Emplacement of granites in a transitional regime from volcanic arcs to post-collisional: an evidence from the Gabal El-Zeit block, Gulf of Suez region, Egypt

Gabal El-Zeit block at the western coast of the Gulf of Suez region in Egypt consists of two varieties of Precambrian granites, namely monzogranite and a peripheral zone of alkali granite. Both granitic varieties display moderate (60.1-68.5 wt%) and high silica (73.36-76.16 wt%) contents with high Na₂O+K₂O (7.21-8.61 wt%), respectively. The investigated monzogranite is calc-alkaline metaluminous whereas the alkali granite is peralkaline. The primordial mantle normalized plots of the Gabal El-Zeit granites indicate that they are characterized by Ba, Sr, P and Ti negative anomalies and a moderate to weak negative Nb anomaly. These geochemical features are similar to those expected for magmas extracted during post-collisional high-K granites from the late Neoproterozoic Arabian Nubian Shield. Tectonic discrimination diagrams show that the monzogranite is formed in a volcanic-arc setting, whereas the alkali granites is a characteristic of post-collisional tectonics. The geochemical characteristics of the monzogranite are consistent with magmatism derived by partial melting of a slightly LILE-enriched amphibole-bearing (i.e. hydrated) mafic source. However, the studied alkali granite perhaps was originated by fractional crystallization process from the monzogranite due to the relative depletion in the contents of Al₂O₃, MgO, CaO, Ba, and Sr with increasing silica which is also strengthened by other geochemical relationships. The collective field investigations and available geochemical data are consistent with a late- to post-collisional stage magmatism formed at the transition from volcanic-arc to within-plate tectonics. The monzogranite is ilmenite- and magnetite-bearing, either homogenous or as exsolution, whereas alkali granites contains homogeneous magnetite only. Therefore, the Fe-Ti content is a very useful tool to distinguish the two granitic varieties.