PETROLOGICAL AND GEOCHEMICAL INVESTIGATIONS ON DOKHAN VOLCANICS AT WADI UM SIDRA-WADI UM ASMER AREA, NORTH EASTERN DESERT, EGYPT

Abstract

The present work provides the geology, geochemistry, and petrogenesis of the Dokhan volcanics rocks exposed at the areas surrounding Wadi Um Sidra and Wadi Um Asmer in the northern Eastern Desert of Egypt, between Latitudes 27° 14 ́ and 27o 26 ́ N and Longitudes 33o 8 ́ and 33o 22 ́ E. The Dokhan volcanic rocks are exposed in the central (Gabal Dokhan) and the eastern parts of the mapped area. They cut by several sets of faults causing distinctive rugged topography, whereas narrow wadis run through these rocks. Based on the field and petrographical studies, the Dokhan volcanics are represented by a thick sequence of stratified lava flows of andesites, dacites, rhyolites and scarce amounts of basaltic rocks with pyroclastics which are composed of tuffs, agglomerates and ignimbrite. The Dokhan volcanics are intruded by monzogranites, syenogranites and alkali feldspar granites with sharp and irregular contacts, and sometimes are represented by large apophyses and several offshoots in syenogranites and as roof pendant over the monzogranites. Geochemically, these rocks have shown metaluminous to slightly peraluminous, medium to highK calcalkaline affinities and are unmetamorphosed with dominantly porphyritic texture and they enriched in LREE. The REE patterns of the rhyolites are characterized by wing shape with strong negative Eu anomaly, reflecting plagioclase fractionation. The studied Dokhan volcanic rocks have geochemical characteristics similar to the rocks developed in volcanic arc setting in subduction environment rather than transitional or post-collisional setting. Rhyolites have geochemical characteristics of lavas erupted in collisional setting.The systematic variations of major oxides and trace element abundances along continuous trends of decreasing Al2O3, CaO, MgO, Fe2O3, TiO2, P2O5, Ni, Cr and Sr and increasing K2O, Rb, Nb and Zr with increasing SiO2can be interpreted in terms of fractional crystallization. The similarity and parallel nature of the normalized REE patterns with increasing total REE contents and decreasing Eu/Eu∗ with increasing SiO2 imply a major role of crystal fractionation during the evolution of these rocks.