Geochemistry and Petrogenesis of Late Ediacaran Rare-Metal Albite Granite of the Nubian Shield: Case Study of Nuweibi Intrusion, Eastern Desert, Egypt

The Nuweibi albite granite (NAG) is a postcollisional intrusion emplaced as a high-level magmatic cupola into metamorphic and syntectonic calc-alkaline country rocks. It consists of two cogenetic intrusive bodies: the western, nonporphyritic, albite granite was emplaced deeper than the eastern, fine-grained, porphyritic, albite granite. In places the nonporphyritic phase crosscuts the earlier porphyritic phase, but the occurrence of gradational contacts between the phases implies a near coincidence in time, with the nonporphyritic phase emplaced before crystallization of the porphyritic phase was complete. The steeply dipping slope of the western contact of the Nuweibi pluton against country rocks, in contrast to the gently dipping contacts above the eastern and northeastern parts, indicates the probable location of the cupola apex in the eastern part of the pluton. The NAG intrusion is highly evolved meta- to peraluminous leucocratic rare-metal albite granite. The NAG intrusion is chemically zoned, with upward increases of Al2O3, Na2O, Sr, Ga, and Ta concentrations, alongside upward decreases in SiO2, K2O, Rb, Nb, Zn, Zr, Th, Sn, and rare earth element concentrations. These trends are interrupted by a compositional gap with discontinuities in evolutionary trends of both compatible and incompatible elements, suggesting multiple pulses of intrusion. The NAG was generated via partial melting of a juvenile crust that had undergone extensive fractional crystallization combined with late-magmatic fluid overprint. Accumulation of residual volatile-rich melt and exsolved fluids in the apical part of the magmatic cupola produced stockscheider pegmatite, greisen, and quartz veins that cut the peripheries of the NAG pluton. Metasomatic activity by late- to postmagmatic fluids drove changes in the bulk composition of the cupola, removing K and driving the alkali feldspars toward pure albite.