A proposed new classification of the granites of egypt

Abstract

Granites and granitoids constitute an important rock group that covers vast areas of the Arabian-Nubian Shield in Egypt. They range in composition from quartz diorite and tonalite, through granodiorite and quartz monzonite to true granites and alkaline-peralkaline granites. Several workers tried the identification and classification of these granites. They were classified according to relative age (old and younger granites), dominant colour (grey, red and pink granites), type localities (Shaitian, Gattarian and Gharib granites) or their apparent relation to orogeny (syn-, late- and post-orogenic granites) and so on. Two plate-tectonic models for the evolution of the shield were proposed: (a) opening and closing of sialic crust; or (b) suturing of a number of island arcs with a continent through the subduction of oceanic lithosphere. The present authors favour the second, and here relate the Egyptian granites to this evolutionary model. Accordingly, the granites of Egypt may be grouped as: 1. (a) Subduction-related calc-alkaline granodiorites. 2. (b) Suture-related granites, formed in a thickened crust due to folding and thrusting. 3. (c) Intraplate, anorogenic granites, related to hot spots and incipient rifting. In order to find out specific features that may be used to characterize each of these groups, data (petrological, geochemical, geochronological and field relations) of as many as possible of the granite masses were collected, processed and correlated with bodies from well known geotectonic settings and from neighbouring countries. On the basis of this, a new classification of the Egyptian granites is here proposed.