الفهرس 
GeologyOnly 14 pages are availabe for public view
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Abstract
The Egyptian basement in the Eastern Desert is exposed over a triangular area and forms mountain ranges parallel to the Red Sea. Recently, the basement rocks have been distinguished into two major assemblages. The older assemblage (pre-Pan-Ahcan) comprises medium to high grade gneisses exposed in the cores of large anticlinal or domal structures constitullllg the infrastructure of the basement (Bennett and Mosely, 1987; Hermina et al, 1989 and El-Gaby et al, 1990). The pre-Pan-African rocks were tectonized and remobilized during the Panffican orogeny and are now commonly present as rnigmatites and granite m eisses. This assemblage also contains ultramafic, mafic and intermediate igneous rocks and metasediments. Their metamorphic grade is characterized by relatively hlgh temperature (rnigmatites) and pressure (garnet and kyanite grades), indicating deeper crustal conditions (Rashwan, 1991, Abdel-Khalek et al, 1992). The younger Pan-&can assemblage structurally overlies the former one forming the suprastructure. It is essentially composed of low grade metamorphosed successions of metasediments and metavolcanics (subgreenschist to biotite grades with local hlgh pressure or high temperature assemblages) and associated plutonic rocks. It also contains late tectonic molasse-type sediments (Hammamat Group), Dokhan Volcanics, and Attala felsites (Stem et al, 1988). The time-orogenic evolution of the Pan-African igneous and metamorphic events vary between about 900-600 Ma (Stem and Hedge, 1985; Kroner et al, 1992). The early compressional tectonic pilase was characterized by the emplacement of calc-alkaline, collisional, I-type granitoid batholiths rangmg in composition from quartz-diorite, tonalite to granodiorite with minor granites. This phase was subsequently followed by an extensional transitional phase (6 15-600 Ma) characterized with emplacement of A-type granites (Beyth et al. 1994). The end of the Pan-Ahcan activity is characterized by extensive emplacement of posttectonic granitoids and related rocks. Some of these rocks intrude and seal the shear zones related to the compression regimes of the last phase (Greiling et al, 1994). The Egyptian granitoids have been differentiated into: a) Older Granites of tonalitic to granodioritic composition and grey in color, and b) Younger Granites of granitic to alaskitic composition and pink and red in color, (El-Ramly and Akaad, 1960). Also, the granites were classified according to their time-relation to orogeny into syn-, late-, and post-orogenic, (El-Shazly, 1964). However, El-Gaby (1975) showed that the Egyptian granites constitute one continous series in whlch granitic rocks become more enriched in SiOz and K20 with younger age. The earlier plutons show oceanic or lower crustal geochemical characters, whereas the younger plutons show more involvement of continental material in their formation. Structurally, the Egyptian granitoids show a range of styles, modes and depths of emplacements. However, the majority do not conform with their established tune and dep* relationships with orogeny, e. g., plutons with catazonal modes of emplacement occur in mesozonasl metamorphosed country rocks only up to amphibolite facies but no granulites. The epizonal plutons which occur throughout the Egyptian basement are not related to any marginal arc. Most of these plutons and their enveloping country rocks show either insignificant or no post-emplacement metamorphism. The Egyptian Younger Granites and calc-alkaline to alkaline in nature which are mainly S-type, spread over the Central ilrnenitc series and Northern Eastern Desert and are smoetirnes associated with Sn, W, Mo, Nb-Ta, REE, Be, U-Th, and F- mineral deposits, (El-Gaby et al, 1988; Hussein, 1990). However, the Xu, Au-Cu, and Cu mineralization are mainly associated with the older subduction related granitoids (El-Gaby et al, 1988). Ln ths study four granitoids in the Central Eastern Desert (Umrn Rus ganodiorite, Umm Samra and Umm Bakra alkali fedspar granites and El-Ineigi u monzogranite) were chosen to study their petrography and geochemistry. All the plutons are associated with a network of quartz veins; Umm Rus is associated with Au-mineralization. While the other three bodies are not known to bear any mineralization. Tischendorf (1977) introduced the terms tin-specialized granite or t~n-cenerating u granite for felsic-granitoids with which rare-elements mineralization IS specially and/ or genetically associated. He recopzed the following four types: a) calcic-type whch is rich in Ca, b) agpiatic-type which has (Na+K/AI> l), c) plumasitic-type whch has Li-rich micas and metaluminous to peralurninous compositions, and d) miaslutic-type in which Ta, P, Ba, Sr and Nb are typically enriched. It is intended in thrs . , study to fully characterize these specialized granitic plutons and to classify them accordingly. Also, the quartz veins and their alteration zones will be tested for any possible rare-metal mineralization. The structural patterns will be studied and compared across the Central Eastern Desert to assess their relation to the controlling tectonics.