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Along the west coast of Western Australia, three blocks of Mesoproterozoic metamorphic and magmatic rocks crop out within the Phanerozoic sediments of the Perth and Southern Carnarvon basins: the Northampton, Mullingarra and Leeuwin complexes. The origin of these basement blocks is controversial: Traditionally, they are interpreted as outcrops of a poorly exposed Mesoproterozoic orogen along the coast of Western Australia. However, an alternative model regards them as allochthonous blocks that originated further south and were translated to their present position during the Neoproterozoic (Fitzsimons, 2001). A metamorphic history has been previously established for the Northampton Complex, the biggest and northernmost of the basement blocks (Bruguier et al., 1999): Granulite facies metamorphic conditions were reached at 1080 Ma, followed by the intrusion of late-tectonic granites at 1068 Ma. Rapid cooling is assumed for the first 20 million years following peak metamorphism. The intrusion of post-tectonic pegmatite dykes at 990 Ma marks the end of deformation and magmatic activity. However, a small number of zircon rims analysed during a provenance study on metasediments from the Northampton Complex (Ksienzyk et al., this conference) indicated a more complex metamorphic history and initiated further investigations. In the present study, 62 zircon rims and 51 monazites were dated by SIMS analyses. The new data confirm that the metamorphic history is more complicated and involves several stages of zircon and monazite growth. Monazite ages range from 1100-1020 Ma, with the majority of ages between 1090-1050 Ma. Metamorphic zircon rims yielded a wide range of ages from 1730-1020 Ma. The majority of ages fall into three groups between 1100-1020 Ma, 1225-1170 Ma and 1330-1280 Ma. The youngest group is characterized by low Th contents and low Th/U ratios and is interpreted as in-situ metamorphic zircon growth in the Pinjarra Orogen. The older groups show higher Th contents and Th/U ratios and probably represent earlier metamorphic events in the source regions of the sediments. The two groups correspond to the ages of the two tectonic stages of the Albany-Fraser Orogen in southwestern Australia (1214-1140 Ma and 1345-1260 Ma; Clark et al., 2000), which is therefore considered a likely source region for the sediments. This is also supported by provenance studies on detrital zircon cores from the Northampton Complex and can be tentatively regarded as evidence for a more southerly origin of the complex and later transport to its present position.
O. Bruguier et al., Contrib. Mineral. Petr. 136, 258-272 (1999)
D. J. Clark et al., Precambrian Res. 102, 155-183 (2000)
I. C. W. Fitzsimons, Geol. Soc. Australia, Abstr. 65, 39-42 (2001)
A. K. Ksienzyk et al., this conference
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