Late precambrian volcanism at Wadi Allaqi, south Eastern Desert, Egypt: evidence for transitional continental arc/margin environment

Abstract

The Dokhan volcanics at Wadi Allaqi, situated in the south Eastern Desert of Egypt, range in composition from basaltic-andesite to dacite. Geochemically, they have a transitional character from low K-tholeiite to calc-alkaline with a relatively high Zr/Y ratio that characterises a continental arc/margin setting. The most basic sample has extremely low Mg# (40) and Ni (55 ppm) values, indicating significant fractionation of olivine and clinopyroxene and that it is not a direct partial melt of the mantle-peridotite. The examined andesites and dacites might have been formed, respectively, via low-pressure fractional crystallisation of basaltic-andesite and andesite melts with about 53% and 35% removal of mostly hornblende and plagioclase with little magnetite and biotite. The enrichment of LILE (supposed to be derived from the subducted slab) and the relative depletion of HFSE (Ta, P and Ti) seem to be inherited from the mantle source. Hence, the examined volcanics are affected by both slab-derived partial melts and aqueous fluids in their generation.