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.