Geochemistry and Sr- and Nd-isotopic study on rare-metal-bearing granitic rocks, central Eastern Desert, Egypt

Abstract

Three post-orogenic granitic plutons exposed in the Egyptian basement complex, namely Homr Akarem, Homrit Mukbid and Nuweibi were selected for geochemical and NdSr isotopic studies. Based on petrographic and geochemical considerations, the granitic rocks are divided into medium-grained pink granites and albite-rich granites. Both types exhibit peraluminous character with many chemical similarities to the high Y/Nb A-subtype granites. Owing to the extreme fractionated nature and complex magma source of these granitic rocks in addition to the high volatile activity, their compositions resemble within-plate magma originated in attenuated crustal lithosphere. Whole-rock RbSr isochron ages of the medium-grained pink granites from Homr Akarem and Homrit Mukbid plutons are 541 ± 4 Ma and 527 ± 26 Ma, respectively. The Nuweibi albite granite (NAG) represents the youngest granite phase and yields a RbSr age of 490 ± 27 Ma. The medium-grained pink granites show a moderate and narrow range of initial 87Sr/ 86Sr ratio (0.7064−0.7081) and negative ϵ t Nd values (−1.31 to −6.68). The NAG has an initial 87Sr/ 86Sr (0.7071) and ϵ t Nd (−3.26 to −4.72), comparable to those of the medium-grained pink granites. The SmNd data yield model ages (T DM) of 0.96 to 1.70 Ga. Similar Nd model ages (1.2−1.8 Ga) have been reported for other post-orogenic granites from southwest Egypt and northern Sudan. The Nd and Sr isotopic compositions of the studied mineralized granites suggest the involvement of old, LREE-enriched continental crust of Pre-Pan-African age and lower crust- or mantle-derived magmas. The Nd and Sr isotopic variations can be explained by a mixing hypothesis.