(oral p.) I.C.W. Fitzsimons
Tectonics SRC, Applied Geology, Curtin University, GPO Box U 1987, Perth WA 6845, Australia;
<ianf@lithos.curtin.edu.au>.
Grenville-age metamorphic belts are widespread in East Antarctica and its Gondwana neighbours of Australia, India and southern Africa. These belts preserve evidence for multiple tectonic events be-tween 1350 and 900 Ma, and although previously regarded as a single continuous collisional orogen,
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-they are now known to comprise a number of distinct provinces juxtaposed by 550 Ma tectonism.
Three major Grenville-age domains exist in Antarctica, each with a different age for high-grade collisional tectonism: namely the Maud (1090-1030 Ma), Rayner (990-900 Ma) and Wilkes
(1330-1130 Ma) provinces, which are closely related to similar rocks in southern Africa, the eastern Ghats of India and the Albany-Fraser Orogen of Western Australia, respectively. A fourth group of Grenville-age rocks occurs as displaced blocks in the Neoproterozoic Pinjarra Orogen of Western Australia, which preserve evidence for high-grade tectonism at 1090-1030 Ma. This age range corresponds closely with that of the Maud Province, raising the possibility that the two are related.
Grenville-age rocks in the Pinjarra Orogen comprise 1090 Ma granitic orthogneiss in the Leeuwin Complex and psammitic to pelitic paragneiss in the Northampton and Mullingarra complexes, deformed and metamorphosed to amphibolite or granulite facies at I 080-1030 Ma. This tectonism is traditionally believed to reflect collision of Australia with India at c. 1100 Ma, but this need not be the case given that East Gondwana is now known to have assembled al 550 Ma and it is quite likely that India and Australia only attained their Gondwana positions at this time. The Maud Province is interpreted as an l 150-1 I 00 Ma magmatic arc and back -arc basin, developed at the margin of an unexposed craton that collided with the southeastern margin (present-day coordinates) of the Kaapvaal-Zimbabwe Craton of southern Africa at c 1100 Ma, resulting in pervasive deformation, granulite-facies metamorphism, and magmatism at 1090-1030 Ma. The unidentified craton is widely assumed to be the East Antarctic Shield, but again this need not be the case given widespread evidence that East Antarctica did not assemble until 550 Ma.
Evidence for the possible identity of the colliding cratons in both cases is provided by detrital zircon.
Comparison of SHRIMP U-Pb zircon age data for three paragneiss samples from the Pinjarra Orogen (BRUGUIER et al. 1999, COBB 2000) and two samples from the Maud Province (ARNDT et al. 1991, HARRIS 1999) reveals a number of striking similarities. 2 I 00-L 110 Ma detrital grains dominate all samples, with a marked lack of c 1500 Ma grains. Although different samples are dominated by dif-ferent populations within this range, significant detrital populations at 1100-1115, 1160-1220, 1280-1310, 1350-1390, and 1420-1440 Ma occur in samples from both regions. Age spectra from the Pinjarra Orogen and Maud Province are indistinguishable within the uncertainties of the data, and imply that paragneisses in both areas were part of the same sedimentary sequence eroded from the same source rocks; i.e. they are fragments of the same collisional orogen. Pre-1130 Ma detrital populations in both areas correspond to the ages of basement rocks in the Albany-Fraser Orogen and Wilkes Province, whereas 1130-1100 Ma grains were eroded from the magmatic arc exposed in the Maud Province, consistem with deposition at an active margin of an Australian-Antarctic Craton.
The sedimentary rocks were then deformed and metamorphosed as the Kaapvaal-Zimbabwe Craton collided with this convergent margin at 1100 Ma.
Arndt, N.T., Todt. W .. Chauvel, M .. Tapfer, M. & Weber. K. (1991): Geo!. Rundschau 80: 759-777.
Bruguier, 0 .. Bosch. D .. Pidgeon. R .. Byrne. D. & Harris. L. (1999): ConLrib. Miner. Petrol. 136: 258-272.
Cobb, M.M. (2000): unpubl. BSc (Hons) Thesis. Cunin University of Technology, Penh.
Harris, P.O. ( 1999): unpubl. PhD Thesis. Rand Afrikaans University, Johannesburg.
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-Correlation of the 1.1 Ga Maud Province with its Gondwana neighbours and the continuation of the East African Orogen into Antarctica
(poster p.) I.C.W. Fitzsimons
Tectonics SRC, Applied Geology, Curtin University, GPO Box U1987, Perth WA 6845, Australia;
<ianf@lithos.curtin.edu.au>.
The East African Orogen has long been interpreted as a north-south trending continental collision zone resulting from late Neoproterozoic closure of the Mozambique ocean, but the continuation of this suture into Dronning Maud Land of Antarctica remains enigmatic (SHACKLETON 1996). The location of a collisional suture is commonly identified by rock types typical of the suture zone itself, such as ophiolites or eclogites, but such features may be poorly preserved in deeply eroded Precambrian orogens. Given that a major suture zone is unlikely to juxtapose crustal blocks with similar histories, another constraint on the location of a suture is that it should not pass through a region of apparently consistent geology.
The Maud Province of western Dronning Maud Land is characterized by high-grade gneiss with metamorphic ages of 1090-1030 Ma (ARNDT et al. 1991). Further east, in central Dronning Maud Land, magmatism and granulite-facies metamorphism at 650-500 Ma is attributed to closure of the Mozambique ocean, but this region still preserves protoliths with 1090-1030 Ma metamorphic ages (JACOBS et al. 1998). Similar ages have been retrieved from gneisses along the eastern edge of the Zimbabwe craton in western Mozambique (MANHICA et al. 2002) and in the Lurio Foreland of northeastern Mozambique (KRONER et al. 1997), which lie adjacent to Dronning Maud Land in Gondwana reconstructions. It is likely that these high-grade rocks represent dispersed fragments of an originally contiguous late Mesoproterozoic terrane. JACOBS & THOMAS (2002) suggested that these rocks formed the Lurio-Maud Microplate within the Mozambique ocean, and that the Mozambique suture had two strands passing either side of this microplate. However, lack of a pervasive Neoproterozoic overprint in western Dronning Maud Land and western Mozambique implies that late Mesoproterozoic rocks in these regions developed in situ adjacent to the eastern margin of the Archaean Kaapvaal-Zimbabwe craton. Indeed these rocks have been interpreted as a Mesoproterozoic collision zone between the Kaapvaal-Zimbabwe craton and another unidentified craton (GROENE-WALD et al. 1995). It follows that the Lurio-Maud terrane is likely to lie on the southern African side of any Mozambique suture zone.
In fact there is some doubt whether a Mozambique suture extends into Antarctica at all. Dominant late Neoproterozoic structural trends in the Lurio Belt, central Dronning Maud Land and S0r Rondane Mountains are sub-parallel to the Antarctic coastline in Gondwana reconstructions, although trans-current structures are developed at a higher angle to the coastline close to the margin of the Kaapvaal-Zimbabwe craton. These coast-parallel structures follow the trend of the Neoproterozoic Zambezi belt of south-central Africa, where there is new structural, petrological and isotopic evidence for Neoproterozoic ocean closure and continental collision (DEWAELE et al. 2003, JOHN et al. 2003).
These data tum traditional models for the Neoproterozoic assembly of Gondwana on their head, and suggest that Neoproterozoic tectonism in Dronning Maud Land could have developed along the southern margin of an extension of an east-west "Zambezi" suture, rather than along a north-south
"Mozambique" suture. If this suture passes into Antarctica it must do so in the Ltitzow Holm Bay region of eastern Drenning Maud Land.
Arndt, N.T .. Todt, W .. Chauvel. M .. Tapfcr. M. & Weber, K. (1991): Geo!. Rundschau 80: 759-777.
De Waele. B .. Wingate, M.T.D .. Fitzsimons. l.C.W. & Mapani, B.S.E. (2003 in press): Geology.
Groenewald, P.B., Moyes. A.B., Grantham, G.H. & Krynauw, J.R. (1995): Precambrian Res. 75: 231-250.
Jacobs, J. & Thomas, R. (2002: In: Antarctica at the Close of the Millennium, R. Soc. N.Z. Bull. 35: 3-18.
Jacobs, J .. Fanning, C.M .. Henjes-Kunst, F .. Olesch. M. & Paech. H.-J. (1998): J. Geo!. 106: 385-406.
John, T .. Schenk, V., Haase, K., Scherer, E. & Tembo, F. (2003): Geology 31: 243-246.
Kroner. A., Sacchi, R., Jaekel. P. & Costa, M. (1997): J. African Earth Sci. 25: 467-484.
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-Manhica. A .. Grantham, G.H, Amistrong. R.A., et al. (2002): Spec. Publ. Geol. Soc. London 184: 303-322.
Shackleton. R.M. (1996): J. African Eanh Sci. 23: 271-287.
IGCP 440 geodynamic map of Rodinia - draft map of Antarctica (poster p.)
I.C.W. Fitzsimons' & J. Jacobs2
1Tectonics SRC, Applied Geology, Curtin University, GPO Box Ul 987, Perth WA 6845, Australia, ianf@lithos.curtin.edu.au
2Universitiit Bremen, Fachbereich Geowissenschaften, P.O.Box 330440, D-28334 Bremen, Germany;
<jojacobs@uni-bremen.de>.
One of the principal aims of IGCP 440 (Rodinia Breakup and Assembly) is to produce a set of maps depicting the tectonic evolution of Rodinia, the supercontinent believed to have assembled at the close of the Mesoproterozoic. Central to this endeavour will be a I: 10 million scale geodynamic map of Rodinia, and a number of regional compilers have been approached to draft maps for their areas of expertise, before these maps are combined into one or more possible Rodinia configurations.
The Rodinia Map Steering Committee has developed a legend, which classifies mappable units on the basis of tectonic setting and age. Map colours are based primarily on tectonic setting, with the following broad categories: intracratonic magmatic rocks, AMCG suite, continental arc rocks, island arc rocks, rift related rocks, foreland basin rocks, intracratonic basin rocks, passive margin rocks, and high-grade metamorphic rocks of unspecified or unknown tectonic setting. Each of these categories is subdivided into age ranges of 1600-1300 Ma, 1300-1100 Ma, 1100-900 Ma and 900-700 Ma. Rocks younger than 700 Ma will not be shown, since they did not exist in Rodinia times, and those older than 1600 Ma are depicted as cratonic blocks with some further subdivision into age ranges of greater than 2500 Ma, 2500-2200 Ma, 2200-2000 Ma, 2000-1800 Ma, and 1800-1600 Ma. There are means of depicting a later metamorphic overprint on units of a particular tectonic setting, and of showing protolith ages in regions of later high-grade metamorphism. Each unit will be linked to tables of geochronological data and relevant references.
The task of the regional compilers is to produce a map of their region using this legend. Antarctica presents a number of special problems in this regard. Most units within the age range of interest are deeply eroded metamorphic rocks whose regional context is clouded by sparse outcrop and minimal geophysical data, making tectonic interpretations problematic in many cases. Antarctica is, however, a critical component in many of the suggested reconstructions of Rodinia, and it is important that the Antarctic regional map is as complete and accurate as possible. We have compiled a preliminary map of the region, at greater detail than the final version to be incorporated into the Rodinia Map. The map is relatively simple, with poor exposure restricting the number of units that can be shown, but in some cases our interpretations have been based on limited and/or conflicting data and we seek feedback from the Antarctic Earth Science community. We would like comments on all aspects of the map including our choice of mappable units, the location and orientation of boundaries between them, and our interpretations of their tectonic setting.
An important outcome of the Antarctic map will be to identify priorities for future work. Problems already highlighted during our preliminary compilation include: limited knowledge of basement protolith ages in the LUtzow Holm Bay and Sl?lr Rondane area; almost no knowledge of the lateral extent, field relationships and tectonic setting of 750 Ma granitoid plutons reported from eastern Dronning Maud Land; little understanding of the age and relationships between different components of the southern Prince Charles Mountains, and a need for geophysical surveys to be focused around and inland from regions where the presence of a major crustal boundary is suspected, including
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-Llitzow Holm Bay, the Denman Glacier region, the Shackleton Range, and the central Transantarctic Mountains.
Obviously Antarctica should not be considered in isolation, and some of our interpretations may be influenced significantly by data from better-exposed rocks in Africa, India, Australia and elsewhere, but at this stage we hope to derive a map that is at least compatible with available data from Antarctica. Regional maps will be combined and finalized early in 2004, with plans for a preprint of the final Rodinia Map to be presented at IGC in Florence in August later that year.