Precambrian basement around Wadi Halfa,Sudan: a new perspective on the evolution of the East Saharan Craton |
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Authors: | R. J. Stern A. Kröner R. Bender T. Reischmann A. S. Dawoud |
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Affiliation: | (1) Center for Lithospheric Studies, University of Texas at Dallas, PO Box 830688, 75083-0688 Richardson, TX, USA;(2) Institut für Geowissenschaften, Johannes-Gutenberg Universität, Postfach 3980, D-55099 Mainz, Germany;(3) Department of Geology, University of Khartoum, PO Box 321, Khartoum, Sudan |
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Abstract: | This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African ( 500–900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northernmost Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episodes of crust formation, inferred from zircon ages, are preserved in the gneisses : 2.6, 2.4, 2.0, 1.7, 1.2 and 0.72 Ga. Nd model ages for the same units are invariably older than the zircon ages, yet still record a predominantly late Archaean and Palaeoproterozoic history, with depleted mantle model ages between 1.3 and 2.8 Ga. The earliest recorded Pan-African magmatic event is about 720 Ma and dates the beginning of collisional deformation. A younger Pan-African volcanic sequence ( 650 Ma) has isotopic compositions of Sr and Nd compatible with derivation from late Prote rozoic asthenospheric mantle. A 530 Ma anorogenic A-type granite also has isotopic compositions suggesting derivation from a primitive source. The inferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction leading to collision at about 700 Ma, accompanied by post-orogenic rifting at about 650 Ma. |
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Keywords: | Precambrian Geochronology Geochemistry Crustal evolution Africa |
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