The East Antarctic Shield

Location

The East Antarctic Shield is an ancient Precambrian craton or continental shield which once formed a central part of the Gondwanan supercontinent. Despite only limited and intermittent outcrop, the current general consensus is that the East Antarctic Shield comprises a number of discrete Archaean terranes separated by a Proterozoic mobile belt.

Within the Australian Antarctic Territory, Archaean terranes of the East Antarctic Shield are represented by the Vestfold Hills of Princess Elizabeth Land, the Napier Complex of Enderby Land and the southern Prince Charles Mountains of MacRobertson Land. Outcrops of the intervening Proterozoic mobile belt have been recognised within the Rayner Complex of Enderby and Kemp Lands, the Stillwell Hills of Kemp Land, the Mawson coastline of Kemp and MacRobertson Lands, the Ingrid Christensen coastline of Prydz Bay, Princess Elizabeth Land, and the northern Prince Charles Mountains of MacRobertson Land.

General geology

Exposures of the Proterozoic mobile belt comprise reworked Archaean rocks as well as rocks of Middle to Late Proterozoic origin. Basement lithology's of both the Archaean terranes and the Proterozoic mobile belt were variably subjected to amphibolite to granulite facies metamorphism, intense deformation, and granitoid intrusion at ~1000ma. Evidence of subsequent geological events, including the localised deposition of Permo-Triassic sedimentary sequences and the intrusion of Mesozoic and Cenozoic dykes, stocks and sills, is well documented within the Prince Charles Mountains.

Although the Proterozoic Mobile Belt was originally considered to comprise a single terrane which underwent granulite facies metamorphism at ~1000ma, subsequent research has revealed that certain areas, exposed along the Prydz Bay coastline, comprise granulite mineral assemblages formed during a second pervasive thermal event in the Early Cambrian, the so-called 500ma or Pan-African event, with little or no evidence of an earlier 1000ma event. This widespread event, which is known to have affected other areas of Gondwana including southern Africa, is thought to have caused exhumation of the East Antarctic shield from as much as 15 km depth, pegmatite and late granite intrusion, greenschist facies metamorphism and shear zone formation. In addition, alkaline dykes of ~500ma age intruded most of the Prydz Bay area, the Prince Charles Mountains and large parts of MacRobertson and Enderby Lands.

The cooling history (sub-120°C) of the East Antarctic Shield subsequent the 500ma event is poorly understood. Reconnaissance apatite fission track analyses of Precambrian basement samples, including Prince Charles Mountains samples, indicate discrete cooling episodes at various times during the Phanerozoic. Fission track data from the continental margin west of Prydz Bay as well as the Prince Charles Mountains indicate that a major phase of cooling was initiated during the Mesozoic (~150ma). This cooling phase could have been associated with the separation of East Antarctica and India during Gondwanan break-up.

Information sources

Arne, D. C., Kelly, P. R., Brown, R. W. and Gleadow, A. J. W. (1993) Reconnaissance apatite fission-track data from the East Antarctic Shield. In: Findlay, R. H., Unrug, R., Banks, M.R. and Veevers, J.J. (editors) Gondwana Eight: Assembly, Evolution and Dispersion, p. 605-611. A.A. Balkema, Rotterdam.

Sheraton, J.W. and Black, L.P. (1988) Chemical evolution of granitic rocks in the East Antarctic Shield, with particular reference to post-orogenic granites. Lithos 21, 37-52.

Stüwe, K. and Sandiford, M. (1993) A preliminary model for the 500ma event in the East Antarctic Shield. In: Findlay, R. H., Unrug, R., Banks, M.R. and Veevers, J.J. (editors) Gondwana Eight: Assembly, Evolution and Dispersion, p. 125-130. A.A. Balkema, Rotterdam.