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Introduction

Australia comprises numerous elements of Archaean crust, which are considered to have been organised into three main crustal segments by 1.8 Ga, namely the North, West and South Australian Cratons. All three cratons subsequently record a broadly comparable succession of first intracratonic (1.6-1.3 Ga) and then convergent/collisional (1.3-1.0 Ga) events, which combined with the available palaeomagnetic evidence at the time, led many authors to suggest that the three cratons formed part of a single intact continent by ca. 1.7 Ga (e.g. Veevers and McElhinny, 1976, Idnurm and Giddings, 1988). Alternative views suggest a more dynamic interpretation, which saw the birth of the Australian continent from its three cratonic parts through collisional processes along late-Mesoproterozoic belts which include the Musgravian and Albany-Fraser Orogens (e.g. Myers et al., 1996).

At present, the few reliable age-matched palaeomagnetic data for the crustal blocks and the limited firm geochronological constraints on the tectonothermal development of the separating orogenic belts, do not lend themselves to an unbiased assessment of the validity of either the static or dynamic model of the formation of the Australian continent (see Wingate and Evans (2003)). Recent years have however seen a significant increase in available age data on both the cratons and various parts of the Albany-Fraser and Musgrave Orogens, but rather than resolving the issue, these data have provoked renewed debate on the amalgamation history of Australia, mainly because of the confirmation of matching geological events between the terranes. Most authors however seem to agree that the final formation of Australia from its component cratons occurred within a Mesoproterozoic time-frame, forming the Albany-Fraser and Musgrave Orogenies.

Aims, significance and expected outcomes

This project proposes to study the largely unknown northeast part of the Albany-Fraser Orogen, which forms the missing link between the western and central Albany-Fraser Complex and the central Australian Musgrave Complex (Fig. 1). Expected outcomes include the first geochronological data for this region, which will be critical to allow correlation between the Albany-Fraser and Musgrave Complex and a better understanding of the amalgamation history of the Australian continent. Secondly, new palaeomagnetic data will be obtained to better constrain the position of Australia at around 1200 Ma (i.e. during the time-frame of the amalgamation of the Rodinia Supercontinent). Thirdly, the proposal includes cutting-edge geochronological techniques, and the first application of the zirconolite chronometer to date magnetisation in basic rocks. Zirconolite geochronology has very recently been developed by researchers at UWA (Drs. Rasmussen and Fletcher) and provides a very innovative approach to geochronology.

Results from this study will not only enhance our understanding of the NE Fraser Complex specifically, but of the Albany-Fraser-Musgrave Orogen in general.

The scientific outcomes will be published in the Australian Journal of Earth Sciences and/or international journals such as Chemical Geology (zirconolite geochronology) and Precambrian Research (Mesoproterozoic amalgamation of Australia). The results will also be presented at suitable international conferences.

Research plan

This project proposes a dual approach to obtain critical information on the NE Fraser Complex. Firstly, new palaeomagnetic data will be obtained for the Fraser Complex and reported basic dykes intruding both the Fraser Complex and the Yilgarn Craton, in an effort to refine the position of the Australian Cratons vis-à-vis other continental blocks at ca. 1200 Ma. The proposed targets are selected from a preliminary survey conducted by Dr. Sergei Pisarevsky on one sample of gabbroic dyke, dated at ~1212 Ma by Wingate et al. (2000), emplaced in the Yilgarn Craton foreland to the northwest of the Cundeelee Fault. A preliminary palaeomagnetic contact test indicated a stable primary remanence similar to those of the 1.2 Ga metasediments of the Mt. Barren Group. In this study, large samples will be collected to allow U-Pb SHRIMP zircon/baddeleyite/zirconolite geochronology, while numerous dykes and the country rock they intrude will be sampled using a hand-held drill to allow a full palaeomagnetic study.

Secondly, a reconnaissance U-Pb SHRIMP geochronological study will be conducted on the northeastern parts of the Fraser Complex, in an attempt to fill in the regional gap in age-data and allow direct comparison between the Fraser Complex and the Albany Belt to the southwest and the Musgrave Complex to the northeast. Mainly igneous lithologies will be targeted, including units from the deformed foreland (Yilgarn Craton) and deformed units within the Fraser Complex itself which are expected to include 1.7-1.6 Ga and post-1.3 Ga suites. High-grade gneisses will be given preference during the study in an attempt to constrain peak metamorphic conditions through the identification and dating of low Th/U zircon overgrowths, in an attempt to confirm and refine published accounts of a two-stage metamorphic development of the southwestern part of the complex (Clark et al., 2000). The obtained crystallisation and metamorphic ages will allow a direct comparison with dated units in the southwestern Albany-Fraser Complex (Clark et al., 1999, 2000, Nelson et al., 1995) and units dated in the Musgrave Complex (e.g. White et al., 1999). One sample of quartzite within the Fraser Complex (Cundeele sheet), which could either be a correlative to the Woodline Beds further northeast or the Mt. Barren Group to the southwest, will be collected for a detrital zircon SHRIMP analysis to resolve this issue. This data will at the same time constrain a derivation pattern for these metasedimentary rocks, providing a fingerprint against which metasedimentary units from the Australian and Antarctic blocks can be compared.

Logistics and cooperation

The proposed research ties in well with efforts of the Geological Survey of Western Australia (GSWA) to promote exploration in so-called “greenfields” areas. There is a lot activity at present on the Fraser complex, where there are some indications of chromitite and possible nickel sulphide mineralisation, and the GSWA plan to map the region at a scale of 1:100,000 in the coming years. GSWA endeavoured to enhance the prospectivity of the area extending southwest from Zanthus through a regional regolith geochemistry survey (Morris et al., 2000), which certainly had some positive influence on exploration activity.

In preparation for the planned mapping, the acquisition of a high-resolution aeromagnetic coverage will be completed in 2006, which will also greatly benefit the research proposed here. Although the GSWA does not envisage to be directly involved, some in-kind support and extensive research cooperation will be possible.

The access to the area is made difficult by the remote nature and lack of potential support in the event of accidents of equipment failure. For this reason, the support from GSWA, with on-going communications and possible emergency response facilities, are of great value to this proposal. The planned fieldwork will take the team (Drs. De Waele and Pisarevsky) into remote areas for periods of time. A well-serviced 4WD vehicle will therefore be used, equipped with food, water and emergency supplies, and a satellite phone for emergencies. Wherever possible, the team will stay at farmsteads or stations, but camping gear will be taken to allow overnight stays in remote areas. Close contact and cooperation with the team of GSWA will further enhance the safety aspect of this project.

Methods and techniques

Palaeomagnetic sampling: The main targets for the palaeomagnetic study are gabbroic dykes and metasedimentary units (quartzite) from the metasedimentary rocks within the Fraser Complex. Outcrop will be located on the published geological maps and from aerial photography. Standard palaeomagnetic sampling techniques, using a hand-held drill, and analyses methodologies will be applied. Baked-contact tests to establish the primary nature of magnetisations will be applied wherever possible. To isolate primary magnetisations from possible overprint components, laboratory analyses will include detailed thermal and alternating field demagnetisation.  Rock magnetic studies will be carried out to characterise fully the nature of magnetic carriers (Dunlop and Ozdemir, 1997).

Overview map of Western and Central Australia showing the Albany-Fraser-Musgrave Complexes, and the approximate location of dated samples and palaeomagnetic sample sites. Note the absence of geochronological or palaeomagnetic data in the study area.

U-Pb Geochronology: Target minerals (zircon, baddeleyite and monazite) will be separated from crushed rock using both heavy liquid and magnetic separation, while in situ microprobe analysis and drilling will be carried out on polished sections of basic dykes to identify zirconolite. Zircon, baddeleyite and monazite will be mounted with appropriate standards, polished mid-section and imaged on a Scanning Electron Microscope fitted with cathode-luminescence detector to reveal zoning and internal fabric. Compositions of monazite will be determined on electron microprobe to allow standard matching. Isotopic measurements will be conducted on the Sensitive High Resolution Ion Microprobe of the Perth Consortium at Curtin University of Technology, following methodologies similar to those reported by Nelson (2001) and Wingate and Compston (2000). Data reduction strategies follow Claoué-Long (1994) using software by Ludwig (2001a; 2001b) for zircon and baddeleyite, and follow Rasmussen and Fletcher (2002) for monazite. Zirconololite has recently been found to be an excellent chronometer for mafic igneous rocks (Rasmussen and Fletcher, 2004). The mineral, once identified, will be extracted from polished sections using a microdrill, and mounted separately. The methodologies used to date zirconolite are described by Rasmussen and Fletcher (2004).

Proposed timing

Fieldwork will be conducted by both Bert De Waele and Sergei Pisarevsky, and part in conjunction with Dr. Bruce Groenewald of the Geological Survey of Western Australia, in the first half of 2006. Palaeomagnetic sampling sites will be identified from published geological maps and reports, and from previously located and sampled dykes. A detailed outcrop map will be made for each sample site, and a sufficient sample set collected to allow the determination of a reliable palaeopole and establish the primary or secondary nature of magnetisation (baked contact test). Palaeomagnetic sampling of the quartzites will entail a detailed outcrop map, and careful measurement of primary structures. The quartzites and associates metapelites will be sampled for monazite and detrital zircon studies. A reconnaissance geochronological survey will target all mapped lithologies across the Zanthus, Cundeelee and Plumridge map sheets, including high-grade ortho- and paragneisses and intrusive units mapped as Archaean basement (Yilgarn craton) and Mesoproterozoic Fraser Complex. Dr. Pisarevsky will continue sampling for palaeomagnetism during these transects.

20 days in the field are planned to achieve sampling of all primary palaeomagnetic targets (Fraser dyke swarm and country rocks; 10 days) and a series of reconnaissance traverses covering the three map sheets.

References

Clark, D. J., Hensen, B. J. and Kinny, P. D. (2000) Precambrian Research, 102, 155-183.

Clark, D. J., Kinny, P. D., Post, N. J. and Hensen, B. J. (1999) Australian Journal of Earth Sciences, 46, 923-932.

Dunlop, D. J. and Ozdemir, O. (1997) Rock Magnetism. Fudamentals and Frontiers, Cambridge University Press, Cambridge.

Idnurm, M. and Giddings, J. W. (1988) Precambrian Research, 40/41, 61-88.

Morris, P. A., Sanders, A. J., McGuinness, S. A., Coker, J. and King, J. D. (2000) In 1:250 000 RegolithGeochemistry SeriesGeological Survey of Western Australia, pp. 45.

Myers, J. S., Shaw, R. D. and Tyler, I. M. (1996) Tectonics, 15, 1431-1446.

Nelson, D. R., Myers, J. S. and Nutman, A. P. (1995) Australian Journal of Earth Sciences, 42, 481-495.

Rasmussen, B. and Fletcher, I. R. (2002) Earth and Planetary Science Letters, 197, 287-299.

Rasmussen, B. and Fletcher, I. R. (2004) Geology, 32, 785-788.

Veevers, J. J. and McElhinny, M. W. (1976) Earth Science Reviews, 12, 139-159.

White, R. W., Clarke, G. L. and Nelson, D. R. (1999) Journal of Metamorphic Geology, 17, 465-481.

Wingate, M. T. D., Campbell, I. H. and Harris, L. B. (2000) Australian Journal of Earth Sciences, 47, 309-313.

Wingate, M. T. D. and Evans, D. A. D. (2003) In Proterozoic East Gondwana: Supercontinent Assembly and Breakup, Vol. 206 (Eds, Yoshida, M., Windley, B. and Dasgupta, S.) Special Publication of the Geological Society, London.