South Australian Cratonic Assemblage (SAC)

Australia: The Land Where Time Began

South Australian Cratonic Assemblage (SAC)

There are 2 main cratonic blocks in the SAC, The Gawler Craton, of Archaean-Palaeoproterozoic age and the Curnamona Province of Palaeoproterozoic-Mesoproterozoic age. A detailed summary of the geological history of the Gawler Craton has been published, that includes its southern extension into the Mawson Block, Antarctica (Fitzsimons, 2002). In both provinces most Palaeoproterozoic and older rocks have been deformed extensively and metamorphosed into amphibolite and granulite facies.

In the Lincoln Batholith large volumes of felsic and mafic granitoids, with minor mafic dykes, were emplaced at the southeast margin of the Gawler Craton about 1.85 Ga, that coincides with the final stages of deposition onto the Sleaford Complex basement, of Archaean age, of the sediments of the Hutchinson Group (Mortimer et al., 1988a; Hoek & Schaefer, 1998). At this time there may have been a limited degree of compressional deformation that may have successive emplacement of plutons (Hoek & Schaefer, 1998; Vassallo & Wilson, 2002). It has been argued (Mortimer et al., 1988a) that the Lincoln Granitoids are not compositionally compatible with the processes of subduction, an alternative model being advocated that is similar to one that had previously been proposed (Etheridge et al., 1987), which involved remelting of underplating mafic material in an intracratonic setting that generated magma. Along the Kalinjala Shear Zone, a major structure that was active during the Kimban Orogeny, 1.74-1.70 Ga, the Lincoln Batholith was juxtaposed against the Hutchinson Group. The Kimban Orogeny included regional deformation and metamorphism of the sedimentary rocks of the Hutchinson Group and the underlying basement, of Archaean age, and emplacement of extensive granitoids of the Moody Suite, 1.74-1.70 Ga (Daly et al., 1998; Hoek & Schaefer, 1998; Vassallo & Wilson, 2002). It was suggested that the Curnamona Province was amalgamated with the Gawler Craton during the Kimban Orogeny.

During the Kararan orogeny, 1.65 and 1.57-1.54 Ga, dip-slip structures formed in the NW part of the Gawler Craton that have been interpreted as a collision between the Gawler Craton and a 'proto-Yilgarn Craton' at 1.65 Ga (Daley et al., 1998), the authors¹ suggesting it may also have involved accretion of the Coompana Block that is only known in the subsurface. It has been suggested that a belt of synorogenic granite plutons, of 1.65-1.55 Ga age (the Ifould Complex; Daly et al., 1998) to the southeast of the Karari Fault Zone could represent a magmatic arc that predated collision at about 1.56 Ga (Giles et al., 2001; Fitzsimons, 2002). It has been suggested that mafic and ultramafic bodies that were associated with these mobile belts indicate thinning of the crust (Daley et al., 1998), though they haven't been interpreted in terms of specific tectonic processes. There are extensive outcrops of subaerial felsic and minor Gawler Range Volcanics and co-magmatic Hiltaba Suite Granites in the central part of the Gawler Craton, of 1.6-1.55 Ga age, that are believed to have been formed by extensive underplating of mafic rocks and melting of the crust from the Archaean-Palaeoproterozoic (Flint, 1993; (Daley et al., 1998).

Metasedimentary and bimodal meta-igneous rocks of the Willyama Supergroup, deposited about 1.69 Ga are the oldest rocks that have been recognised in the Curnamona Province (Page & Laing, 1992). The presence of Archaean and/or Palaeoproterozoic basement is suggested by inherited zircon ages of 2.7-1.78 Ga (Page & Laing, 1992; Robertson et al., 1998). During the Olarian Orogeny about 1.6 Ga these rocks were subsequently deformed and metamorphosed to amphibolite and granulite facies (Page & Laing, 1992). There are also abundant synorogenic to post-orogenic granitoid and bimodal volcanic rocks of similar, and according to the authors¹ possibly correlative with, the Gawler Range Volcanics and Hiltaba Suite Granitoids, from 1.6-1.55 Ga, of the Gawler Craton (Daly et al., 1998; Robertson et al., 1998).

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