Zircon geochronology and Sr, Nd, Pb isotope geochemistry of granitoids from Bayuda Desert and Sabaloka (Sudan): Evidence for a Bayudian event (920–900 Ma) preceding the Pan-African orogenic cycle (860–590 Ma) at the eastern boundary of the Saharan Metacraton

Abstract

Results of zircon geochronology and Sr, Nd and Pb isotope investigations carried out on granitoid rocks from the boundary of the Saharan Metacraton with the Arabian–Nubian Shield (ANS) reveal a protracted, partly pre-Pan-African geodynamic evolution and the existence of crustal terranes of different ancestries. (Meta)granitoids and gneisses in north-central Bayuda Desert record a 920–900 Ma orogenic event, hitherto unrecognized in northeastern Africa. This early Neoproterozoic Bayudian event is restricted to a pre-Pan-African crustal terrane which appears only slightly affected by Pan-African tectogenesis and deformation, named here the Rahaba–Absol terrane. Within this terrane, amphibolite-facies metamorphism occurred at 921 ± 10 Ma in the El Melagi muscovite–biotite gneiss of the Rahaba Series, probably during a collisional phase; followed by the intrusion of the Absol medium-K granite–granodiorite pluton discordantly emplaced at 900 ± 9 Ma into high-grade schists and amphibolites of the metavolcanosedimentary Absol Series. Nd T DM model ages (2040–2430 Ma) and ages of rounded zircon cores (1060, 1980, 2540 and 2675 Ma) of the El Melagi gneiss indicate a predominantly late Archaean to Palaeoproterozoic source region and suggest a latest Mesoproterozoic to early Neoproterozoic depositional age of its pelitic precursor sediment. The post-collisional Absol pluton has isotope characteristics ( ɛ Nd 900 of −0.3 to −4.3, T DM model ages: 1300–1830 Ma, SrIR 900 of 0.7028–0.7055, 207Pb/ 204Pb of 15.66–15.81) indicating assimilation of old pre-Neoproterozoic crust. The Rahaba–Absol terrane is part of the Saharan Metacraton and is in tectonic contact with the high-grade metamorphic Kurmut terrane of eastern Bayuda Desert. The Kurmut terrane has juvenile Neoproterozoic isotope characteristics, its granitoids record only Pan-African orogenic events and it is therefore considered a part of the Arabian–Nubian Shield. Within the Kurmut terrane the Dam Et Tor medium-K epidote–biotite gneiss gave a concordant zircon age of 858 ± 9 Ma, interpreted as the crystallization age of the volcanic/subvolcanic precursor of the gneiss. Isotopic data of the Dam Et Tor gneiss ( ɛ Nd 858 of 5.7–6.1, T DM model ages: 860–900 Ma, SrIR 858 of 0.7026–0.7028, 207Pb/ 204Pb of 15.46) indicate a depleted mantle source and an oceanic arc magmatic environment. The age of the amphibolite-facies metamorphism and deformation of the Dam Et Tor gneiss and its enclosing metavolcanosedimentary Kurmut Series is best approached by a Sm–Nd isochron age (806 ± 19 Ma; Küster and Liégeois [Küster, D., Liégeois, J.P., 2001. Sr, Nd isotopes and geochemistry of the Bayuda Desert high-grade metamorphic basement (Sudan): an early Pan-African oceanic convergent margin, not the edge of the East Saharan ghost craton? Prec. Res., 109, 1–23]). Both the Rahaba–Absol and the Kurmut terrane of the Bayuda Desert are intruded by post-collisional high-K granitoid magmatism of late Pan-African age, with no intermediate events recorded. The An Ithnein pluton that intrudes the pre-Pan-African Rahaba–Absol terrane is dated at 597 ± 4 Ma. The isotope characteristics of this pluton ( ɛ Nd 600 of −1.5 to 2.4, T DM model ages: 910–1190 Ma, SrIR 600 of 0.7011–0.7038, 207Pb/ 204Pb of 15.59–15.69) and that of the Nabati pluton intruding the Kurmut terrane ( ɛ Nd 600 of 0.1–4.5, T DM model ages: 750–1050 Ma, SrIR 600 of 0.7032–0.7036, 207Pb/ 204Pb of 15.56–15.60) indicate a derivation from a juvenile crustal source or the mantle with limited assimilation of older crust. Post-collisional late Pan-African granite plutons at Sabaloka, south of Bayuda Desert, are distinguished into high-K and shoshonitic suites. The Banjedid high-K pluton was emplaced at 605 ± 4 Ma, while the Babados shoshonitic pluton intruded at 591 ± 5 Ma. Magmatic zircons from both plutons have rims with low Th/U ratios, suggesting a HT metamorphic event contemporaneous with magma emplacement. Both granitoid suites have isotopic characteristics ( ɛ Nd 600 of −8.3 to −2.1, T DM model ages: 1160–1560 Ma, SrIR 600 of 0.7029–0.7069, 207Pb/ 204Pb of 15.72–15.81) indicating the assimilation of older pre-Neoproterozoic crust. Sabaloka is thus clearly identified as a part of the Saharan Metacraton but is distinguished from the Rahaba–Absol terrane by a strong Pan-African high-grade metamorphic imprint [Kröner, A., Stern, R.J., Dawoud, A.S., Compston, W., Reischmann, T., 1987. The Pan-African continental margin in northeastern Africa: evidence from a geochronological study of granulites at Sabaloka, Sudan. Earth Planet. Sci. Lett. 85, 91–104] indicating the likely existence of a discrete Sabaloka terrane.