Abstract
Dyke swarms traverse Neoproterozoic rocks in the Hawashiya region in the extreme northern part of the Eastern Desert of Egypt. They are a bimodal suite of mafic dykes including basaltic andesite and andesite, and felsic dykes including dacite and rhyolite. The mafic dyke suite is more abundant in the younger granites than in the older granitoids in which the felsic dykes are the commonest. The dyke swarms trend predominantly NE-SW. The felsic dyke suite is older than the mafic dyke suite. Phenocrysts of plagioclase, clinopyroxene, amphibole, biotite, potash feldspar and quartz embedded in a fine-grained groundmass are the main components. Clinopyroxene is augite in basaltic andesites, while it varies from diopside to augite in andesites. Amphiboles from the mafic dyke suite comprise either magnesio-hornblende and actinolitic hornblende or magnesio-hastingsite. Plagioclase composition varies from labradorite - oligoclase (An58-12) in the mafic dyke suite, to oligoclase (An13-24) in the felsic dyke suite. Both dyke suites are calc-alkaline (Alkaline dykes are rare) and are relatively depleted in TiO2 and Nb but enriched in the incompatible elements and HFSE. The felsic dyke suite is enriched in REE and strongly LREE fractionated relative to the mafic dyke suite. Although the Hawashiya dykes were emplaced at the end of the Neoproterozoic in an extensional tectonic setting, they have geochemical characteristics that are consistent with a subduction-related regime. These chemical signatures were inherited from the lithospheric rocks that produced their host Hawashiya granitoids. The felsic dyke suite magma can be derived from crustal rocks (essential source component) by partial melting. The mafic dyke suite magma was generated from a lithospheric mantle and has undergone a limited degree of fractional crystallization.
