SHRIMP zircon dating and Nd isotopic systematics of Palaeoproterozoic migmatitic orthogneisses in the Epupa Metamorphic Complex of northwestern Namibia

The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia bordering the Kunene River and extending into southern Angola It consists predominantly of granitoid gneisses which are migmatized over large areas This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite We have undertaken reconnaissance geochemical studies and single zircon U-Pb SHRIMP and Pb-Pb evaporation dating of rocks of the Epupa Complex The granitoid gneisses migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous calc-alkaline granitoids predominantly with volcanic arc geochemical signatures The zircon protolith ages for the orthogneisses range from 1861 +/- 3 to 1758 +/- 3 Ma Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 +/- 4 Ma and the anatectic Otjitanda Granite has a zircon age of 1757 +/- 4 Ma Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at similar to 1760 Ma which has not been recorded elswhere in northern Namibia The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2 4-2 0 Ga ago A major Archaean component in the source area is unlikely Structural reworking of the Epupa gneisses during the transpressional Neoproterozoic to early Palaeozoic Kaoko orogeny led to partial or complete obliteration of the older structures and resulted in spectacular low- to high-grade shear and mylonite zones This reworking did not affect the U-Pb isotopic system in the zircons but documents partial destruction of the Congo cratonic margin The Epupa granitoid rocks formed during an event generally referred to in Africa as the Eburnian orogeny but the nature and tectonic setting of the Congo craton of southwestern Africa during this time remain largely unknown (C) 2010 Elsevier B V All rights reserved