks are nevertheless ubiquitously present in the form of magmatic flow rocks  known as komatiite . The upper parts of the flows are characterised by the  presence of olivine or, more rarely, clinopyroxene, in the form of coarse, platey,  sheaf-like aggregates that propagate downwards from the upper surface of the  flow. The aggregates resemble the Australian spinifex grass, and the olivine is  therefore said to be spinifex textured. The presence of ultramafic liquids in  Archean volcanic sequences is used as evidence that geothermal gradients were  higher during the Archean than at present, and that the mantle was subjected to  relatively high degrees of melting at this time. Archean volcanic sequences are  important sources of Ni, Cu, Zn, Fe, and Au, but not tin.
        Rocks of the Abitibi sub-province of the southern Superior province are  very well preserved, and little exhumed. Along its southern margin the Abitibi  volcanics are overlain by belt of turbiditic sediments that pass from proximal to  distal from north to south. The source of the sediments, and particularly the  2900-2800 Ma detrital zircons found in the Pontiac, likely lies at least in part to  the north within the Opatica gneiss belt. The boundary zone of the volcanic and  sedimentary sequences is also the locus of deposition of shallow water coarse  conglomerate, cross bedded sandstones, and K-feldspar rich alkali syenites  (shoshonites), granites and rhyolites. The boundary zone is also the locus of  deposition of hydrothermal gold at c. 2693 Ma.
        Within the belts dominated by plutonic rocks, Archean supracrustal  sequences are commonly preserved as relict mafic keels within a sea of  granitoid intrusive rocks of the 'Tonalite-Tronhdjemite-Granodiorite' suite.
         [In the type area for this mode of occurrence, the Barberton Mountain  Land granite-greenstone terrane of South Africa is composed of small wedges of  tonalitic gneiss (Theespruit gneiss, 3538 Ma), komatiitic, mafic and felsic  volcanics (Onverwacht Group, 3472 - 3416 Ma) injected by 3352 Ma old gabbro  and diabase (rifting), a middle sequence of greywackes, cherts, and ignimbritic  volcanic rocks (Fig Tree Group), and an upper sequence of shallow marine to  subaerial quartzose sediments. One Archean model suggests that the  Onvervacht represents Archean ocean crust obducted onto the Theespruit  gneiss as continental crust. Studies of the geochronology of zircons from  Archean rocks of South Africa indicate that the main phase thrust and recumbent  fold deformation of the Barberton Mountain Land was a very rapid event initiated  during the later stages of deposition of the Fig Tree Group, and coeval with  upper Fig tree ignimbritic volcanism at 3227 Ma and with the very rapid  deposition of the molasse-like Moodies Group. The latest stage in the evolution  of the Barberton Mountain Land involved formation of upright Archean folds and  extensive intrusion of potassic granites at c. 3105 Ma.]         In Canada a representative example of a relict greenstone keel would be  the c. 3023- 2900 Ma North Spirit Lake greenstone belt of the northern Superior  Province (Sachigo sub-province). This belt is unusual however, in that the mature  quartz arenite Nemakwis assemblage - analogous to the Moodies of South  Africa - overlying the basal ultramafic-mafic-felsic volcanic sequence is overlain  by younger spinifex textured komatiite units, followed in turn by coarse alluvial  sediments and c. 2735 Ma  mafic-felsic volcanic rocks and associated turbidites.  As in the Abitibi belt, the youngest assemblage at North spirit Lake includes  shoshonitic rocks and course fluvial sediments.
         The oldest zircons found in the Archean are 4300 Ma detrital zircons in  Archean rocks of Western Australia. However the 3962 Ma zircons from the Acasta gneisses (tonalite and granite protoliths) of the Slave Province are the  oldest zircons from intact terrestrial rocks. The gneisses form the basement on  which were deposited 2810 to 2600 Ma old volcanic and sedimentary rocks.  Deposition of c. 2600 ma turbidite and conglomerate sequences across the  entire Slave Province was synchronous with or closely followed by regional  deformation, metamorphism, and plutonism between 2600 and 2580 Ma.
         Where the Archean crust has been tilted, as in the case of the Kapuskasing belt of Ontario, it can be observed that low grade volcanic and  sedimentary rocks are separated from high grade mafic rocks by extensive  areas of tonalitic gneiss. The latter are possibly formed by the partial melting of  large amounts of only partially subducted volcanic material that because of  higher geothermal gradients during the Archean was relatively buoyant and less  easily subducted into the deep mantle.

        Overheads
    Structural Provinces of the Canadian Shield (14strprv.gif)
    Subprovinces of the Superior Province Timmins Kirkland Lake Noranda area (arch2sup1.jpg)
    The Kirkland Lake area (no image)
    The Slave Province (slavecolmap.jpg)
    The Kapuskasing uplift within the Abitibi belt (archkapus.jpg)
    Subduction or no subduction - the thermal buoyancy problem (arch2supbuoy.jpg)
    Continental Growth through time (15cntgrw.gif)

    Isotopic ages - Abitibi belt
        2980 single grain in Timiskaming, Davis 1991
        2900 oldest Michipicoten volcanics, Corfu and Sage 1992
        2840 - 2760 20% of detrital zircons from Abitibi belt sediments, Davis 1991
        2750 - 2720 xenocrystic zircons in porphyry and lamprophyre of Larder Lake
        2747 Pacaud tuffs, Mortensen 1993
        2736 Older Temagami volcanics, Bowins and Heaman 1992
        2730-2725 Normetal - Hunter Mine, Lac Abitibi region = Deloro, Timmins  area
        2727 Upper Deloro, Corfu et al. 1989
       2725 Majority of detrital zircons in Abitibi belt sediments
        2724 Mattagami volcanics, Ep = 2.5, Vervoort et al 1991
        2717 Kidd Creek, Barrie et al. 1988 (quoted by Davis et al 1991); Schandle et  al. 1990
        2714-2713 Hunter Mine volcanics, Corfu et al 1989
        2710-2700 Youngest Michipicoten volcanics, Corfu and Sage 1992
        2705 Larder Lake; Corfu et al. 1989; Blake River volcanics, Machado et al.  1991
                = Columbiere rhyolite of Val d'Or, Mortensen 1993
        2703 Upper Tisdale Corfu et al. 1989
        2701 Blake River Gp., Skead Gp., Corfu et al 1989
        2698 Upper Tisdale Krist Pyroclastic = Clericy rhyolite of the Blake River,  Mortensen 1993
                trondjemite bounder in Dore conglomerate of Michipicoten, Corfu and  Sage 1992
        2696-2670 Sanukitoids, Ep 1.8-3.7
        2695-2688 thrusting of 2700 volcanics over 2750-2720 volcanics in the Larder  Lake region
        2693 (2703-2683) hydrothermal zircons, quartz-tourmaline-gold veins (Kerrich  & Kyser, 1994)
        2687 lamprophyre, Barrie & Tucker 1990 (Au is younger); youngest zircons in  the Kewagama;
                 Younger Temagami volcanics, Bowins and Heaman, 1992
        2686 Baie de Lys gneiss, Lac Simard Nord monzonite Machado et al. 1991;
               to 2680 detrital zircons in the Timiskaming, Corfu et al. 1991
        2685+3 Bidgood qtz porphyry, Corfu et al. 1991
        2683 minimum age of Kewagama
        2682 Clericy syenite (cuts Kewegama), Mortensen 1993
        2681 Younger detrital zircons in Central and Northern Michipicoten; seds cut  by 2671 granodiorite Corfu and Sage 1992
        2680 Otto stock ?; 2679 maximum age of Timiskaming
        2678 Lac Maple granodiorite (has inherited zircons of 2692), Machado et al  1991
        2677 +3 porphyry dike cutting Timiskaming of Larder Lake region
                = volcanic agglom. of the Bear Lake Fm north of Larder Lake
                =minimum age of Timiskaming
        2673 titanite in lamprophyre, Wyman and Kerrich 1987
        2671+8 Otto stock, ion probe age, Ben Othman 1990 Ep volcanics = 2.5+1.5
                Michipicoten granodiorite, Corfu and Sage 1992
       2669 Belleterre monzodiorite Machado et al. 1991
        2660 concordant titanite in Opatica gneiss = metamorphic episode, Machado et  al. 1992
        2645 Most S-granites in the Pontiac, range 2663-2611, Feng and Kerrich 1991
        2637 concordant titanite in Baie de Lys gneiss = metamorphic episode,  Machado et al. 1992
        2632 Halle intrusive (inherited zircons of 2702) = ae of Au mineralization,  Machado et al. 1992
        2624+6 Hydrothermal rutile, Kidd volcanic centre, Schandle et al. 1990
        2630-2579 U-Pb ages on gold vein rutile and titanite (Kerrich and Kyser, 1994)

FIGURES
 
 

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