Controls on the emplacement of the arc-related Um Rus pluton, central Egyptian Nubian Shield

The emplacement of small, compositionally zoned, arc-related, gabbro-diorite plutons is controversial, particularly where voluminous older batholiths are emplaced in volcanic arc settings. The Um Rus zoned pluton (URZP) features a zone of hybrid quartz diorite at its outer margin, containing abundant mafic xenoliths, and a core that grades inward from tonalite to granodiorite. New U–Pb geochronologic data indicate that the URZP was emplaced at 675.6 ± 5.1 Ma. Magnetic data shows a curved contact zone for the gabbroic intrusion that hosts the URZP at shallow depths. The URZP and the host rocks were emplaced entirely within the wadi Mubarak shear zone (WMSZ), but lack penetrative deformation at shallower depths, with regional structural trends related to the Najd fault system (NFS). Moreover, the intensity of the WMSZ structural framework increases at greater depths in relation to the NFS, and their distribution pattern suggests the transition from the NFS structural framework at shallower depths to the WMSZ framework at deeper crustal levels. The small gabbro-diorite arc magmas show a gradual increase in SiO₂ content with time but exhibit an abrupt change in MgO, K₂O, and Na₂O contents, suggesting successive intrusive pulses from a single parental magma with distinctive crustal assimilation. The gradual increase in crustal assimilation northward in the URZP is consistent with increasing arc maturity northward, away from the Allaqi-Heiani suture zone. The URZP is the most fractionated pluton, displaying a peraluminous and felsic nature. The compositional zoning in younger arc plutons, marked by a distinctive gap, led to their bimodal tectonic affinity. The magma chamber was emplaced in a pre-collision setting, predating the transition to a syn-collision setting at approximately 650 Ma.