Unraveling the Genesis of Highly Fractionated Rare-Metal Granites in the Nubian Shield via the Rare-Earth Elements Tetrad Effect, Sr-Nd Isotope Systematics, and Mineral Chemistry

The rare-metal granites of the Nubian Shield are commonly associated with Nb-Ta and Sn mineralization. The magmatic and hydrothermal evolution of these granites and their relation to the Nb-Ta and Sn mineralization need further investigations. This study reports new mineral chemistry, whole-rock geochemical, and Sr-Nd isotopic data for the Abu Dabbab granites in the Central Eastern Desert of Egypt to provide new insights into the formation mechanisms of the granites and their associated rare-metal mineralization. The granites consist mainly of albite, orthoclase, quartz, and Li-phengite, with accessory minerals including cassiterite, columbite, tantalite, wodginite, and Hf-zircon. Geochemically, they are peraluminous with high SiO2, alkalis, Rb, Nb, Ta, Sn, F, and low contents of MgO, CaO, and P2O5. The granites are characterized by distinct rare-earth element tetrad effects (TE1,3 = 1.8-2.5), with strongly negative Eu anomalies and are extremely depleted in Ba, Sr, P, and Ti. These features imply that they are typical muscovite peraluminous highly evolved granites. Isotopically, they display positive ϵNd(t) values (+1.11 to +8.32) and two-stage Nd depleted model ages (tDM2) between 0.68 and 1.13 Ga, suggesting a juvenile magma source of the Neoproterozoic age. The Abu Dabbab granites could be formed by dehydration melting of muscovite-rich crustal materials followed by extensive fractional crystallization with a significant role of magmatic-hydrothermal fluids during the transitional orogenic stage of the Nubian Shield.