Petrogenesis and geodynamic model for (Ta, Nb)-fertilized Nuweibi albite granite, Egyptian Nubian Shield: Juvenile crust-mantle mixing and metasomatic enhancement

Abstract

Nuweibi Albite Granite (NAG, 1.3 km² as total area) is considered one of the well-known highly evolved, rare metals-bearing pluton in the Egyptian Nubian Shield. The Nuweibi area constitutes ophiolite assemblage (metagabbros, serpentinites and related talc-carbonates), tuffaceous metasediments with their associated hornfelse, granodiorite-tonalite and post-tectonic albite granite, the latter is cross cutting by aplite and basaltic dykes. The present work focuses on the geochemical characteristic of NAG, using new data of whole-rock geochemistry and mineral chemistry to address-out approach on its origin, geodynamic evolution and genetic relation with the Ta-Nb mineralization. The main (Ta, Nb)-bearing minerals include; tantalite, columbite, wodginite and calciomicrolite. The geochemical analyses revealed that, the studied NAG has peraluminous, within-plate, post-collisional, A-type characters. It exhibits an enrichment of Na₂O (up to 6.56 wt%) reflecting the alkaline affinity (σ up to 6.19), closing to alkaline trend of an extensional regime. The REEs tetrad effect is well pronounced with negative Eu anomaly, which is most remarkable toward the closing of magma at the late magmatic stage of fractionation. The analytical criteria support the evolution of NAG through highly evolved magmatic fractionation (in major) and later metasomatic replacement (in minor). The abundance of rare metals such as (Ta, Nb), suggests the strong influence of fluorine and aqueous fluid system in the residual melt. With respect to the magmatic origin of NAG through advanced degree of fractionation, a complex set of geological processes including crust-mantle mixing, partial melting and fluid flux-volatiles interaction (i.e. metasomatism) have been involved during the geodynamic evolution.