Archaean Zircon U-Pb Age Paradox in Juvenile Neoproterozoic Granitoids, Central North Sudan, Saharan Metacraton


Abstract: It has long been shown that central North Sudan with its heterogeneous isotopic composition, unlike neighbouring Archaean cratons and the Neoproterozoic Arabian-Nubian Shield, is part of the Saharan metacratonic crust that was remobilized in the Neoproterozoic. In this paper, we report an Archaean zircon U-Pb age in the Saharan Metacraton that paradoxically does not indicate the presence of an Archaean crust. The second paradox concerns geochemical data that show Archaean tonalite-trondhjemite-granodiorite (TTG) features and also post-Archaean granitoid features, yet the rocks are Neoproterozoic. The granitoids studied are from north of Delgo in the Halfa terrane. They are calc-alkaline and meta- to peraluminous and have negative Nb and Ti anomalies. Zircon morphology, cathodoluminescence (CL) and U-Pb age data define four magmatic zircon populations. The oldest is characterized by a 3025 Ma Archaean U-Pb age. The 728 to 702 Ma ages of the second zircon population suggest that the studied area was involved in the Neoproterozoic intraplate magmatism that was induced by the delamination of the thickened asthenospheric mantle due to the first collisional contact between East and West Gondwana. Zircons of the third group yield peak Pan-African orogeny Neoproterozoic ages (630 to 600 Ma) that are identical with titanite age data, and show that the studied rocks were intensely remobilized by the Pan-African tectono-metamorphic regional event. Lastly, the 554 Ma concordant zircon of the fourth population suggests that the area thereafter never again experienced such high temperature and pressure regional orogenic effects. Sr initial values (0.702389-0.704011) and \epsilonNd values (+5.05±8.66) indicate juvenile sources with insignificant crustal contribution. Nd TDM ages are Neoproterozoic (917-653 Ma) and identical within error and/or slightly older than the Neoproterozoic zircon ages, confirming the primitive nature of this magmatism. This sets a paradox with the Archaean zircon population in these rocks, implying that this zircon population is xenocrystic and has no bearing to the age of the tract of terrane investigated. These results show that combining zircon U-Pb ages and Nd isotope systematics is a very powerful tool in unequivocally defining the petrogenesis of rocks and geological terranes.

Keywords: zircon populations, Archaean zircon U-Pb age paradox, Nd TDM, petrogenesis, juvenile sources, Neoproterozoic

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