Composition,age and tectonic setting of amphibolites in the central Bushmanland Group,Western Namaqua Province,southern Africa |
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| Institution: | 1. Department of Geochemistry, University of Cape Town, Rondebosch 7700 South Africa;2. Bernard Price Institute of Geophysics, University of the Witwatersrand, Johannesburg 2000 South Africa;3. Department of Geology, University of Cape Town, Rondebosch 7700 South Africa;1. Institute of Earth Sciences, University of Graz, Graz, Austria;2. Center for Earth Sciences, University of Vienna, Vienna, Austria;3. Department of Earth Sciences, University of Bergen, Bergen, Norway;4. Department of Geology, University of Dar es Salaam, Dar es Salaam, Tanzania;1. Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa;2. Saskatchewan Isotope Laboratory, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada;3. Department of Geology, University of Pretoria, Pretoria 0001, South Africa;1. Department of Geology & Geography, University of North Carolina at Pembroke, Pembroke, NC 28372, USA;2. Department of Mining and Environmental Geology, University of Venda, Private Bag X5050, Thohoyandou, Limpopo Province, 0950, South Africa;3. Department of Geology, University of Georgia, Athens, GA 30602, USA;1. Department of Agroecology in the Tropics and Subtropics (490f), Institute of Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute), University of Hohenheim, 70599 Stuttgart, Germany;2. Biodiversity Conservation and Ecosystem Management, The Nelson Mandela African Institute of Science and Technology (NM-AIST), PO Box 447, Arusha, Tanzania;3. Centre for Agroecology, Water and Resilience, Coventry University, Ryton Gardens, Ryton-on-Dunsmore, Coventry CV8 3LG, UK;4. Department of Societal Transformation and Agriculture (430b), Institute for Social Sciences in Agriculture, University of Hohenheim, 70593 Stuttgart, Germany;1. Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa;2. Council for Geoscience, P.O. Box 572, Bellville, South Africa |
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| Abstract: | The Western Namaqua Province (WNP) of southern Africa is composed of several supracrustal assemblages and intrusive granitoid suites that were formed during the mid-Proterozoic 2.0-1.0 Ga ago. The Bushmanland Group (BG) has been the subject of intense study recently because of its considerable strategic metal potential, hosting several giant stratiform base metal deposits (e.g., Gamsberg and Aggeneys). The BG contains amphibolites which represent metamorphosed tholeiitic basalts with MgO (9-4%) and enriched in Ni relative to modern basalts, a feature reminiscent of Archaean tholeiites. Post depositional mobility of elements is probably widespread and hampers recognition of parental magma composition and source region characteristics. However, the amphibolites have yielded a Sm-Nd isochron age of 1649 ± 90 Ma (ϵNd(T) = −0.48 ± 0.89), which is interpreted as the time of extrusion of the basalt precursors, and by inference a minimum sedimentation age for the underlying metalliferous BG sequence. It follows that the BG sequence evolved during an intermediate period in the history of the Namaqua Province, after major crustal stabilisation around 2.0-1.9 Ga and before the last major tectonothermal event around 1.2-1.1 Ga. Results of a parallel Rb-Sr and U-ThPb study of the BG amphibolites indicate strong metamorphic resetting 1.2-1.1 Ga ago, thereby confirming earlier work on the WNP. Characteristics of the source region to the BG basalts have been inspected with MORB normalised trace element abundances, with the salient result being the BG basalts require derivation from a source that experienced variable geochemical modification similar to that proposed for the mantle wedge overlying modern subduction zones. Such a process is currently preferred over crustal contamination, since the Sm-Nd isochron for the BG amphibolites is remarkably linear, but independent confirmation of the Sm-Nd age is still required before this alternative model can be discounted. To reconcile the requirement of subduction related metasomatism in the source to tholeiitic basalts erupted within what appears to be a cratonic sedimentary basin, we suggest that back are continental extension could provide a satisfactory tectonic setting, as such an environment is intrinsically linked to subduction. |
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