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Abstract The Precambrian rocks of Egypt (l00.000 Km2 ) outcrop over an extensive area in the Eastern Desert, south western part of the Western Desert and south Sinai. The Precambrian belt represents the NE part of the Arabian Nubian shield which is exposed in both sides of the Red Sea, and belonging to the Pan-African organic belt. The Egyptian Precambrian belt of the Eastern Desert IS composed of an upper-Proterozoic volcano- sedimentary complex containing scattered ultramafic bodies and intruded by syn-to late - tectonic plutonites (mainly tonalite to granodiorite) with extensive senes of calc-alkaline volcanic rocks. This sequence IS unconformably overlain by molasse sediments of the· Hammamat group. The sequences are cut by alkaline to sub-alkaline, late - to post tectonic granites. In the central and southern Eastern Desert , the deformed and largely mylonitized rocks of the infrastructure crop out in gneiss domes disposed along the axes of genetic lines trending NNW-SSE which also functioned as magnetic area (EI Gaby, 1983 ) . Remobilize infrastructure rocks are well displayed at Wadi Feiran, SW Sinai ( EI Gaby & Ahmed 1980), along the Qena Safaga road (Sabet et. a!., 1972 , Akaad et a!., 1973). Wadi Beida area lies in the southern part of the Eastern Desert about 30Km west of Shalatein town between latitudes 22° 54’ _23° 04’ 30” N and longitudes 35° 10’ - 35° 22’ 30” E and covers an area Cfiapter 0/1 The present thesis involves detailed field, geological, geochemical, mineralogical and structural studies for the rock units at Wadi Beida area. These rock units comprise dismembered ophiolities, arc volcanic assemblage, Arc-grantitoid and younger gabbros. The ophiolites occupy and extend in NW-SE direction in the northeastern part of the mapped area. They embrace ultramafic rocks, metagabbros, pillow lavas and cherts. These rocks are set, in places, inside a schistose matrix of highly sheared ultramafic rocks, foliated metasiltstone and metamudstone. Elsewhere, this matrix encloses small bands of quartz carbonates. All these components show tectonic contact in-between and alltogether were tectonically transported and intermingled with the metavolcanics. At Bir Beida, the contact of ophiolites against the metavolcanics is characterized by strong shearing in NW-SE direction. Veinlets of talc, chromite lenses and magnesite bands are aligned along this contact. , The Ultramafic rocks consist of intensively sepentinized hurzbergite, lherzolite and subordinate dunite. They are represented by main outcrop on the right side at the entrance of Wadi Beida and by unmappable slices east and southeast of G. Beida. These rocks are brown, dark grey to black and have a general NW -SE elongation. In localities, the ultramafics contain veinlets and pockets (5-11 em thick) of chrysotile indicating the tectonic effect. Petrographically, the ultramafic rocks are represented by serpentinites (with relics of hurzbergite, lherzolite and dunite). The serpentinites are composed of antigorite, low contents of chrysotile, lizardite and ankerite together with opaques (about 15-20% of the whole rock). The opaques are mainly iron oxides and chromite with traces of sulfides. The iron oxides are represented by magnetite that is densely altered Cfiapter’VI Summary and Condusions into goethite. The chromite occurs as disseminated crystals and is partly altered into magnetite. The metagabbros are exposed in the east of the mapped area and cover an area of about 60 km2 • They form low to moderate relief and have, along their eastern and western sides, tectonic contacts against the ultramafics and their highly sheared derivatives respectively. In places, these rocks exhibit layering , and rosette feature. The layering is mostly disturbed and is not continued for long distances. The layers are generally striking NW-SE and dipping at moderate and steep angles towards plagiogranites. The latter forms white bodies in the rocks. The metagabbros are coarse-grained but they are fine-grained in the uppermost parts. Petrographically, the metagabbros are mainly composed of plagioclase, actinolitic hornblende and pyroxenes. Opaques and sphene are the main accessory minerals. Biotite, chlorite, epidote and calcite are the secondary minerals. Quartz is found in the strongly sheared varieties and is mainly of secondary origin. Plagioclase (An 36-44) reaches to 60% of the rock. It shows lamellar and composite twinning and zoning. In samples, the core of plagioclae crystals are charged with epidote while the peripheries are clear indicating high and low calcic compositions respectively. Occasionally, these crystals are obscured by dense kaolinitization and sericitization. Actinolitic hornblende is partly or completely chloritized and still shows relics of pyroxenes. These relics include augite, enstatite and sometimes diopside; being altered to chlorite and epidote. Opaques (2-10%) are dominated by ilmenite (70 % of the opaques), magnetite (30 % of the opaques) and traces of pyrite. Ilmenite is occasionally homogeneous and shows two types of alterations; high temperature alteration to sphene and/or hematiterutile graphic intergrowths and low temperature alteration to goethite. Magnetite is Ti-poor variety. Hematite-ilmenite exsolution intergowth (metamorphic origin; Takla et al., 1981) is widespread in most samples. Magnetite external granules in ilmenite are rarely encountered. The pillow lavas are encountered near Bir Beida and trending NNW-SSE .The pillows are generally dark green and exhibit finegrained basaltic composition with amygdales filled with epidote, carbonates and iron oxides. The individual pillows are circular or oval-shaped with massive cores and a zone of vesicles near the periphery. The pillows range in size from 2S cm to 1 m and show general dipping toward the NE. Cherts occur as dark grey to grayish, yellowish and greenish gray bands. They are tectonically interleaved with the ultramafic rocks and pillowed basalt. The thickness of the chert bands vary from S to 30 ern. Petraographically, the pillow lavas are composed of plagioclase laths, tremolite, epidote and clorite together with few amounts of sphene, quartz, calcite and opaques. The opaques constitute about S% of the rock and are dominated by magnetite which is partly martitized. They are characterized by schistose and amygdaloidal textures. Some amygdales are filled with calcite, epidote and chlorite. |