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Anna Ksienzyk, Universitetet i Bergen (Norway)
Joachim Jacobs, Universitetet i Bergen (Norway)
Jan Kosler, Universitetet i Bergen (Norway)
Keith Sircombe, Geoscience Australia (Australia)
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The Maud Belt (MB) and the Pinjarra Orogen are both late Mesoproterozoic orogens which formed during continent-continent collision. Based on palaeomagnetic data, recent Rodinia models suggest a collision between Kalahari and Western Australia along a combined MB/Pinjarra Orogen. Although attempts have been made to support this model with geochronological data, the amount of available data is as yet insufficient to allow statistically sound conclusions. In this contribution, we present 941 new LA-ICPMS and SHRIMP ages of detrital zircon cores from the MB and the Pinjarra Orogen. Seven samples from the Northampton Complex (NHC, Pinjarra Orogen) show similar age distributions with the majority of ages between 1300-1050 Ma, other prominent zircon populations between 1700-1400 Ma, and only few ages older than 1800 Ma. Probable source regions for these ages are the Albany-Fraser Orogen and the South Australian Craton. The age of regional metamorphism in the NHC has been constrained at 1080 Ma and the youngest ages are therefore attributed to a metamorphic overprint.
Three samples from the MB show variable age distributions, but all are dominated by ages between 1250 and 1000 Ma and contain older age groups between 1450-1250 Ma and 2050-1650 Ma. Ages between 1650-1450 Ma are virtually absent. Both Palaeo- and Mesoproterozoic rocks are currently exposed in Kalahari and could have contributed the corresponding zircons, while the young ages between 1100-1000 Ma are interpreted as metamorphic or magmatic events during collision along the MB. The absence of data between 1650-1450 Ma is in good agreement with the potential source regions in Kalahari, none of which could have supplied zircons of this age. However, this gap in the age distribution is inconsistent with a contribution of the Australian Craton to the sediments of the MB, as significant magmatic and metamorphic activity occurred in SW Australia between 1650-1450 Ma and is recorded in the sediments of the NHC. The MB and Pinjarra Orogen show therefore distinct zircon populations and the argument that they are part of the same sedimentary sequence and thus belong to the same orogenic belt is not supported by the new data. The dominant age group (1300-1000 Ma) occurs in both regions, but these ages are to be expected in Grenville-age orogens. As the latter occur worldwide, this match cannot be considered strong evidence for a correlation. More important are the older zircons, and here the MB is characterised by ages between 2050-1650 Ma and an absence of data between 1650-1450 Ma, while the samples from the NHC yielded a number of ages between 1650-1450 Ma, but contain very few ages older than 1800 Ma. The samples from the MB show therefore a clear Kalahari signature, while the sediments of the NHC were probably derived from sources within Australia. Both regions show significantly different age distributions and no clear evidence for a Kalahari-Western Australia connection was found.
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