PETROLOGICAL, GEOCHEMICAL AND ECONOMIC ASSESSMENT OF THE BASEMENT ROCKS IN WADI UM KHARIGA AREA, EASTERN DESERT, EGYPT

Abstract

The present work is a geological and geochemical study of the Pan African basement rocks in a critical area on the boundary between the central and southern parts of the Eastern Desert of Egypt, namely; the area between the southern reaches of Wadis Um Khariga and Garf.

Detailed field studies revealed that the area is covered by a variety of low grade regionally metamorphosed volcanic rocks pertaining to two different tectonic settings, associated with dismembered remnants of an ophiolitic sequence and metasediments, and intruded by older and younger granitoids and few dykes.

Most of the contacts between the metamorphosed rocks are tectonic and so a tentative chronological sequence is established, whereby the different rock units are grouped into three main petrotectonic assemblages, namely; a- the ophiolitic melange, b- the island arc assemblage and c- the late intrusives.

The ophiolitic melange includes slabs of serpentinite and amphibolite and metagabbro masses incorporated into foliated metabasalts and metasediments. The serpentinites are derived from peridotites of harzburgite composition, whereas the amphibolites and metagabbros exhibit mafic cumulate composition and together with the foliated metabasalts show that they are oceanic tholeiites of subalkaline character. The metasediments are essentially pebbly chlorite schists which geochemically seem to have been derived from tholeiitic or calc alkaline arc. The Atud Conglomerates which were thrusted over the pebbly chlorite schists are here considered to represent the lower part of a thick metasedimentary sequence with the chlorite schists forming the upper horizons. These schists enclose reworked cobbles and boulders of greywacke and fine conglomerate simulating petrographically those encountered in the succession of the Atud Conglomerates.

The island arc assemblage is represented by a sort of continuous series of slightly metamorphosed volcanic rocks intruded by arc granitoids. The metavolcanics include, a- massive metabasalts, b- the meta-andesites and metadacites of Gabal El Homr, c- banded metatuffs of metadacite and metarhyolite composition and d-rhyolitic lapilli metatuffs.