North Australian Cratonic Assemblage (NAC)

Australia: The Land Where Time Began

North Australian Cratonic Assemblage (NAC)

During the Barramundi Orogeny, 1.89-1.87 Ga, there was synchronous deformation, metamorphism and magmatism across a large part of the NAC, that included the Pine Creek Block, Tennant Creek Block, northern Arunta Block and Mt Isa Block, as well as possibly the Georgetown Block (Etheridge et al., 1987; Page, 1988). It has been argued that the Barramundi Orogeny was intracontinental, based on the predominance low-p metamorphism and the lack of features diagnostic of intercontinental orogeny of the present (Etheridge et al., 1987). The model they proposed included mafic underplating and continental extension that was driven by delamination of the crust-mantle and A-subduction. It has been argued (Wyborn, 1988) that most of the Australian continent geochemical signatures of felsic igneous rocks, 1.88-1.84 Ga, are consistent with such a model.

It has been thought that the Kimberley Block accreted to the NAC along the Halls Creek Orogen about 1.85-1.82 Ga (Bodorkos, 1999). In the Tickalara Complex layered mafic-ultramafic bodies and associated felsic sheets have chemical signatures that are consistent with either an oceanic island-arc/back-arc basin or an ensialic marginal basin setting (Sheppard et al., 1999a). Volcanics and sediments were deposited in a passive margin setting on the eastern edge of the NAC, on continental crust of Archaean-Palaeoproterozoic age (Sheppard et al., 1999a). It has been argued that their observations are more consistent with processes similar to those of plate margin interactions of the present than with ensialic models that were proposed previously (Etheridge et al., 1987; Page & Hancock, 1988).

In the northern Arunta Inlier both early events, 1.78-1.77 Ga, and late events, 1.745-1.73 Ga, are included in the Strangways Orogeny (Collins and Shaw, 1995), and along the southern margin of the NAC has been inferred to reflect oblique convergence and accretion of magmatic arcs (Myers et al., 1996). Sedimentation and volcanism took place across the NAC in a series of intercontinental basins between about 1.8-1.6 Ga, the MacArthur Basin containing the best preserved of these, and in the Mt Isa Inlier and Georgetown Inlier (Plumb et al., 1990;). It has been suggested that along the southern margin of the NAC these basins formed during back-arc continental extensions above a north-dipping subduction zone, that was long-lived (Giles et al., 2001). It has been suggested (Zhao & Cooper, 1992) that the Argilke Event, 1.68-1.66 Ga, may reflect the accretion of a strip of continental crust to the same margin. It has been argued on chemical grounds that during these events the subduction and consumption of oceanic crust, though this has been questioned (Collins & Shaw, 1995), citing the scarcity of subduction-related rock types that are diagnostic, the apparent correlation of strata across the Arunta Block and Tennant Creek Block, suggesting it is evidence against accretion. During the Chewings Orogeny, about 1.6 Ga, in the northern Arunta Inlier, compression occurred that may have been intracratonic (Myers et al., 1996), that was synchronous with pegmatite intrusion further north that was post tectonic, that suggests the inlier was intact by that time (Collins & Shaw, 1995).

There was orogenesis and high-T metamorphism that was broadly synchronous in northeast Australia at 1.6-1.5 Ga in NE Australia; 1.58 in the eastern fold belt and 1.54-1.53 Ga in the western fold belt of the Mt Inlier

NE Australia 1.6-1.5 Ga

Mt Isa Fold Belt-eastern 1.58 Ga (Connors & Page, 1995; Page & Sun, 2000)

Mt Isa Fold Belt-western 1.54-1.53 Ga (Connors & Page, 1995; Page & Sun, 2000)

Coen Inlier 1.59-1.55 Ga (Blewett & Black, 1998)

Georgetown Inlier about 1.55 Ga (Black & Withnall, 1993; Black et al., 1998)

It has been suggested that these events possibly reflect the accretion to the eastern margin of the NAC of the Coen Block and the Georgetown Block (Myers et al., 1996). Others have disagreed, suggesting that the absence of ophiolites, paired metamorphic belts, calc-alakaline magmatism and a significant degree of vertical uplift, and the presence of low-pressure (P) high-temperature (T) metamorphism, anticlockwise P-T-time (P-T-time) paths, bimodal magmatism and basins that were broad and shallow, all suggest that these were intraplate events (Wyborn, 1998; Blewett & Black, 1998; Oliver et al., 1998).

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