PETROLOGY AND GEOCHEMISTRY OF FELSIC DYKE SWARMS IN SOUTHWEST SINAI, EGYPT

The southwestern part of sinai massif is formed of schist, gneiss and migmatite; a metagabbro - diorite association and granitic rocks. They are dissected by dyke swarms of several different types, ages and styles, including mafic (old) and felsic (young) suites. The felsic dyke suite is studied in terms of field, petrography and geochemistry. The majority of these dykes trend in the ENE - WSW direction. They range in composition from comendite to alkali rhyolite and true rhyolite, with 73 - 81\% SiO2 and 3.2 - 7.6\% K2O. They consist of rhyodacite, rhyolite, quartz porphyry, granite porphyry and granophyre. Porphyritic varieties contain phenocrysts of plagioclase and minor biotite and hornblende. The intrusion of these dyke swarms has occurred during the emplacement of late precambrian younger granites. Chemically, these felsic dykes are peraluminous, enriched in K and their sialic nature suggests a derivation from upper crustal materials. Tectonomagmatic discrimination probably suggests a continental margin environment for these volcanics.