U-Pb-Hf isotopic systematics of zircons from granites and metasediments of southern Ouaddaï (Chad), implications for crustal evolution and provenance in the Central Africa Orogenic Belt

Abstract

We present the first combined U-Pb ages and Lu-Hf isotope data of zircon grains from the southern Ouaddaï area in Eastern Chad in the Central Africa Orogenic Belt. Zircon xenocrysts from granites and detrital zircons from quartzites of the Ouaddaï document crustal accretion from depleted mantle sources at 3.0 ± 0.5 Ga, rarely up to 3.8 Ga, and for subsequent reworking during four magmatic events, during the Neoarchean at ca. 2.6 Ga (εHft = −7.0 to 4.6), Paleoproterozoic at ca. 1.9–2.1 Ga (εHft = −17.7 to 5.3), Mesoproterozoic at ca. 1.1 Ga (εHft = −15.3 to 5.9), and Neoproterozoic/early Cambrian at 635–540 Ma (εHft = −31.1 to 1.4). Superchondritic εHft values point to new crust addition during all four events. Metamorphic zircon rims yield a Concordia age of 600 ± 2 Ma, and provide evidence that greenschist to amphibolite-facies metamorphism affecting the quartzites occurred between syn-tectonic (635 Ma) and post-kinematic (540 Ma) granite intrusions. Our new set of zircon data, combined with field relationships, also indicate that the quartzite precursor rocks were deposited between 1045 and 635 Ma, possibly after 670 Ma in a basin that subsequently became the Central Africa Orogenic Belt. The detritus was supplied from surrounding cratons, although the sources of the Mesoproterozoic zircons remain ambiguous. Comparison of age-εHft data from worldwide sources suggests sediment supply from the Irumide, Katanga and Mozambique belts, located to the south-east of the Congo Craton. Paleogeographic reconstructions further require sediment transport prior to final closure of several oceanic basins surrounding Central and Northern Africa during the Neoproterozoic, comprising the Khomas and Mozambique oceans