Australia: The Land Where Time Began
Gawler Craton ca 1550-1450 Ma see Mawson Continent
The Gawler Craton occupies about 440,000 km2 of central South Australia. Prior to about 1550 million years ago it was composed of a number of active Proterozoic orogenic belts that had existed back to at least 2450 Ma. It is one of the largest blocks of Archaean-Proterozoic crystalline basement in Australia.
The geological history of this craton adds to the knowledge of the reconstruction of the supercontinent because it contains evidence of the earliest Palaeoproterozoic collisional orogenies, about 2460 to 2430 Ma, and together with the North Australian Craton and Antarctica, has often been correlated with the western margin of Laurentia.
The early history of the Gawler Craton comprised 3 major tectonic events.
Between about 2700 and 2300 Ma, Late Archaean sedimentation and volcanism was followed by early Palaeoproterozoic plutonism and metamorphism of the Steafordian orogeny.
Between 2000 Ma and 1700 Ma, a period of initial basin-platform sedimentation was followed by widespread plutonisn, metamorphism and deformation during the Kimban Orogeny, with local volcanism and continental sedimentation.
Between 1690 and 1450 Ma, during the Kararan Orogeny.
Crust of Mesoarchean age, about 3.15 Ga has been found outcropping at or near the surface in the eastern section of the Gawler Craton, pushing back the age of the craton by more than 500 million years. The wedge-shaped area of about 1,500 km2 has been found to underlie much of the South Australian Heat Flow Anomaly. The presence of this wedge of crust indicates that crust older than 3 Ga is not restricted to the Yilgarn and Pilbara Cratons.
The supracrustal and magmatic lithologies of the Gawler Craton are surrounded, overlain and intruded by units from the Palaeoproterozoic (2000-1610 Ma) and Mesoproterozoic (1590-1490 Ma) (Daly et al., 1998; Ferris et al., 2002; Swain et al., 2005b; Fanning et al., 2007; Hand et al., 2007). Interpretation of Total Magnetic Intensity (TMI) and gravity datasets have been combined with geological evidence to delineate tectonic domains for the Gawler Craton, that mostly represent structural trend variations and the extent to which the crust has been reworked, not to fundamental boundaries of terranes (Hand et al., 2007). See Source 2.
Late Archaean - Early Palaeoproterozoic see Source 2 p. 5
In the central parts of the Gawler Craton metasedimentary, volcanic and granite-greenstone lithologies comprise the stratigraphy of Late Archaean age, about 2560-2500 Ma (Daly & Fanning, 1993; Swain et al., 2005b) deformed during the Sleafordian Orogeny, 2460-2430 Ma (Daly et al., 1998; McFarlane, 2006). A magmatic arc is believed to have been the setting for the formation of the Devil's Playground Volcanic and the Dutton Suite, of about 2560-2500 Ma age, that terminated during or shortly before the Sleafordian Orogeny (Swain et al., 2005b). The Kiana Granite Suite, about 2460 Ma, is included in the magmatism of the Sleafordian Orogeny (Fanning et al., 2007) and the Whidbey Granite, leucogranites, about 2445 Ma (Jagodzinski et al., 2006). The grade of the metamorphics of the Sleafordian Orogeny ranges from sub-greenschist to granulite facies (Daly & Fanning, 1993). P-T estimates of 800-850o C and about 7.5 kbar (Tompkins & Mavrogenes, 2002) and 750-800o C and 4.5-5.5 mbar (Teasdale, 1997) record peak metamorphism for localities within in the Mulgathing Complex. In the central Gawler Craton, structures from the time of the Sleafordian Orogeny (Mulgathing Complex) are comprised of folds, that plunge to the NNE-NE at a shallow angle. Later shear zone movement has subjected these folds to a degree of block rotation (Teasdale, 1997; Direen et al., 2005).
About 2 Ga. The Miltalie Event see Source 2 p. 5
After the Sleafordian Orogeny there was a tectonic quiescent period of about 400 million years, the first tectonic activity occurring after this quiescent phase was the Miltalie Event (Webb et al., 1986; Daly et al., 1998). Protolith magmatic ages determined for the Miltalie Event are 2002 ± 15 Ma and 1999 ± 13 Ma (Fanning et al., 2007). According to field observations of the authors the Miltalie Gneiss map unit (Parker, 1993) appears to incorporate metasedimentary lithologies, though the age has not been determined, the Miltalie Gneiss and its protoliths setting is still to be constrained.
2000-1860 Ma. Sediment deposition and the Cornian Orogeny of about 1850 Ma see Source 2 p. 5
Sequences of the Hutchison Group overlie the Miltalie Gneiss, that have been interpreted to have been deposits that accumulated on a passive margin between about 2000-1860 Ma (Parker, 1993Schwarz et al., 2002). Before the start of the Cornian Orogeny, the volcanics of the Bosanquet Formation relatively tightly constrained the final sedimentation at about 1866 ± 10 Ma (Fanning et al., 2007). In the Corny Point region, Yorke Peninsula, there is evidence of sediment deposition that appears to be time equivalent (Howard et al., 2007). Large volumes of felsic magmatism of the Domington Suite (Hoek & Schaefer, 1998; Reid et al., 2008) are associated with the Cornian Orogeny, 1850-1840 Ma, (Reid et al., 2008). Structural fabrics striking east-southeast were produced by this orogenic event, that are overprinted by folds that strike east-west, and late south-side extensional ductile shearing.
1800-1740 Ma. Magmatism and sedimentation see Source 2 p. 5
1730-1690 Kimban Orogeny see Source 2 p. 7
1660 Ma. Ooldean Event see Source 2 p.7
Transition events - Palaeoproterozoic-Mesoproterozoic
Evidence for a complex sequence of events that occurred between 1630-1540 Ma is preserved in the Gawler Craton. In the St Peter Suite, 1620-1608 Ma (Flint et al., 1990), there are co-magmatic and felsic intrusions that have been interpreted as petrogenesis related to subduction (Swain et al., 2008). Large volumes of metallogenically significant material of the the Gawler Range Volcanics (GRV), about 1592 Ma, and the Hiltaba Suite intrusives, 1595-1575 Ma, followed the St Peter Suite magmatism (Flint, 1993; Daly et al., 1998; Budd, 2006). The Gawler Range Volcanics and the Hiltaba Suite were interpreted as an anorogenic magmatic event linked to a plume (Flint, 1993; Creaser, 1995). A syntectonic setting for the Gawler Range Volcanics-Hiltaba Event has been suggested, based on later evidence of high-grade-metamorphism and deformation, that has been found in the Mt Woods and Coober Pedy Ridge domains (Skirrow et al., 2006; Fanning et al., 2007). The potential role of a mantle plume in the generation of magma has not been negated by this syntectonic setting, and it does not require the tectonism and magmatism to be linked causally (Betts et al., 2007). In the Eyre Peninsula region (Foster & Ehlers, 1998), and in the Walleroo Region (Conor, 1995) there is also deformation and low-grade metamorphism that has been reported (Direen & Lyons, 2007; Hand et al., 2007). Shortly before, and within uncertainty of, the Kararan Orogeny, about 1565-1540 Ma, deformation and metamorphism occurred that was associated with the Gawler Range Volcanics-Hiltaba Event (Hand et al., 2007).
The final episode of high-grade metamorphism and deformation within the Gawler Craton is represented by the Kararan Orogeny, as defined by Hand et al. (2007) (Teasdale, 1997; Fraseer & Lyons, 2006; Fanning et al., 2007; Payne et al., 2008). This was followed by a final period of shear zone activity, and about 1450 Ma, subsequent cratonisation (Webb et al., 1986; Fraser & Lyon, 2006). Evidence of the Kararan Orogeny is mostly confined to northern and western parts of the Gawler Craton. In the Fowler Domain, peak metamorphic conditions of 800o C and 10 kbar are recorded (Teasdale, 1997). Granulite grade metamorphism occurred in the Cooper Pedy and Mabel Creek Ridge regions (Fanning et al., 2007; Payne et al., 2008). Regional aeromagnetic data from the northern part of the Gawler Craton, and outcropping in the Peake and Denison Inliers, shows east-west to northeast trending structures that have been interpreted to have formed during the Kararan Orogeny (Hopper, 2001; Payne et al., 2008). Constraint is lacking, at the time of writing, for the extent of the shear zone activity at about 1450 Ma (Fraser & Lyons, 2006), and the influence it had on the geometry of the Gawler Craton.
See Source 2 for more detailed information and illustrations
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|