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Windhoek, Namibia

Hofmann M.,Private Bag X607 | Linnemann U.,Senckenberg Naturhistorische Sammlungen Dresden | Hoffmann K.-H.,Geological Survey of Namibia | Germs G.,Private Bag X607 | And 6 more authors.
Precambrian Research | Year: 2015

The transition from supercontinent Rodinia to Gondwana took place in the Neoproterozoic. The western margin of the Kalahari Craton in southern Namibia underwent rifting at c. 750. Ma, caused by the break-up of Rodinia, followed by drift-events and ongoing sedimentation throughout the Cryogenian (at least from 750 to 630. Ma) in Namibia. These sediments comprise at least three different deposits of glacio-marine diamictites (Kaigas at c. 750-720. Ma, Sturtian at c. 716. Ma and Marinoan at c. 635. Ma). The Ediacaran is characterised by collision during the assembly of Gondwana and includes a fourth glacial deposit (post-Gaskiers Vingerbreek glaciation at c. 547. Ma). This study presents more than 1050 single zircon grain U-Pb analyses of different diamictite horizons from southern Namibia and discusses their correlation. For all samples from sediments related to the Kaigas, Sturtian and Marinoan glacial events, the youngest obtained zircon ages were at c. 1.0. Ga, making differentiation by the maximum age of sedimentation impossible. But a correlation was still possible by using the complete detrital zircon U-Pb age patterns, with a significant change in the relative abundance of concordant Mesoproterozoic to Paleoproterozoic zircons. This P/M ratio seems to be a good tool to distinguish the Cryogenian diamictites (Marinoan: P/M. <. 0.4, Sturtian: 0.4. <. P/M. <. 10, Kaigas: P/M. >. 10). Although all the observed ages from the detrital zircons can be explained by derivation of local material, none of our samples correspond to the Cryogenian rifting events in southern Namibia. Therefore the source area cannot be local and more probably is located in the east of the studied areas. The constancy of the main U-Pb ages suggests a constant sediment supply direction throughout the Cryogenian. The same age populations occurring in the Ediacaran Aar Member indicate the same sediment transport direction from the east, but with an increased proportion of zircon grains older than 2.2. Ga. This marks a transition to the unconformably overlying Vingerbreek (post-Gaskiers) diamictite horizons, which show a significant change in the age spectra. Probably due to mixed input from the east (Kalahari Craton) and from the west (Gariep Belt), the Vingerbreek diamictites show a wider range of zircon ages with youngest ages at c. 590. Ma. This time is characterised by collision events and the Gondwana formation. The Hf isotope record shows that the only input of juvenile material in our samples occurred in the Mesoproterozoic during the Namaqua Natal Orogeny (formation of the Namaqua Belt). In total, four Archaean to Proterozoic crustal growth events are recognised in the western part of the Kalahari Craton: (1) Meso- to Paleoarchean (c. 3.42-2.8. Ga), (2) lower Paleoproterozoic to Neoarchaean (c. 2.8-2.27. Ga), (3) lower to upper Paleoproterozoic (c. 2.27-1.7. Ga) and (3) Mesoproterozoic (c. 1.6-1.0. Ga). © 2014 Elsevier B.V. Source

Kasemann S.A.,University of Bremen | Pogge von Strandmann P.A.E.,Birkbeck, University of London | Prave A.R.,University of St. Andrews | Fallick A.E.,Scottish Universities Environmental Research Center | And 2 more authors.
Earth and Planetary Science Letters | Year: 2014

A marked ocean acidification event and elevated atmospheric carbon dioxide concentrations following the extreme environmental conditions of the younger Cryogenian glaciation have been inferred from boron isotope measurements. Calcium and magnesium isotope analyses offer additional insights into the processes occurring during this time. Data from Neoproterozoic sections in Namibia indicate that following the end of glaciation the continental weathering flux transitioned from being of mixed carbonate and silicate character to a silicate-dominated one. Combined with the effects of primary dolomite formation in the cap dolostones, this caused the ocean to depart from a state of acidification and return to higher pH after climatic amelioration. Differences in the magnitude of stratigraphic isotopic changes across the continental margin of the southern Congo craton shelf point to local influences modifying and amplifying the global signal, which need to be considered in order to avoid overestimation of the worldwide chemical weathering flux. © 2014 Elsevier B.V. Source

Evans R.L.,Woods Hole Oceanographic Institution | Jones A.G.,Dublin Institute for Advanced Studies | Garcia X.,Barcelona Center for Subsurface Imaging | Muller M.,Dublin Institute for Advanced Studies | And 9 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200-250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly "damp" (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab. Copyright 2011 by the American Geophysical Union. Source

Kasemann S.A.,University of Bremen | Prave A.R.,University of St. Andrews | Fallick A.E.,Scottish Enterprise | Hawkesworth C.J.,University of St. Andrews | Hoffmann K.-H.,Geological Survey of Namibia
Geology | Year: 2010

The Neoproterozoic Earth underwent at least two severe glaciations, each extending to low paleomagnetic latitudes and punctuating warmer climates. The two widespread older and younger Cryogenian glacial deposits in Namibia are directly overlain by cap carbonates deposited under inferred periods of high atmospheric carbon dioxide concentrations. Oceanic uptake of carbon dioxide decreases ocean pH; here we present a record of Cryogenian interglacial ocean pH, based on boron (B) isotopes in marine carbonates. Our data suggest a largely constant ocean pH and no critically elevated pCO2 throughout the older postglacial and interglacial periods. In contrast, a marked ocean acidification event marks the younger deglaciation period and is compatible with elevated postglacial pCO2 concentration. Our data are consistent with the presence of two panglacial climate states in the Cryogenian, but indicate that each had its own distinct environmental conditions. © 2010 Geological Society of America. Source

Van Schijndel V.,Gothenburg University | Cornell D.H.,Gothenburg University | Hoffmann K.-H.,Geological Survey of Namibia | Frei D.,Stellenbosch University
Geological Society Special Publication | Year: 2011

The African continental crust was assembled by a series of orogenies over a period of billions of years mainly in Precambrian times. Tracing the build-up history of this stable crust is not always straightforward due to multiphase deformation and regions with poor outcrop. Episodes of metamorphism and magmatism associated with multiple Wilson cycles are recorded in zircons, which found their way into sediments derived from the hinterland. Dating of zircon populations in detrital rocks can hence provide age spectra which reflect the metamorphic and magmatic events of the region. Microbeam dating of detrital zircon is used to characterize the crustal development history of the Rehoboth Province of southern Africa. We investigated a quartzite of the Late Palaeo-Early Mesoproterozoic Billstein Formation, formed in a continental basin, and a quartz-feldspar arenite layer of the late Mesoproterozoic Langberg Formation conglomerates, immature sediments formed within a felsic volcanic system (both close to Rehoboth Town). The combined data indicate three episodes of crustal evolution in the Rehoboth Province. The oldest phase is only documented in the Billstein quartzite by three 2.98-2.7 Ga Archaean zircons. A Palaeoproterozoic phase between 2.2 and 1.9 Ga is older than any known exposures of the Rehoboth Province. The Billstein quartzite shows a main peak at 1.87 Ga, corresponding to the 1863±10 Ma Elim Formation. The Langberg sample reflects magmatism related to the entire Namaqua-Natal Wilson cycle between c. 1.32 and 1.05 Ga. The absence of zircons of that age range in the Billstein quartzite indicates a pre-Namaqua age for the Billstein Formation. Our data shows that there were at least three episodes of crustal development at 2.98-2.7 Ga, 2.05-1.75 and 1.32-1.1 Ga. We have documented the existence of a previously unrecognized 2.98-2.7 Ga Archaean crustal component, which was probably exposed in the Rehoboth Province during the Palaeoproterozoic and thus indicates a much longer geological history for the Rehoboth Province than previously known. © The Geological Society of London 2011. Source

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