PETROGENESIS OF HOMRAT EL-GIRIGAB ALKALI-FELDSPAR GRANITES, NORTHERN EASTERN DESERT, EGYPT

Abstract

Homrat El-Girigab area located at Northern Eastern Desert which, characterized by abundant intrusion of calc-alkaline and alkaline/peralkaline granitoids and their associated volcanics. These granitoids have a particular geodynamic interest as they provide an outstanding opportunity to tell how continental crust of ANS was formed. Homrat El-Girigab area is covered by Dokhan volcanics (andesite & dacites), which intruded by alkali-feldspar granites. The chemistry of biotites indicates that, the alkali-feldspar granites were crystallized from alkaline crustal source under oxidized conditions (i.e. nickel-nickel oxide buffer or NNO). They were crystallized under conditions including, temperatures range from 700 to 750 ̊C, pressures 3 to 4 kbar, depths of emplacement range from 7 to 11 km and under Oxygen fugacity (log fO2) ranges from -15 to -16. Homrat El-Girigab alkali-feldspar granites (HGAFGs) are alkaline, ferroan anorogenic (i.e. extensional) A-type granites. They were emplaced during the late post-collisional crustal extensional stage at which the effect of lithospheric delamination, and thus asthenospheric uprise, likely diminishes. At this stage the mantle-derived mafic melts start intraplating the lower crustal levels, that facilitated by the abundance of strike-slip faults and shear zones. This lithospheric intraplating caused widespread melting producing the alkaline magma of HGAFGs. The studied granites were derived from lower crustal amphibolitic source and evolved mainly by fractional crystallization.