Esam S. Farahat

We report here, for the first time on the Nubian Shield, the western half of the Arabian–Nubian Shield (ANS), pegmatite-hosted pockets with a unique mineralogy, including pyroxmangite. It represents the second discovery on the ANS, where the first one was at Jabal Aja on the Arabian Shield, the eastern half of the ANS. One of the most remarkable aspects of pyroxmangite is its rarity and the potential economic value of its use in jewelry and decorative applications. Pegmatites are associated with A-type granites of the Gabal El-Bakriya intrusion (GEBI), Eastern Desert, Egypt. Mineralized pegmatites occur at the margin of the alkali-feldspar granite and exhibit gradational contacts with the host rocks. The pegmatites were emplaced as plugs and dikes within the intrusion and along its periphery. Pyroxmangite appears as coarse-grained, massive black aggregates or as disseminated crystals. The pegmatites are composed of K-feldspars and quartz, with subordinate amounts of albite, micas, and mafic minerals. Accessory phases include monazite-(Ce), zircon, fergusonite, xenotime, fluorite, pyrochlore, allanite, thorite, bastnäsite, samarskite, cassiterite, beryl, and pyrochlore. Pyroxmangite-bearing assemblages consist essentially of pyroxmangite and garnet, with accessory pyrochroite, quartz, zircon, magnetite, and fluorite. Geochemically, the pegmatites are highly evolved, with elevated SiO₂ content (76.51–80.69 wt.%) and variable concentrations of trace elements. They show significant enrichment in Nb (Nb > Ta), Y, REE, Zr, Th, U, and F, consistent with NYF-type pegmatites. REE contents range from 173.94 to 518.21 ppm, reflecting diverse accessory mineral assemblages. Tectonically, the pegmatites crystallized in a post-collisional setting, representing a late-stage differentiate of the A-type GEBI magma. Mineralization is concentrated in the apical and marginal zones of the granitic cupola and is dominated by barite, fluorite, Nb-Ta oxides, REE minerals, and uranium-bearing phases. The highly evolved granites, greisens, pegmatites, and quartz-fluorite veins of the GEBI have a high economic potential, deserving further exploration.

The Neoproterozoic granitic rocks of Mount Abu Kibash and Tulayah in the central Eastern Desert of Egypt are of geodynamic interest and provide us with important information about the evolution and growth of the northern part of the Arabian–Nubian Shield (ANS) continental crust. They are primarily composed of granodiorites and syenogranites based on new field, mineralogical, and geochemical analyses. The granodiorites are marked by an enrichment of LILEs such as Sr, K, Rb, Ba compared to HFSEs like Nb, Ta, Ti and show a higher concentration of LREEs relative to HREEs. This composition suggests a subduction-related setting and aligns with the characteristics of subducted I-type granites in the ANS. Chemistry of the analyzed primary amphiboles in the investigated granodiorites support a calc-alkaline nature, mixed source and subduction-related setting. The granodiorites represent an early magmatic phase in this setting, likely formed from a mix of mantle-derived mafic magmas and lower crust material, with subsequent fractional crystallization. On the other hand, syenogranites exhibit high SiO₂ (72.02–74.02 wt%), total alkali (7.82–8.01 wt%), and Al₂O₃ (13.79–14.25 wt%) levels, suggesting their derivation from peraluminous (A/CNK > 1) parental magmas. Their REE-normalized patterns are flat with a pronounced negative Eu anomaly, typical of post-collisional A2-type granites worldwide. These rocks originated from the partial melting of a juvenile lower crustal source (tonalite) in a post-collisional setting, driven by lithospheric delamination that facilitated mantle upwelling and underplating to the lower crust. Interaction between the upwelled mantle and lower crust led to fertilization (enrichment with HFSE and alkalis) of the lithosphere before partial melting. Fractional crystallization coupled with less considerable crustal assimilation are the main magmatic processes during the evolution of these rocks. The transition from subduction to post-collisional setting was accompanied by crustal uplifting, thickening and extensional collapse of ANS continental crust that caused emplacement of large masses of A-type granites in the northern ANS.

The widely distributed granitic intrusions in the Nubian Shield can provide comprehensive data for understanding its crustal evolution. We present new bulk-rock geochemistry and isotopic (zircon U-Pb and Lu-Hf) data from the Haweit granodiorites in the Gabgaba Terrane (NE Sudan). The dated zircons presented a ²⁰⁶Pb/²³⁸U Concordia age of 718.5 ± 2.2 Ma, indicating that they crystallized during the Cryogenian. The granodiorites contain both biotite and amphibole as the main mafic constituents. The samples exhibit metaluminous (A/CNK = 0.84–0.94) and calc-alkaline signatures. Their mineralogical composition and remarkable low P₂O₅, Zr, Ce, and Nb concentrations confirm that they belong to I-type granites. They exhibit subduction-related magma geochemical characters such as enrichment in LILEs and LREEs and depletion in HFSEs and HREEs, with a low (La/Yb)_N ratio (3.0–5.9) and apparent negative Nb anomaly. The positive Hf(t) values (+7.34 to +11.21) and young crustal model age (T_DM^C = 734–985 Ma) indicates a juvenile composition of the granodiorites. The data suggest that the Haweit granodiorites may have formed from partially melting a juvenile low-K mafic source. During subduction, the ascending asthenosphere melts might heat and partially melt the pre-existing lower crust mafic materials to generate the Haweit granodiorites in the middle segment of the Nubian Shield.