GEOCHEMISTRY AND PETROGENESIS OF THE CALC-ALKALINE GRANITOIDS FROM THE NORTHERN ARABIAN–NUBIAN SHIELD. A CASE STUDY FROM WADI EL-AKHDER GRANITOIDS, SOUTHERN SINAI, EGYPT

Abstract

I-type granitoids emplaced during pre-collision stages of the Neoproterozoic Pan-African Orogeny, are widely distributed in the southern Sinai Peninsula and constituting 30% of the basement outcrops. Petrological, geochemical, and mineral chemistry data are presented for the I-type, Wadi El-Akhder granitic suite (AGR), in order to determine their origin and geotectonic implications. The tonalite– granodiorites of Wadi El-Akhder have geochemical characteristics of medium-K calc-alkaline, metaluminous to mildly peraluminous granitoids formed in an island-arc environment, which are conformable with well-known Egyptian older granitoids rocks (G1). They have low Al2O3, FeO*, MgO, MnO, CaO, TiO2, Sr, Ba, and V contents, but possess high contents of Na2O, K2O, Nb, Zr, Th, and Rb. The studied AGR are generally characterized by enrichment of LILE and LREE and depletion of HFSE relative to N-MORB values, and follow assimilation and fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source. The studied granitic rocks show high CaO/Na2O ratios (0.82-1.54) being decrease from tonalite to granodiorites, whereas they have also high Al2O3/TiO2 ratios (16.69-27.61) increasing toward the granodiorites. These features indicate that emplacement of the granitoid were strongly affected by magma mixing of felsic and mafic melts. The chemical characteristics of the AGR are remarkably similar to those of subduction-related granitoids from the Arabian-Nubian Shield (ANS).