Crust‒mantle interaction and fluid evolution in the Arabian‒Nubian Shield: evidence from Abu Aqarib A-type granites, Eastern Desert of Egypt

Abstract

Post-collisional A-type granites provide important insights into crust-mantle interaction and late-stage hydrothermal activity in the Arabian-Nubian Shield. This study integrates petrography, mineral chemistry, and whole-rock geochemistry to investigate Abu Aqarib alkali feldspar granites in the Eastern Desert of Egypt. The aim is to evaluate magma sources, magmatic evolution, tectonic setting, and fluid-driven alteration. The granites are ferroan hypersolvus A-type varieties characterized by high SiO₂ (up to 76.5 wt%), elevated total alkalis (up to 8.6 wt%), enrichment in light rare-earth elements (LREEs), and negative Eu anomalies. Zircon saturation thermometry indicates high crystallization temperatures (875-1018 °C), whereas pressure estimates suggest shallow crustal emplacement (< 6.5 km) under reduced conditions (log ƒO₂ ≈ - 10). Geochemical signatures indicate derivation predominantly from partial melting of metasedimentary crustal sources, with limited mantle involvement during post-collisional extension. The generated magma subsequently evolved through extensive fractional crystallization, with possible minor crustal assimilation during its ascent. Petrographic observations and isocon mass-balance analyses indicate that late-stage fluid-rock interaction led to episyenitization. This process is marked by alkali metasomatism, quartz dissolution, and redistribution of several rare-metal and trace elements, including Nb, Ta, Th, U, Y, and heavy REEs (HREEs). A conceptual model is proposed in which evolved A-type magmatism was followed by structurally controlled fluid circulation and hydrothermal alteration. The coupled effects of magmatic differentiation and fluid-rock interaction contributed to rare-metal redistribution in Abu Aqarib AFGs. These findings offer novel insights into the processes controlling rare-metal concentration in post-collisional A-type granites and contribute to a broader understanding of post-collisional magmatic-hydrothermal evolution in similar granite provinces worldwide.