Petrography of the migmatites from the High Dam Western Quarry, Aswan, South Egypt

Abstract

The migmatites of the High Dam Western Quarry are part of a crystalline massif south of Aswan and comprise granitoid and trondhjemitic varieties. Both comprise leucocratic (leucosome) and melanocratic (melanosome) fractions. The granitoid migmatites are more abundant and their mineralogy is similar to that of biotite gneisses whereas that of the trondhjemitic migmatites is similar to that of the hornblende gneisses of Aswan. The migmatites show stromatic, ptygmatic and phlebitic structures. The plagioclase is normally zoned and usually carries fine inclusions of sphene, apatite and iron oxide that lie in planes parallel to the long axes or the twin planes of these plagioclase crystals. K-feldspar megacrysts show intensive perthitization of exsolution origin and some include xenomorphic quartz crystals between the idiomorphic cores and their outer rims. Porphyritic, myrmekitic, graphic and rapakivi textures are common in the migmatites. These features suggest crystallization of the leucosomes from silicate melts formed by partial melting in small closed systems in water-undersaturated conditions through high grade regional metamorphism up to the K-feldspar-sillimanite-almandine sub-facies.