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Department of Geology, Faculty of Science, University of Zagazig, Zagazig, Egypt

ABSTRACT

The massif of the Aswan region consists of granitic rocks of Precambrian age, the earlier phases of which are more mafic, coarse grained, and sometimes show porphyritic and rapakivi textures. The K-feldspar megacrysts show intensive perthitization of exolution origin and occur in more than one generation, sometimes replacing each other. The K-feldspar includes oriented plagioclase crystals that lie parallel to the re-entrant angles at the Carlsbad twinning planes of the K-feldspar megacrysts. The plagioclase megacrysts also contain oriented biotite, magnetite and apatite inclusions that are concentrated in the cores and between the cores and their outer rims. Occasionally these included crystals lie parallel to the re-entrant angles at the Carlsbad twinning planes of the plagioclase megacrysts. These features suggest crystallization of the K-feldspar and plagioclase megacrysts from silicate melts and lend some support to the magmatic hypothesis of the Aswan batholith