Superimposed tectonic events at 2450 Ma, 2100 Ma, 900 Ma and 500 Ma in the North Mawson Escarpment, Antarctic Prince Charles Mountains
Introduction
Located in Antarctica’s Southern Prince Charles Mountains (Fig. 1) between latitude 72°30′ and 73°42′S, and longitude 68° and 69°E, the Mawson Escarpment forms west-facing scarps in the order of 1000 m tall that extend for a distance of 130 km. It is built up predominantly of Archaean and Palaeoproterozoic basement complexes dated between 3.5 and 2.1 billion years old (Tingey, 1991, Mikhalsky et al., 2001, Mikhalsky et al., 2006a, Mikhalsky et al., 2006b, Boger et al., 2001, Boger et al., 2006, Boger et al., 2008). The rocks show overall structural continuity of E/W oriented transposition fabrics and folds, and metamorphic grade increases progressively from amphibolite facies in the south to lower granulite facies in the north (Tingey et al., 1981, Kamenev et al., 1993).
The area recognised as the North Mawson Escarpment, 72°30′–73°10′S, consists mainly of complexly deformed grey gneisses and metasupracrustals that are tectonically interlayered and structurally repeated (Fig. 2). Limited radiometric work has indicated major crustal development during the Palaeoproterozoic and overprinting by regional tectonothermal events during the Meso–Neoproterozoic and Early Palaeozoic (Tingey, 1991, Mikhalsky et al., 2006a, Mikhalsky et al., 2007, Corvino and Henjes-Kunst, 2007). Evidently, this area represents a transitional zone from stable Archaean cratonic components in the south to the approximately 1000 and 500 m.y. old metamorphic belts that characterise exposures to the north, namely the Rayner and Prydz Belts (Fig. 1). The relative importance of these belts on the structural and metamorphic evolution of the North Mawson Escarpment is still poorly understood.
The purpose of this paper is to present SHRIMP ion-microprobe U–Pb zircon dating results for the North Mawson Escarpment. In doing so, we establish that this area records evidence of at least four major tectonothermal events that occurred within the intervals 2490–2420 Ma, 2180–2080 Ma, 940–880 Ma and 530–490 Ma (Table 1). These seem to overlap with almost all of the principal tectonic cycles that have affected geographic regions between Princess Elizabeth and Enderby Lands since the beginning of the Proterozoic. But rarely are they clearly preserved together in such close proximity. From our results, it is suggested that much of the high temperature structural evolution occurred at least 900 m.y. ago and reflects a southern continuation of the Rayner Belt.
Section snippets
Regional geology
The main tectonic subdivisions of the Prince Charles Mountains and surrounding regions are shown in Fig. 1. Basement rocks in the North Mawson Escarpment are thought to constitute part of the Lambert Complex (Kamenev et al., 1993) that underwent major crustal development in the period 2500–1600 Ma (Boger et al., 2001, Boger et al., 2008, Mikhalsky et al., 2006a, Mikhalsky et al., 2006b). The main rock types recognised so far include c. 2475 Ma and c. 2425 Ma granodioritic–granitic gneisses, c. 2120
Previous dating—North Mawson Escarpment
Amongst the first radiometric dating results for the North Mawson Escarpment were Rb–Sr whole-rock isochron ages of 941 ± 94 Ma for felsic gneiss at Lawrence Hills, and 551 ± 71 Ma for muscovite–biotite granite at Harbour Bluff (P.A. Arriens, unpublished data, cited in Tingey et al., 1981, Tingey, 1982, Tingey, 1991). Other reconnaissance Rb–Sr model ages of c. 2989 Ma, c. 2575 Ma and c. 1755 Ma, and several TIMS Pb–Pb model ages of c. 1310–705 Ma, were obtained by Russian workers (Mikhalsky et al., 2001
Sample selection and analytical methods
Absolute age determinations were made for eight samples from the North Mawson Escarpment, the results of which are described below in order of latitude moving southwards. Three samples, including two grey gneisses (69 and 102), and one metapelitic schist (94), were chosen because they represent basement rocks that constitute a large volume of the study area (Fig. 2). Another sample, 21.1, is from an amphibolite lens occurring within voluminous grey gneiss in the Rofe Glacier area. The remaining
Sample 66 granite dyke, Waller Hills
Sample 66 was collected from a 0.5 thick leucogranite dyke at 72°48′20″S, 68°04′19″E, that intrudes multiply deformed migmatitic paragneisses and provides a minimum age for their timing of formation and high-grade metamorphism (Fig. 3). Both the orientation of the dyke and its internal biotite foliation are axial planar to a late phase of upright, open-tight folds. Late synkinematic emplacement of the dyke with these folds is inferred. The sample is composed of 1–5 mm antiperthitic plagioclase
Interpretation of results
Our results, together with radiometric work presented by Mikhalsky et al. (2006a), Corvino and Henjes-Kunst (2007) and Mikhalsky and Roland (2007), demonstrate that the North Mawson Escarpment was affected by tectonothermal events at 2490–2420 Ma, 2180–2080 Ma, 940–880 Ma and 530–490 Ma (Table 1). These time intervals are constrained by the spread of the most concordant U–Pb zircon age spectra, plotted in Fig. 10. The Palaeoproterozoic events are related primarily to the production and initial
Regional implications
It is not possible to make a direct correlation of any of the ages presented here with the Archaean Ruker Complex, as illustrated in Fig. 11. Boger et al., 2001, Boger et al., 2008 have emphasised that the Ruker and Lambert complexes evolved separately throughout the Proterozoic, and were not assembled until events related to Gondwana construction around 530–490 m.y. ago. This is to some extent supported by the increasingly intense reactivation of Palaeoproterozoic rocks at c. 500 Ma marginal to
Conclusions
The North Mawson Escarpment records a complex Proterozoic crustal evolution dominated by tectonothermal events at 2490–2420 Ma, 2180–2080 Ma, 940–880 Ma and 530–490 Ma. The 2490–2420 Ma and 2180–2080 Ma events involved the generation and partial reworking of volumetrically important sialic crust, now manifest as thick bodies of transposed granodioritic–granitic grey gneiss. Early structures and metamorphic assemblages associated with these events are as yet unresolved or have been largely
Acknowledgements
Fieldwork for this study was part of the 2002–2003 Prince Charles Mountains Expedition of Germany and Australia funded by the Australian Antarctic Division and Bundesanstalt für Geowissenschaften und Rohstoffe. We thank members of the Australian National Antarctic Research Expeditions for their friendship and logistic support, in particular Bill Baxter and Gary Kuehn for their field guidance. Eugene Mikhalsky and Nigel Kelly are thanked for their thorough and helpful reviews. Peter Cawood is
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