Geochemistry and petrogenesis of I-type granitoid rocks around Nasb-Zurar intrusion, West Wadi Allaqi, South Eastern Desert, Egypt

Abstract

Granitic rocks surrounding Nasb-Zurar intrusion at North West Wadi Allaqi comprise two varieties: tonalite and granodiorite of older granite (OG), and monzogranite of younger affinity (YG). Plagioclase crystals in OG are generally zoned and range in composition from andesite to oligioclase with An (21.0–46.4), Ab (52.9–78.1), and Or (0.7–1.1); meanwhile, they have albite composition in YG with Ab (87.4–93.3). Alkali feldspars are perthitic microcline and orthoclase. Muscovite has phengite composition with Si:Aliv being greater than 3:1. Biotite varies between Al-poor biotite end-members annite and phlogopite. Chlorite in OG has ripidolite composition, while that from YG is brunsvigite. The OG granite has SiO2 wt.% of 67.7 to 71.9, with high Al2O3 wt.% of 12.9 to 16.2 and low TiO2 wt.% of 0.26 to 0.52, MgO wt.% of 0.47 to 2.07, Fe2O3 wt.% of 1.91 to 4.5, P2O5 wt.% of 0.05 to 0.13, CaO wt.% of 2.01 to 3.92, K2O wt.% of 0.36 to 3.63, and Na2O wt.% of 3.63 to 5.64. YG is rich in SiO2 wt.% (73 to 75), and Al2O3 wt.% ranges 13.3–14.4, K2O wt.% elevated between 3.7 and 4.6, CaO wt.% ranges 0.18–1.59, and Na2O wt.% is high (3.52–4.18), while TiO2, Fe2O3, and MgO wt.% contents show relatively lower values. All studied granites have peraluminous characters and evolve as I-type granites within volcanic arc environment. The OG are believed to form by melting of metagreywackes surrounding the area at about 680 °C; meanwhile, the YG is supposed to crystallize under relatively lower temperature (average ~ 630 C°) by re-melting and differentiation of OG.