Geochemistry and petrogenesis of late Neoproterozoic Nuweibi and Atawi rare metals bearing granites, central Eastern Desert, Egypt

Abstract

Nuweibi albite granite (NAG) and Atawi two feldspar granites (AG) are silica rich, highly evolved intrusions and rare metals-bearing in the Central Eastern Desert of Egypt exhibiting different stages of magmatic-late magmatic evolution. Both are evolved from low-P2O5 metaluminous to peraluminous granitic magma resulting from partial melting of crustal source. The chemical features of both of NAG and AG as Na2O + K2O, FeOt/MgO, Ga, Nb and Ta high concentrations are compatible with an anorogenic within plate tectonic setting. However, NAG have higher concentration of Al2O3, Na2O, MnO, Ga, Rb, Hf and Ta and lower concentrations of SiO2, TiO2, Fe2O3t, MgO, CaO, K2O, P2O5, Ba, Zr, Y, Sn, W, F and REEs. NAG shows obvious enrichment of heavy rare earth elements (HREEs) relative to light rare earth elements (LREEs), negative Eu anomaly and tetrad effect. On the other hand, AG has higher concentration of REEs and its pattern is slightly enriched in LREEs relative to HREEs with negative Eu anomaly. For NAG, the fluid-mobilization of REEs and other trace elements is supported by very low ratios of Eu/Eu* (0.17–0.3), (La/Yb)n (0.06–0.4), Nb/Ta (0.44–1.09), Zr/Hf (1.84–2.61), K/Rb (66.8–109.9) and Y/Ho ratios (2.9–6.3) together with high Sr/Eu (160–440). On the other hand, AG represents moderately fractionated granitic melt in regard to chondrites still showing chondritic or almost chondritic Nb/Ta (5.5–15.9), Zr/Hf (14.8–28.6), Y/Ho (26.2–45), K/Rb (99.24–346.28) and Sr/Eu (56.6–520) ratios indicating the lower effect of late magmatic fluid-rock interaction compared to NAG. Fluid phase enriched in alkalis from a nearly solidified granite causes local albitization in Atawi and Nuweibi areas where Nuweibi granite is more albitized. Moreover, greisenization (decomposition of feldspar and Fe-mica) is also more common in NAG.